The small (diameter: < 13="" mm),="" short-lived,="" four-plated,="" australasian="" barnacle="">A. modestus was introduced to Britain in the 1940s, and from there it spread to continental Europe and Ireland. In continental Euro...
The small (diameter: < 13 mm), short-lived, four-plated, Australasian barnacle A. modestus was introduced to Britain in the 1940s, and from there it spread to continental Europe and Ireland. In continental Europe, its current invasive range is from Denmark to Portugal and at two sites in the Mediterranean (Thau Lagoon, Marseilles, and the lagoon of Venice). It spreads by remote and marginal dispersal. Its success in Europe has been attributed to its being eurythermal and euryhalineand its rapid growth, high reproductive capacity, long settlement period, generalist feeding habits and tolerance of turbid waters. It is particularly common on sheltered shores and in estuaries, but can survive on more exposed shores and occurs in the intertidal and shallow subtidal vones. It competes with native barnacle species and oyster spat. Recently it has started to expand its geographic range and increase in abundance. A single specimen was recorded in South Africa in 1949 but it did not become established.
This species was originally described as Elminius modestus by Darwin (1854, p. 350, plate 12, figs 1a-1e). Foster (1982) designated the lectotype from Darwin’s syntypic series that originated from New Zealand, Lectotype BMNH Reg. No 1981.274 (ex BMNH Reg. No. 184.108.40.206), one of several specimens attached to an intertidal limpet Cellana ornata, which are endemic to New Zealand. Moore (1944) found Darwin’s description “fairly full”, but see also plate 46D in Moore (1944), fig. 57 and plate 12 in Foster (1978), fig. 25 in Jones (1990), Southward (1976) and descriptions therein.
Buckeridge (1983) proposed three subgenera for Elminius, with Austrominius for E. modestus Darwin. Buckeridge and Newman (2010) have undertaken a review of the subfamily Elminiinae and the species formerly known as Elminius modestus is now known as Austrominius modestus (Darwin, 1854). According to Buckeridge (1983 in Jones, 1990, p. 402) “Austrominius may be distinguished by the presence of thin compartments with the inflected inner basal margin absent, the scutum with depressor muscle pits and the adductor ridge absent, but the articular ridge prominent, the tergal spur confluent with the basi-rostral angle, and the articular furrow of the tergum wide”.
Southward’s (2008) key to the intertidal barnacles of Britain and Ireland, with colour plates showing the differences in the colouration of tergo-scutal flaps, is useful for comparing A. modestus with other species found there. Southward (2008) summarises how to identify it in the field: “Identification: The four symmetric wall plates are diagnostic, usually tinged with slaty grey lines. The plates are thin but often carry rounded ridges giving the shell a sinuously octoradiate outline. The basis is membranous. In young and eroded specimens each scutum carries a slaty grey line…The tergo-scutal flaps of live specimens are held flat, basically white, with brown marks at the pylorus and two blackish bands in the rostral half…Size: A small conical barnacle up to 10 mm rostro-carinal diameter when fully grown”.
The change of the scientific name of this species from Elminius modestus to Austrominius modestus has not yet entered the literature. Elminius modestus is still used in most databases (WoRMS: World Register of Marine Species; DAISIE: Delivering Alien Invasive Species Inventories for Europe; NOBANIS: European Network on Invasive Alien Species; ITIS: Integrated Taxonomic Information System; GenBank; GISP: Global Invasive Species Programme). Buckeridge & Newman (2010) also transferred the subfamily Elminiinae to a new family, Austrobalanidae, from the Archaeobalanidae; WoRMS does include the Elminiinae in the Austrobalanidae.
Austrominius modestus is native to New Zealand, where it is very common in harbours and estuaries, on a range of substrates, in the midlittoral and shallow subtidal (Moore, 1944; Morton and Miller, 1968; Foster, 1978). According to Foster (1982) it is the commonest fouling barnacle in New Zealand harbours. There has been some debate as to whether it is native or introduced into Australia, since it is mostly found in port areas (see Foster, 1978; Jones, 1990). Foster (1982) suggested that it might have been introduced to Australia via shipping, some time before 1836 (as Darwin, 1854 collected a specimen from Sydney Island, dated 1836), while Flowerdew (1984) noted that it probably spread to Australia by remote dispersal before 1832. Southward (2008) states that in northwest Europe this species is an immigrant from New Zealand and does not mention Australia. Jones (1990) though points out that in Western Australia small numbers are also found “in sheltered estuaries and inlets in south-western areas...as well as in the port area at Albany”, suggesting that it may be an Australasian species. In Australia, it occurs along the southern Australian coast from the Spencer Gulf to Sydney and in Tasmania (Foster, 1982). Newman (1979) noted that it is unlikely that it ever inhabited Antarctica and its present distribution in Australasia is most likely entirely via the West Wind Drift.
It was introduced into the northern hemisphere via shipping (Foster, 1982) or flying boats (M Barnes, personal communication in Eno et al., 1997). It was first recorded in Europe in 1943 from Portsmouth, England (Stubbings, 1950). It is now ranges from Denmark to southern Portugal (Southward, 2008), and it has been recorded in Thau Lagoon, Marseilles, where oyster culture is practiced (H Zibrowius, Centre de Océanologie de Marseille, CNRS 6540-DIMAR, Station Marine d'Endoume, Marseille, personal communication, 1998), and in the lagoon of Venice (Cassellato et al., 2007). Its presence in the Mediterranean has been confirmed by Streftaris et al. (2005). Gruvel (1907) recorded Elminius cristallinus in the Azores (Ponta Delgada). Newman and Ross (1976) suggested that the single specimen that he found was probably Austrominius modestus; however, Foster (1982) thought that it migght have been a juvenile Balanus, while Alan Southward (Marine Biological Association of the UK, personal communication) proposed that it was probably a cryptic Chthamalus stellatus with plate abnormalities, and W.A. Newman (Scripps Institution of Oceanography, USA) and J.S. Buckeridge (RMIT University, Melbourne, Australia) concur with the latter suggestion (personal communication, 2009). Outside Europe, although a single specimen was recorded in South Africa in 1949 (Sandison, 1950), it did not become established there. Flowerdew (1984) suggested that it was “presumably introduced by ships stopping off on the voyage from the Antipodes to Europe”.
When Egan and Anderson (1985) compared the larvae of New Zealand samples (Barker, 1976) with Knight-Jones and Waugh’s (1949) descriptions of the species found in European waters, there were some anomalies, e.g. in the illustration with respect to the arrangement of spines on the thoraco-abdominal process of the naupliar stages III and IV were like those of Austrominius covertus (Foster, 1982) (formerly known as Elminius covertus Foster, 1982) (see Similarities to other Species/Conditions section). However, Egan and Anderson (1985) also spotted discrepancies between Knight-Jones and Waugh’s (1949) illustration and their text, but Barker (1976) has pointed out that differences in naupliar setation may occur with specimens collected from different locations or with interpretation of what constitutes a group of setae. Egan and Anderson (1985) had suggested that with the “reclassification of the Australian E. modestus into three separate species (Foster, 1982), a reassessment of the larvae of E. modestus occurring in European waters might prove of interest.” However, it should be noted that the adults of A. covertus and A. modestus are very different in colouration, that of A. covertus being red, and there are no published accounts to suggest that anything other than (white) A. modestus have been found on European shores.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
First found (a dense population) in 1978 in the intertidal area in the Danish Wadden Sea, but had disappeared by March 1978 (after a severe winter). According to Jensen and Knudsen (2005) repopulates after winter
A list of Australian museum specimens, which he has confirmed are the specimens A. modestus, not E. covertus. See Foster (1978) and Jones (1990) regarding doubts about whether native or introduced into Australia from New Zealand
This species is native to Australasia. Bishop (1951) described it originally as “one of the most geographically confined of all barnacles”. However, he pointed out that Darwin was aware of the important role shipping can play in the dissemination of barnacles. It was first recorded in Europe in 1943 from Portsmouth, England (Stubbings, 1950), but Crisp (1958) pointed out that it is impossible to establish exactly when it arrived in England (Bishop and Crisp (1948) say that the first record was from Chichester Harbour, near Portsmouth, in 1945). Bishop (1947) suggested that it was carried to Britain from its native coasts as fouling on ships or on flying boats (M Barnes, personal communication in Eno et al., 1997). Crisp (1958) described the pre-requisites for the establishment of an invasive species, such as A. modestus, which included ships being laid up for a sufficient period of time in an enclosed area, so that sufficient larvae could be released and subsequently recruit. Such a scenario (which Flowerdew (1984) called “a freak event”) arose during the start of World War II, when large numbers of vessels (including Australasian vessels), assembling as convoys, were anchored around Southampton for periods of time. Based on the spread of the species along the coast of southern England, Crisp (1958) suggested that the two most likely areas for its original introduction from Australasian shipping were Southampton Water and the Thames estuary, in particular the former. He suggested that the conditions at the time might have provided “a unique opportunity” for A. modestus to release larvae and as 1940 was a warm summer this may have allowed it to build up to critical densities to allow establishment, after which it spread by marginal dispersal while other populations were established by remote dispersal.
Crisp (1958) explained the means of dissemination of invasive species and defined the two main modes, the limitations governing the two processes and barriers restricting dissemination:
“A species invading new territory may follow either of two possible courses. It may spread only at the boundary of the existing population, by natural processes of dispersal or it may be carried a long distance by means of a vector and so establish a new centre of dissemination. It will be useful to refer to the former as marginal dispersal, and the latter as remote dispersal”.
“The process of marginal dispersal demands a suitable coastline with closely spaced objects such as piers, breakwaters, and boulders, available for settlement…During marginal dispersal a fresh area is colonized from a large well-stocked area nearby, and becomes in turn a source of larvae for further dispersal. The supply of larvae is therefore practically unlimited; the chief factor limiting the spread of the species is the distance which, on average, a larva will be carried during its planktonic life. In remote dispersal, however, the number of larvae introduced is limited. Even under the best conditions, as, for example, when an old hull covered in barnacles is laid up, the actual numbers set free must be very small compared with those which would be produced on a neighbouring well-stocked shoreline. The small number of larvae available in a given area is therefore a feature which in practice distinguishes remote from marginal dispersal. These few introduced larvae become dispersed by water movement, which will vary in magnitude with the topography, currents, tides and winds prevailing.”
With respect to its introduction into Great Britain, Crisp (1958) noted that, at that time, ships coming from Australia were occasionally found to be fouled by A. modestus and that even if even if not found on the hull, they could be found with other non-indigenous species in the condenser boxes and ducts. Crisp (1958) suggested that barnacles do not suffer adverse effects being carried on ships unless they visit fresh or polluted water; rather the steady water movement may be beneficial to growth and reproduction. A number of authors (e.g. Bishop, 1947; Crisp, 1960; Barnes and Barnes, 1961) have commented on the role of shipping in spreading the species. Barnes and Barnes (1961) noted that piers, where ships may remain for long periods, often become infected before shores. Barnes and Stone (1972) suggested fishing boats and pleasure crafts as likely vectors for initial spread to the Oban area. As well as fouling on ships’ hulls the larvae may also be transported in ballast water (Eno et al., 1997). Crisp (1958) and Crisp and Southward (1959) pointed out that, with respect to remote dispersal, the most likely areas for successful invasion are enclosed areas, such as docks, small harbours and narrow estuaries. Here infected ships may remain long enough in port to liberate larvae and the area may be suitable for the retention of the larvae during development and/or their later settlement, or during layings of shellfish when the shellfish are bearing spat (Southward and Crisp, 1954).
The spread of A. modestus around the British coast by marginal and remote dispersal has been well documented (e.g. Crisp, 1958). Crisp (1958) explains the different ways in which it spread around the British coast from its introduction in the early 1940s until 1955. After its introduction, it did not spread uniformly, but showed different patterns, depending on hydrographic conditions, including local tidal currents, shore type/substrate availability and exposure, which can affect supply of larvae and settlement. Barnes and Barnes (1961) suggested that it might have reached Kirkcolm in Scotland directly from the south of England, as a result of a seaplane, as a slipway nearby was used during the war. It spread around the British coast at a rate of 20 to 30 km per year from its introduction in the early 1940s until 1955. This figure is an indication of the dispersal range of a larval phase lasting 10 to 15 days along a coast which has no residual currents (Flowerdew, 1983). Barriers to its dispersal may be an unfavourable exposed rocky part of the coast, but more often Crisp (1958) suggested that open deep water was the most effective barrier, though fixed buoys may provide regular oases. Crisp and Southward (1953) suggested that ~48 km was near the critical limit for its crossing a sea barrier by natural means, with the larvae probably existing in the plankton for two to three weeks. Southward (1991) noted that although A. modestus is now a major component of the barnacle zone in estuarine and sheltered localities in southeast England, the Bristol Channel and the Irish Sea, it is not so common on the open coast in south-west Britain. Its most northerly location in Britain (and Europe) was recorded in 1976 in the Shetlands by Hiscock et al. (1978), although by 1986 it could not be found there (Eno et al., 1997); Barnes and Miller (2005) have reported it fouling on drifting plastic as far north as the Shetlands. Southward (2008) summarises its current distribution around Britain.
Although it was not recorded in France until 1950, Den Hartog (1956) proposed that it first settled in France in Normandy during the invasion of 1944, when ships travelling between the English southern coast and Normandy bore A. modestus (Stubbings, 1950). Crisp (1958) suggested that initially on continental Europe there were four, possibly five, main centres of establishment (most in France and one in the Netherlands) and provides a probable history of the dispersal there and the vectors responsible. The centres in France were the estuary of the Seine, shortly after the invasion of Normandy in 1944 (Den Hartog, 1956; Bishop and Crisp, 1957), the Brest area, probably in 1949/1950 (Crisp, 1958; 1959) and perhaps one in the river estuaries of the Morlaix-Roscoff area, around the same time as the one in Brest (Crisp, 1958). Based on their own records and those of other authors, Bishop and Crisp (1957) and Barnes and Barnes (1965a; 1966; 1968a; 1969; Barnes et al., 1972) have summarized its spread along the Atlantic coast of France, as well as in Spain and Portugal. Barnes and Barnes (1968a) proposed a number of different vectors for its spread along the French Atlantic coast, including shipping, local fishing craft and movement of substrata bearing the species during activities associated with oyster cultivation. They also suggested that the spread of A. modestus within the Bassin d’Arcachon was facilitated by the small boat traffic between oyster pounds and the movement of substrata (related to oyster farming) bearing Austrominius. The first location in the Mediterranean where it was recorded – in the Thau Lagoon, Marseilles, is one where oyster culture is practiced (H Zibrowius, Centre de Océanologie de Marseille, CNRS 6540-DIMAR, Station Marine d'Endoume, Marseille, personal communication, 1998), although it has also been recorded from the Venice lagoon (Casellato et al., 2007).
In 1946 it was recorded in the Netherlands (e.g. Meulen, 1946: Bishop, 1947; Boschma, 1948; Leenhouts, 1948a, b; Den Hartog, 1953; Wolff, 2005). Den Hartog (1953) suggested a number of ways by which it spread from the Britain to the Netherlands. He discounted flotsam, although the currents would transport drifting objects in a north-easterly direction from the English Channel, since he suggested that when deposited on the shores it would soon get covered by sand or dry out. He also ruled out larval transport, and suggested that fouling on ships’ hulls was the most likely vector, probably due to remote dispersal between East Anglia and the Hook of Holland (Bishop and Crisp, 1957; 1958), but that subsequent spread was by remote and/or marginal dispersal. Den Hartog (1953) suggested that its initial slow spread along the Dutch coast was due to lack of suitable substrate - just some breakwaters along an otherwise sandy coast. It was first recorded in Belgium in 1949 (Den Hartog, 1953) ((Leloup and Lefevere (1952) report 1950), and from Germany in 1951 (Den Hartog, 1953) (Kühl (1963) reports 1953). By 1978 it had reached Denmark (Rømo, Wadden Sea) (Theisen, 1980), its most northerly location on continental Europe. According to Jensen and Knudsen (2005), this is the only location where it has been found in Danish waters and it probably dies out in cold winters and recolonizes after mild winters; it had not been found in a survey of the Danish coast in 1960. According to Kühl (1963) it reached Helgoland by artificial transport. As well as being spread on ships’ hulls, it was also carried on other crustaceans and on molluscs. For example, Leloup and Lefevere (1952) noted its being found on the carapace of the crab Portunus holsatus, which had been caught off Ostend in Belgium in April 1952. Crisp (1958) attributed its rapid spread (e.g. 52 km per year between 1946 and 1953) along the coast of the Netherlands and Germany to the strong residual currents in that area.
A centre in northwest Spain probably originated between 1950-1953 (Fischer-Piette and Prenant, 1956), whereas the one in Portugal was probably distinct from this one (Fischer-Piette and Prenant, 1957). These authors describe the spread of the species along the Iberian Atlantic coast. According to Bishop and Crisp (1958), it spread at a rate of 50-70 km per year along the coasts of France and Portugal. Fischer-Piette and Prenant (1956; 1957) pointed out that in Spain and Portugal they only found it in sheltered areas, such as ports, bays, estuaries and rias, rather than on the open coast, similar to the situation at that time in Brittany. They suggested that it may be because as it is a cold temperate species the climatic conditions are not favourable for it. Fischer-Piette and Forest (1961) suggested that a reduction in numbers between surveys on this coast might imply a precariousness in the conditions due to the proximity of a climatic barrier, noting that the north Iberian coast is a transition area for quite a few species. The southernmost published record in Europe is at Praia de Faro, southern Portugal (O’Riordan and Ramsay, 1999), while J.J. Castro et al. (Laboratório de Ciências do Mar, Universidade de Evora, Portugal, personal communication, 2009) are currently undertaking a resurvey of the Portuguese coast for non-indigenous species, with 16 sites being examined for A. modestus. Some authors (e.g. Eno et al., 1997) suggest that it has been found in Gibraltar, citing Barnes and Barnes (1966), but although the latter authors have Gibraltar marked on their map of sampling sites (Fig. 1, p. 84), they did not find it there. Lewis (1964, fig. 70) depicts it rate of dispersal in Europe, with probable colonization dates for certain localities, while Southward and Crisp (1963) discussed its spread along Atlantic shores.
Having been absent in 1953 (Southward and Crisp, 1954), it was first recorded in Ireland, in southwest Cork, in 1957 (Beard, 1957); however, it was probably established in Cork Harbour a few years before then (Crisp and Southward, 1959). It was probably introduced as fouling on ships’ hulls. It is now present around the whole Irish coast where there are suitable habitats (e.g. O’Riordan, 1996; Allen et al., 2006), spread by a combination of remote (shipping and shellfish movement, especially oysters) and marginal dispersal (O’Riordan, 2002).
At the limits (western and southern) of the introduced range, a few specimens have been found on certain shores but the species never became established on those shores, e.g. in Ireland (see O’Riordan, 1996) and Portugal (O’Riordan and Ramsay, 1999). As mentioned above, Fischer-Piette and Forest (1961) suggested that a reduction in numbers between surveys might imply a precariousness in the conditions due to the proximity of a climatic barrier, but this may also occur due to stretches of unsuitable coastline. Crisp (1958) pointed out that the initial population density is of critical importance, since individuals further apart than 5 cm never became fertilized, as this species is an obligatory cross-fertilizer (Crisp, 1954; Barnes and Crisp, 1956); if insufficient individuals settle within 5 cm of each other to allow breeding, then the population will not become established. Crisp (1958) called this the ‘critical breeding density’. Crisp (1958) commented that “Obviously the greater the proportion of unfavourable coastline, the smaller will be the numbers of larvae released and scattered along it, and the more likely will these larvae settle in unfavourable places or fail to reach the critical breeding density…These conditions will diminish the rate of spreading of the species, for the majority of larvae produced will be wasted in unsuitable places. Another type of wastage may be important in such an environment, namely, the wastage of larvae offshore.”
There is no suggestion that this species has ever been intentionally introduced. Under climate change scenarios, there is potential for it to spread further and/or increase in abundance at the northern margins of its present distribution and potentially displace arctic-boreal barnacle species. In recent years it has already started to show significant increases in its abundances, having remained at relatively low densities previously (Witte et al., 2010).
Moore (1944) noted that in New Zealand, A. modestus occurs in estuaries and harbours, in mangrove forests and Zostera beds. According to Moore (1944) “Darwin picked out on the most striking ecological attribute of E. modestus, its ability to withstand brackish and very muddy water”. Jones (1990) suggested that it thrives in conditions of little water movement. It occurs on the open coast, but it can only tolerate modest wave action. With respect to tide level, Moore (1944) described it as ‘versatile’. Intertidally, it can occur at a range of shore heights, including high on the shore if there is freshwater or brackish water runoff, as well as subtidally (~5 m depth; Jones, 1990) and it occurs as fouling on harbour vessels and test panels. According to Bishop (1947), this species in its native waters appears to occupy the position held by Amphibalanus improvisus in the British fauna. In southwestern Australia it is associated with Austrominius covertus. Moore (1944) and Jones (1990) reported that, in New Zealand, this species can settle on a wide range of animate and inanimate substrata, noting that “The nature of the substratum is immaterial if conditions otherwise are suitable” and it favours mobile animals as hosts. According to Luckens (1975), it can settle on rock, wood, iron, other barnacles, settled invertebrates, and even algae, both intertidally and subtidally, when cyprids are available in sufficient numbers.
Early on in its invasion of Europe, Crisp and Chipperfield (1958) noted that the habitats favoured by A. modestus were similar to those described for its native range in New Zealand by Moore (1944), and this is still the case. Although A. modestus is more commonly associated with sheltered areas, such as inlets and estuaries, it can survive and grow in exposed areas (Crisp, 1958). Anon. (1948 in Crisp, 1948) suggested that it “seems to withstand muddy waters of our estuaries better than any of our native barnacles”. With respect to wave-exposed shores, Crisp (1958) suggested that possibly the species is uncommon there because its larvae do not settle, rather than because of high mortality (from wave crash or predation). He also suggested that the seawater in tidal estuaries might contain more suitable sized food particles than offshore waters. In France, Crisp and Fischer-Piette (1959) reported A. modestus occurring mostly in very sheltered and sheltered areas, less so in semi-exposed areas, rarely in exposed areas and not at all on very exposed shores. It can flourish in sheltered lochs and bays where summer temperatures can be high (Barnes and Barnes, 1961) and was found to replace native species in sheltered bays and harbours of the eastern part of the English Channel (Crisp and Southward, 1959). As mentioned earlier, in southwest Britain estuaries are now dominated by A. modestus, but in normal salinities on moderately exposed shores A. modestus occurs only at low densities, being outnumbered on different parts of the shore by Semibalanus balanoides, Perforatus perforatus and Chthamalus montagui (Ross et al., 2003). Jenkins (2005) examined the abundance of A. modestus, along with three other barnacle species, at three shore heights on four shores (two sheltered and two exposed) in the Plymouth area. The shores were dominated by chthamalid barnacles with the maximum percentage abundance of A. modestus being less than 25%
According to Crisp (1958) “Elminius tolerates the presence of silt and pollution probably better than any other species of British barnacle, with the possible exception of Balanus [Amphibalanus] improvisus and in warmer situations B. amphitrite [Amphibalanus amphitrite]. In dirty harbours and muddy rivers within the intertidal zone it may have few or no competitors.” In Britain, Knight-Jones (1948) noted that it occurred in some places where native barnacles were absent (e.g. Maldon beach).
As in Australasia, in Europe this species is found intertidally and subtidally. A number of authors have described its vertical zonation, e.g. Moyse and Knight-Jones (1961, 1963) found that around Dale, Pembrokeshire, it could occur nearly as high as Mean High Water Springs (~ 7 metres above Chart Datum) down to the sublittoral. On Bangor Pier, Wales, it occurred from low water (LW) to high water (HW) (Patel and Crisp, 1960a). According to Crisp (1958), A. modestus occurs over a wider intertidal range than S. balanoides, with small numbers growing at levels slightly above the highest S. balanoides, and it penetrates into the subtidal to depths of 5 m below Low Water Springs.
Similarly to its native habitat, Den Hartog (1953) reported that it can occur on whole range of substrates, including a range of rock types, iron, wood, cork, a number of different species of algae, living gastropods and bivalves, as well as on barnacles of its own species and S. balanoides as well as on crabs, tunicates and empty shells. Crisp and Davies (1955) got it to settle on glass plates, and Anger (1978) reported it occurring on Plexiglas panels, suspended at a depth of 1 m, in Helgoland Harbour (Binnenhafen), Germany.
Flowerdew (1984) used electrophoresis to compare 22 enzymes between 10 populations of A. modestus from Europe and three populations from the Antipodes. He found no significant difference at the 19 loci between the 13 populations studied, with the genome of the European population being exactly representative of that of the aboriginal population. His results suggested that the initial settlement in Great Britain was of a very large number of individuals, as had been proposed by Crisp (1958). Flowerdew (1984) reported no significant difference in allele frequencies between samples from northern Spain to Scotland. O’Regan (1980) examined samples from Bantry Bay and Cork Harbour, County Cork, Ireland and found no significant differences between the two populations in their allelic frequencies and phenotype distribution for the two loci studied. Flowerdew (1983) noted that MPI allozymes have indicated a monomeric enzyme structure in A. modestus. Towards the northern limits of introduced range, Harms and Anger (1989) suggested that this species is subject to considerable genetic selection pressure towards cold adaptation. This was supported by Harms’ (1986) findings: with respect to larval development rate, it was found that larvae from Helgoland, Germany, showed a tendency of adaptation to cooler temperatures, when compared with those from New Zealand.
This species is a slightly protandrous hermaphroditic obligate cross-fertiliser (Crisp, 1954; Barnes and Crisp, 1956; Crisp and Patel, 1961). It can produce multiple broods in a year, depending on temperature and food availability, with the ovaries redeveloping as each brood matures (Crisp and Davies, 1955). The percentage bearing embryo masses varies with locality, season and shore height (Barnes and Barnes, 1968a; Barnes, 1989; 1992). According to Barnes (1989 and references therein), in its native and introduced range, where there is sufficient food and water, and the temperature is above 6 °C, the breeding season appears to be continuous, with occasional peaks. After a period of incubation of the fertilized eggs, the first stage nauplii are released. There are six free-swimming and feeding naupliar stages followed by the non-feeding cypris stage, which is adapted morphologically for site selection and settlement. It reaches maturity at a smaller size than Semibalanus balanoides (Crisp, 1964a). It has been suggested that its high fecundity, the ability to produce larvae throughout the year and the early age of reproduction have contributed to its success in Europe.
The brooding season has been examined in Australasia by Moore (1944), Powell (1947), Wisely and Blick (1964), Foster (1967) and Luckens (1970; 1975; 1976). Luckens (1975) described this species as “a fast maturing, continuously breeding species, can settle at any time of the year”. According to Moore (1944), in New Zealand nauplii and juveniles have been found throughout the year. She suggested that within the optimal temperature range and in relatively stable conditions there is some evidence of seasonal breeding superimposed on a general continuous low level of reproduction. In Auckland, breeding was found to be continuous throughout the year and Moore suggested that this may be similar all around the New Zealand coast, which is what Foster (1967) reported for Leigh (North Island) with eyed nauplii being found in at least some individuals throughout the year. In Auckland, they could reach maturity within 2.5 to 3 months of settlement (Moore, 1944). Elsewhere in New Zealand (Port Nicholson, Wellington), settlement on subtidal pine blocks, occurred mainly from April to July (austral autumn) although there was also some in October and November (austral spring) (Ralph and Hurley, 1952), which is probably related to peaks in phytoplankton production (Skerman, 1958). Skerman (1958) suggested that peak settlement periods in autumn and spring may be related to seasonal food availability for the nauplii. The only months lacking settlement were December-February (austral summer), although cyprids, which may have been A. modestus, were found nearby, which supports Moore’s (1944) suggestion that breeding and spawning may occur throughout the year. In Australia (Garden Island, Sydney), Wisely and Blick (1964) only found low levels of brooding embryos, with eyed larvae only present for seven months. They suggested that this species might release low levels of nauplii throughout the year, with a slight tendency to release during the colder months of the year. However, this material may not have been a sample solely of A. modestus (Egan and Anderson, 1985).
The reproductive biology of this species has attracted more research in its Europe than in Australasia. The brooding season has been examined in Europe by a number of authors (e.g. Knight-Jones and Waugh, 1949; Crisp, 1954; 1957; Crisp and Davies, 1955; Wisely, 1960; Crisp and Patel, 1961; Stubbings and Houghton, 1964; Barnes and Barnes, 1966, 1968a; Harms, 1984; Barnes, 1992; O’Riordan and Murphy, 2000). Overall, they found that fertilized embryos could be found throughout the year, but the level of brooding varied, which reflected changes in seasonal temperatures, with breeding most rapid at moderately high temperatures (Barnes and Barnes, 1962). In Britain, Stubbings and Houghton (1964) and Patel and Crisp (1960) reported embryos being present in January and February, but suggested that they would not hatch until temperatures increased, while for Helgoland, Germany (near the north of their continental limits), Harms (1984) reported that the barnacles bred at temperatures between 7-18 °C (May to October), but the main period was July-September. In southwest France (Arcachon), they were found to contain embryos from November to March, with up to 100% gravid (Barnes and Barnes, 1962a). It was suggested that the low level of gravid animals present during May and June, when temperature and food would be suitable for breeding, might be because brooding is synchronous, especially after embryos being retained over winter. Barnes and Barnes (1962a) found that in Scotland temperatures of 15-20 °C favour breeding. However, Crisp and Spencer (1958) noted that sometimes when sampling adults ripe embryo masses may not be recorded, because eyed nauplii can be released as soon as they mature if phytoplankton is abundant. Crisp and Davies (1955) noted that during the autumn and winter the ovaries tend to be small and poorly developed, whereas male organs, though somewhat reduced in development, are still present. Although Crisp and Davies (1955) reported maximum breeding (80%) during mid-summer, they found that in southwest England continuously submerged specimens continued to breed at low levels (15-20%) in the winter, but noted that elsewhere (e.g. southeast England), with lower winter temperatures, breeding would cease in severe winters. Tighe-Ford et al. (1970) noted that although in southern England development of the larvae occurs in the wild in spring, summer and autumn, a small percentage of adults contain embryo masses throughout the winter (January and February). In Cork Harbour, Ireland, where adults can contain fertilized embryos throughout the year, O’Riordan and Murphy (2000), found that the testes and vesiculae seminales showed a distinct annual cycle of development. Furthermore, animals living in proximity to the outfall pipe of a power station (which at that time discharged cooling seawater at 9 to 10 °C above ambient at a rate of 9 m3s-1) showed significantly lower percentages of animals brooding embryos than at another site nearby. However, in Southampton Water, England, close to Marchwood Power Station, Pannell et al.(1962) reported that Austrominius showed an extended breeding period in artificially heated areas. Crisp (1960) suggested that high temperatures in the warm and prolonged summer of 1959 resulted in increased fecundity, which allowed the spread northward of the species on the west and east coast of Britain after little change for five years. Leloup and Lefevere (1952) pointed out that its ability to reproduce for a longer period of time during the year than Balanus/Semibalanus species gives it a distinct advantage.
Temperature and food availability are two of the main factors found to influence reproduction in this species (Crisp and Davies, 1955). According to Crisp and Davies (1955), A. modestus breeds at any temperatures from 6 to 20 °C and even at temperatures as high as 23-25 °C. They suggested that they may survive at temperatures well below 0 °C and breed at temperatures below 6 °C (if food is adequate) but development is very slow. They commented that its “type of breeding is well suited to life in shallow estuaries and sheltered coast of temperate latitudes and accounts for its success in Britain in competition with the indigenous S. balanoides”. Patel and Crisp (1960a) also reported that the lowest temperature for breeding is 6 ºC and in the field embryos capable of being hatched are retained until temperatures increase (Crisp and Davies, 1955). In the laboratory, eggs could be incubated at 3 ºC, but would not hatch naturally and took more than 50 days to develop. The highest temperature for successful breeding was 32 ºC. The optimum temperature for breeding was 22-25 ºC, with the percentage of fertilized eggs and mean ovary index decreasing significantly at 30-31 ºC. In laboratory conditions, Southward (1955a) reported animals copulating at temperatures ranging from 4 °C to 15 °C.
Crisp and Davies (1955) examined the number of broods per year in subtidal populations. Since the ovary redevelops as a fertilized brood matures, the time taken to incubate the eggs (10 days in summer) determines the reproductive rate (Crisp and Davies, 1955; Crisp and Patel, 1961). In southwest England, in spring and summer, when there was abundant food available, a succession of broods could be produced, with the fecundity probably being limited by the rate of development of embryos. A. modestus has a much faster rate of beat of the cirri than many other species in the British Isles, which facilitates its higher reproductive and growth rate (Crisp and Davies, 1955). The interval between broods produced was limited by how long it took for the embryo masses to develop. In winter, the main limit to reproduction was availability of food, rather than any deficiency in the male organs, but broods could take 60-80 days to develop. In Scotland, where summer temperatures are only moderate (maximum 15-16 °C), Barnes and Barnes (1962a) suggested that only a maximum of two or three broods would be able to be produced.
A number of other aspects of the reproduction of A. modestus have been studied in Europe. Barnes (1992) examined changes in body weight and the number of penis annulations. She reported that although a penis was found throughout the year in France, the number of penis annulations (up to 240) followed the breeding season, decreasing towards the end of the year. Barnes et al. (1971) examined the accessory droplet and spermatozoa. There has been a suggestion that older individuals of some species of barnacle show sterility, but Crisp and Davies (1955) found no evidence of this for A. modestus. Hill et al. (1988) identified the chemical (a monohydroxy derivative of eicosapentaenoic acid) that causes hatching of the fertilized eggs of A. modestus. Cawthorne and Davenport (1980) found that reduced salinity and aerial exposure caused adults to isolate themselves from the environment by closure of the opercular valves, resulting in an enhanced release of stored up larvae following a return to suitable environmental conditions. They hatch only at salinities above 21psu. Fluctuations in temperature induce larval release in A. modestus, but not Semibalanus balanoides (Cawthorne and Davenport, 1980).
Crisp and Chipperfield (1948), Crisp and Patel (1961) and Moore (1944) described the age and size at first breeding. Crisp and Patel (1961) reported that, in subtidal animals, sexual maturity was mainly a function of size, but also to a small extent dependent on age. As A. modestus is a slightly protandrous hermaphrodite, the testes, vesiculae seminales and penes can develop in 0+ animals measuring between 3 and 5 mm in rostro-carinal diameter (RCD), whereas ovaries can develop in those of 4 to 6 mm RCD. Crisp and Patel (1961) only found embryos present in animals with a RCD greater than 4.0 mm, but it can reach this size (and hence begin breeding) within 8 weeks of settlement (Crisp and Chipperfield, 1948; Crisp and Davies, 1955; Crisp and Patel, 1961). Crisp and Davies (1955) noted that within 12 weeks of settlement, subtidal individuals reached a breeding equilibrium indistinguishable from that of older individuals. Anger (1978) reported that on subtidal Plexiglas panels, suspended at a depth of 1 m, in Helgoland Harbour (Binnenhafen), Germany, within a month of settling, when water temperatures were 13-16 °C and there was plenty of food available, this species reached 3 mm in RCD. Four months after settling, some of them were large enough to reproduce. Crisp and Davies (1955) and Crisp and Patel (1961) found that in crowded conditions, the onset of breeding was delayed (for about 4 weeks), but the male reproductive organs developed and fertilized embryos were present in barnacles of a smaller size than in uncrowded conditions.
Crisp and Davies (1955) and Foster (1967) described the changes in colour of egg masses with development; Crisp and Patel (1958) and Patel and Crisp (1961) looked at the timing of copulation and ecdysis. The latter authors reported that the moulting phase did not influence the frequency of successful copulation and individuals could become fertilized at any period of the moulting cycle. In the laboratory the moulting rate of A. modestus increased linearly with temperature from 4 to 24 ºC (Patel and Crisp, 1960b), but the intermoult period is shorter, especially for unfertilized animals, if they are fed rather than starved (Patel and Crisp, 1961).
Barnes and Barnes (1968b) reported the number of eggs per brood as well as the metabolic efficiency of egg production. They found that for A. modestus, as well as a number of other barnacle species, there was little variation in the number of embryos produced (1800/1.0 mg oven dry body weight for A. modestus), despite collections being made from Scotland to Spain. They suggested that “the ecological environment [estuarine for A. modestus] which limits the local distribution of these species is sufficiently precise to determine that wherever they are found the conditions and hence egg production are similar”. As mentioned above, in subtidal situations, when food was abundant, A. modestus was estimated to produce a brood in 14 days (Crisp and Davies, 1955); thus it could produce 26 broods in a year (Barnes and Barnes, 1968b), but in the littoral it appears to produce much less frequently (Barnes and Barnes, 1968b). In comparison to temperature, Patel and Crisp (1960b), pointed out that nutrition affects the number of eggs produced, rather than the size of the eggs. Age and size may also affect fecundity in A. modestus (Crisp and Patel, 1961). Barnes (1971) estimated egg production (in the Arcachon Basin, France) as 6.26 g dry wt per m2 surface area per year.
The ovum measures 100-150 mm (Crisp, 1954; Crisp and Patel, 1961) and the egg between 190-192 mm long (Crisp, 1954; 1987; Barnes and Barnes, 1965b). Patel and Crisp (1960a) examined the rate of development of eggs. In the laboratory, egg size and volume were seen to decrease in size with increased rearing temperature (Patel and Crisp, 1960b; Barnes and Barnes, 1965b). Barnes and Barnes (1965b) suggested that a reduction in egg size (relative to other barnacle species) might be associated with euryhaline behaviour, citing A. modestus. Barnes and Barnes (1968b) suggested that “Smaller eggs produced in successive though smaller broods which will settle and reach maturity quickly and so contribute to further egg production puts a eurythermal species such as Elminius modestus at a tremendous advantage over a species such as Balanus balanoides [Semibalanus balanoides] in spite of a ‘constant’ metabolic efficiency in their egg production and this accounts for its rapid spread- often at the expense of the boreo-arctic species. Indeed a boreo-arctic species is only successful because of its capacity to withstand the environment and not because of its interspecific competitive efficiency.”
One of the reasons (see also Harms, 1999) to which the rapid colonization of shores in Europe was attributed was the remarkable fecundity of A. modestus (Crisp and Chipperfield, 1948), since it can breed at a very young age (and size) and become fertilized at any time of year (as soon as ovaries have regenerated), unlike some other species where fertilization can only occur during certain seasons (see Crisp, 1954), produce many broods each year (Crisp and Davies, 1955), and breed during the winter, as well as summer, months, if temperatures remain sufficiently high. Crisp and Davies (1955) suggested that assuming a life span of three years, with 12 broods a year, with 500 nauplii per brood, an individual could produce 20,000 nauplii in its lifespan. In a given year, they also can settle over a much longer period of time than e.g. the arctic-boreal S. balanoides, utilizing any bare space that becomes available.
The nauplii of the species have been described in detail by a number of authors, including Barker (1976) (larvae reared from adults collected in New Zealand), Foster (1967) and Knight-Jones and Waugh (1949). Knight-Jones and Waugh (1949) and Barker (1976) gave the carapace length and width of all of the naupliar stages and the cyprid. Egan and Anderson (1985) described how the nauplii of A. modestus can be distinguished from those of A. covertus, as they may co-occur in the plankton in New South Wales (Australia). Ross et al. (2003) gave a key to identify nauplii of the common barnacles that occur around the British Isles. Those of A. modestus can occur in the water at any time, but are most common from May to October. A. modestus nauplii can be distinguished by the presence of a trilobed labrum, in which the medial lobe extends beyond the two lateral lobes (Knight-Jones and Waugh, 1949). They can be separated from the nauplii of Perforatus perforatus by this feature, but also the larvae of A. modestus are smaller and more pyriform in shape and in nauplii IV-VI, the frontolateral horns are stubby (Norris and Crisp, 1953). Walker (1973) described the frontal horns and associated gland cells of the nauplii. Crisp (1987) tabulated the shape of the naupliar stages.
Wisely (1960) and Tighe-Ford et al. (1970) are among the many authors who described methods for rearing the larvae in the laboratory. Tighe-Ford et al. (1970) found that the rearing temperature affected the rate of development of the embryos and also the size of the larvae. Development was slower at lower temperatures but larger larvae were produced. Moyse (1960, 1963) reared A. modestus on flagellates and diatoms, and the minimum time from hatching to cyprid (at 20±3 ºC) was 6 days (shorter than the 10 days needed for Semibalanus balanoides or ‘C. stellatus’). Foster (1967) also reported being able to rear the cyprids of A. modestus from nauplii in just six days in the laboratory. Culture was more successful on certain diets (Moyse, 1960). Moyse (1963) described the feeding habits of the larvae of A. modestus as more catholic (as it feeds on both flagellates and diatoms), than S. balanoides or ‘C. stellatus’, which can only be reared solely on diatoms or flagellates respectively. Stone (1988, 1989) found that different algal diets affected the rate of development, survival and size of the larvae of A. modestus. She suggested that the increase in mesh size of the antennal filter during growth of the nauplii is one of the reasons for the varying success of the different diets. Previously Stone (1986) had found that A. modestus larvae survive better on flagellates at warmer temperatures and on diatoms at cooler temperatures. However, the fact that nauplii can survive and grow both on flagellates and diatoms allows their production throughout the year, whereas the larvae of the spring breeder S. balanoides fare better on diatoms. Yule (1986) examined changes in the limb beat movements of A. modestus nauplii in the presence of food organisms. When they encounter patches of algae their average beat amplitude (and often beat frequency) increases, so the volume through which the antennae moved increased. The maximum ingestion rate occurred at algal concentrations of >100-150 cells ml-1. Vay et al. (2001) examined the digestive enzymes during development in A. modestus. Neal et al. (1986) examined the changes in lipid composition that occur when the faecal pellets produced by the nauplii of A. modestus are ‘repackaged’ into bigger pellets during coprophagic feeding by adults of a larger species of zooplankton, Calanus helgolandicus.
Barnes and Barnes (1974) and Bhatnagar and Crisp (1965) reported that although in the laboratory larvae could develop at a wide range of salinities, abnormalities occurred at the extreme salinities. At 21.4-42.8 psu viable nauplii developed irrespective of the stage of nauplii at which transfer into the salinity regime was made. Bhatnagar and Crisp (1965) commented that A. modestus larvae were slightly more tolerant to lower salinities than larvae of S. balanoides and ‘C. stellatus’, but still not as tolerant as the A. modestus adults. Cawthorne and Davenport (1980) noted that the larvae are still mobile at 9 psu. Cawthorne (1978; 1980) examined tolerances of the nauplii to changes in salinity and temperature. Although nauplii of both S. balanoides and A. modestus have a similar LT50 in a steady-state experiment, A. modestus is significantly more tolerant of cyclic exposure to high temperatures (Cawthorne, 1980). The nauplii can survive temperatures of 26 °C for short periods of time without any mortality. Crisp and Ritz (1973) found that the nauplii of A. modestus showed dark adaptation, and Harms (1984) suggested that the wide temperature range tolerated during larval development has contributed to the success of the species. Yule (1984) examined the effect of temperature on the swimming activity of the nauplii of A. modestus. It could swim at temperatures as high as 30 °C and as low as 5 °C. Between 5 °C and 25 °C, it showed a two-fold increase in beat rate for every 10 °C increase in temperature. Yule (1984) suggested that its successful colonization of British and other temperate shores may have been aided by its slight but possibly significant response of decreased swimming activity by its nauplii as the temperature and limb beat frequency increase.
Barnacles such as A. modestus which inhabit relatively sheltered areas have smaller larvae than the oceanic and exposed coast species, as they have a shorter planktonic phase (Moyse, 1963). Ross (2001) noted that in Plymouth waters the distribution of nauplii of A. modestus (as well as other barnacle species) mirrored the distribution of adults. Wolf (1973) commented that in the Dutch Wadden Sea, adults are nearly exclusively in the intertidal rather than the subtidal zone, despite the fact that the larvae are mixed through the Wadden Sea. Although Wolf (1973) found variations in the numbers of stage VI nauplii and cyprids at different times of the tide in the western Wadden Sea, unlike other species, they appeared to be independent of the phase of the tide. There were also no significant correlations with current velocity, chlorine levels, silt or sand content nor with total suspended matter, but there was a slight negative correlation of cyprid numbers in the plankton with temperature. He suggested that the fact that the cyprids of A. modestus show a slight association with warm water and that they may have a slightly lower density than water affects their distribution in the water column and might explain why in the Wadden Sea they occur nearly exclusively in the intertidal zone and why the species showed a slow spread along the casts of Western Europe (Den Hartog, 1953). Cassie (1959a; 1959b; 1960; 1962) examined the dispersion and aggregation of A. modestus nauplii in Wellington Harbour, Port Nicholson, New Zealand, where there were abundant adults. He found that the larvae showed aggregation, which was positively correlated with water temperature and negatively with salinity.
Although the cyprids overlap in size (length; 429-646 µm -- see for example Knight-Jones and Waugh, 1949; Jones and Crisp, 1954; Tighe-Ford et al., 1970; Wolf, 1973; Al-Yahya, 1991, based on laboratory and field collections) with co-occurring barnacle cyprids in Europe, they can be distinguished by shape and colour (see O’Riordan et al., 2001), being colourless to a pale straw colour and of glassy transparency (Knight-Jones and Waugh, 1949; Norris and Crisp, 1953). O’Riordan et al. (2001) summarised the main distinguishing features of the cyprids of A. modestus. They are elongated, with a sharp angle between the dorsal and ventral surface at the anterior end. The posterodorsal margin rises steeply to an angle and then is evenly curved and they have a narrowly curved posterior end. Although the cyprids of are about the same size and colour as those of Amphibalanus improvisus, those of the latter species are broader and more oval (Jones and Crisp, 1954). Knight-Jones (1953) suggested that the cyprid cuticle of A. modestus and B. crenatus is strongly hydrophobic resulting in them becoming trapped in the surface film when left in shallow dishes.
Tighe-Forde (1977) found that two analogues of insect juvenile hormone induced abnormalities in the nauplii and cyprids of A. modestus. Nauplii VI metamorphosed morphologically abnormal larvae, which were intermediate in size between the nauplius and cyprid stages. Cyprids either metamorphosed to non-attached adults or formed larvae that were larger or morphologically abnormal. Crisp (1974) found that A. modestus may metamorphose from a cyprid to a miniature adult in as little as 8 hours, but Knight-Jones (1953) and Knight-Jones and Crisp (1953) had found that it could occur in less than 4 hours.
Settlement and recruitment
A lot of research has been carried out on the settlement and recruitment of A. modestus. Knight-Jones and Crisp (1953) described the detailed sequence of behaviour of a cyprid during exploration of a potential surface and at settlement. As well as orientating to light, they also orientate to surface contour and the responses are independent reactions (Crisp and Barnes, 1954). As mentioned previously A. modestus will attach to a very wide range of substrates. It will settle on and remain attached to smooth glass surfaces (like ‘C. stellatus’) (Knight-Jones and Stevenson, 1950; Crisp and Davies, 1955), Plexiglas panels (Harms and Anger, 1989), as well as slate (e.g. Knight-Jones and Stevenson, 1950), bakelite and wood (Crisp and Barnes, 1954). Knight-Jones and Stevenson (1950) noted that a greater number of individuals are lost from a smooth glass surface than a frosted one, indicating that settlement in a small hollow is advantageous, ensuring both an initial key for the cement and some measure of protection to the young barnacle. Crisp and Barnes (1954) named its tactile response to surface contour as rugotrophic. They noted that the cyprids are rugophilic and will settle in both grooves and concavities, while there is some evidence that they will swim off if they encounter sharp angles or ridges. A. modestus orientates to light (Barnes et al., 1961). This orientation occurs at settlement (and has been demonstrated on both horizontal and vertical panels) and no rotation occurs during metamorphosis. In deep grooves, orientation may be due to a response to light, but in shallow grooves, the orientation can be ascribed to rugophilic response (Barnes et al., 1961). Gollety et al. (2008) suggested that higher densities of A. modestus (than Chthamalus montagui) at a site in Roscoff, France, were due to the substrate (predominantly smooth concrete), which might be more favourable for the survival of the former species. In turn, Crisp (1961) noted that the base of A. modestus “with a sinuous octoradiate outline has eight concavities which are depressed in both planes, and so are highly attractive, often being occupied by a small barnacle in each”.
In New Zealand, Skerman (1958) reported that, on subtidal panels, A. modestus prefers to settle on long-immersed surfaces, especially those hosting already established A. modestus. This is unsurprising since Knight-Jones and Stevenson (1950) have found the cyprids to be gregarious during settlement. The cyprids will show higher settlement when slate panels are placed near shells bearing adult A. modestus than when panels are near bare mud with no adult Austrominius. On substrates where adults had previously occurred, settlement occurred at a greater rate than on substrates that had not previously had Austrominius adults. Knight-Jones (1953) pointed out that gregariousness aids breeding in this obligate cross-fertiliser but also brings the cyprids to habitats that have proved suitable for survival, preventing “wastage through individuals settling in unsuitable localities and in isolation” (Knight-Jones and Stevenson, 1950; Moyse and Kinght-Jones, 1967). Knight-Jones and Stevenson (1950) noted that gregariousness “will not prevent the colonization of new areas, for occasional individuals settle in such areas even when settlement is sparse. Groups will gradually form around the majority of these pioneers, so that their breeding capacity will not be wasted and suitable areas will eventually become fully colonized. Far from restricting the spread of Elminius [Austrominius] in this country, gregariousness will make it more certain, though more gradual”. Knight-Jones (1955) found that settlement is stimulated most by the same species, but members of the same family induce settlement more than members of other families, e.g. Austrominius will evoke a slight gregarious response in Semibalanus balanoides, but the latter species will not react to the more distantly related species. Crisp and Meadows (1962) noted that for wild-caught cyprids A. modestus is more sensitive (shows higher settlement) to panels soaked in an extract of its own species than other barnacle species and the same authors later identified the substance responsible as arthropodin (see also reviews of gregarious settlement in barnacles in Crisp, 1984 and Gabbott and Larman, 1987). However Larman and Gabbott (1975) found that laboratory-reared cyprids of A. modestus were not selective between extracts of their own species and those of S. balanoides. Since it is less discriminating at the settlement stage (Larman and Gabbott, 1975), it invades already established assemblages more readily than other barnacle species (Hui and Moyse, 1982). Hill and Holland (1985) reported that fractionated extracts of oil shale enhances settlement of A. modestus in response to an adsorbed layer of a fraction containing metalloporphyrins. They suggested that it acted similarly to arthropodin, binding the proteins associated with the cyprid attachment disc.
Brown and Eaton (2001) and Brown et al. (2001) reported that A. modestus recruitment was found to be higher on wood panels treated with the anti-marine-borer timber preservative chromated copper arsenate (CCA) than on untreated wooden panels. They suggested that the enhanced settlement may be a result of modifications to the surface properties of the timber by the CCA preservative (Brown and Eaton, 2001).
Crisp (1961) and Knight-Jones and Moyse (1961) discovered that the larvae of A. modestus space out from individuals of their own species at settlement. The cyprids show territorial behaviour, with the spacing between individuals decreasing with greater densities. Although one can get densities of 230 cm-2 on smooth slates (Waugh, 1957), their spacing out on crowded surfaces prevents overcrowding and smothering during their short life span (Knight-Jones, 1951; Knight-Jones and Moyse, 1961). Knight-Jones and Crisp (1953) reported however that a crowded surface is repellent. Conversely, Barnett et al. (1979) and Barnett and Crisp (1979) found that cyprids of A. modestus were gregarious even at very close range, in the field and in the laboratory. Although Moyse and Hui (1981) pointed out certain problems with the latters’ work, Hui and Moyse (1982) found that A. modestus cyprids settle readily in contact with adult barnacles in the field and do not discriminate between conspecific and other adult barnacles. Territorial spacing only occurred when the established barnacle was less than 3 mm.
Neal and Yule (1974) found that the cyprids of Austrominius modestus showed greater tenacity on relatively thin, dense bacterial films associated with high shear (83 s-1) than to relatively thick, less dense films associated with low-shear (15 s-1), but they attached as strongly to the latter as they did to unfilmed surfaces. Crisp (1955) found from experiments in glass tubes that moderate velocity gradients exceeding a shear of 500 sec-1 sweep the cyprids past the surface before they can attach.
The timing of settlement shows some variation. In New Zealand, Luckens (1975) noted that A. modestus can settle at any time of the year, forming dense, even layers over rock, wood, iron, on other barnacles, settled invertebrates, and even algae, both intertidally and subtidally, when cyprids are available in sufficient numbers. Settlement occurs from May to September with a main peak in June or early July in southeast England (Knight-Jones and Waugh, 1949; Knight-Jones and Stevenson, 1950) and north Wales (Wisely, 1960). Southward (1991) recorded it settling predominantly in late summer and autumn. Bennell (1981) reported that although A. modestus could be found all round the island of Anglesey, north Wales, the only place where it made a significant contribution to the barnacle cover was in the sheltered Menai Strait. She reported that Austrominius larvae could be found in the plankton throughout the year, but with higher abundance in late summer and autumn. The timing of maximum settlement varied with sometimes heavy settlement in July to September, but in other years between October and December. According to Barnes and Barnes (1962a), in Scotland settlement was highest in the summer with negligible levels in the winter. Lawson et al. (2004) reported that the settlement rate of A. modestus may be over 300 times that of Semibalanus balanoides in Lough Hyne, southwest Ireland, with the greatest settlement in April-May and August-September.
In Germany Kühl (1963) reported that A. modestus settled between May-June and September-October. Harms and Anger (1989) examined annual variation in the subtidal settlement of A. modestus on Plexiglas panels, at Helgoland (North Sea). In general they found lowest settling intensity and success in the summers following unusually cold winters (air temperatures <-20 °C), which had significantly reduced the adult populations both intertidally and subtidally in the German Bight.
In southwest Ireland, Watson et al. (2005) reported differences in the recruitment of A. modestus (maximum mean ~1-2 cm-2 ) relative to other barnacle species, when he examined intertidal (at 0 m, with only ~2 hours of emersion) and subtidal (6 m and 12 m depth) panels, and between panels submersed for a month, versus those left for 60-120 days (seasonal panels) or 370-400 days (annual panels). In general, intertidal recruitment was dominated by A. modestus. Recruitment was measured on the underside of intertidal panels, where desiccation would be less. Two other barnacle species, Balanus crenatus and Verruca stroemia, dominated the longer-immersed panels, highlighting the importance of post-recruitment processes to the survival of A. modestus. No A. modestus were found on subtidal annual panels. Intertidally and subtidally A. modestus showed peaks of recruitment in spring and autumn, with recruitment also occurring in winter, but at a lower level. Watson et al. (2005) suggested that the sheltered conditions of Lough Hyne (County Cork, Ireland) allowed high larval settlement and recruitment. In southern England, Herbert et al. (2007) recorded highest counts of recruitment of A. modestus of 16 and 23 cm-2 (mean and maximum) along the Sussex coast (Selsey Bill), with recruitment greater at the lower tide level. Southward (1991) showed significant year on year, as well as interdecadal, effects for A. modestus in the southeast England intertidal. At Roscoff, France, Gollety et al. (2008) reported that recruitment extended from late spring to the middle of autumn, with two peaks in June and another in August and October, with individuals reaching up to 9.4 mm in RCD. In subtidal samples in Helgoland, Germany, Anger (1978) recorded recruitment from July to September-October, with maximum recruitment occurring in September-October. Meadows (1969a) examined mortality of recently recruited A. modestus on subtidal panels at Rosyth, Scotland and found quite high levels (e.g. up to 51%). Crisp and Davies (1955) had maintained A. modestus on subtidal panels for 21 months, but they constantly manipulated the panels to prevent any other settlement/recruitment. Golléty et al. (2008) noted that A. modestus is expected to live for at least 22-24 months, whereas C. montagui is estimated to live until at least 19 months.
Size, shell growth and densities
In its native range A. modestus reaches a largest diameter of 13 mm and greatest height of 9 mm, but the average size is approximately 5x4 mm (Moore, 1944). Southward (1991) noted that it is a rapidly growing species. In Europe, A. modestus usually grows to a RCD of 8 mm (Crisp, 1955), but Bishop (1954) has found individuals over twice this size (up to 17 mm in some locations) “on bridges spanning the narrow drowned river valleys of north Finisterre, where the large tidal ranges produces exceptionally fast currents”. He suggested that the “most advantageous positions for barnacles to settle are therefore those where the water movement is as great as they can tolerate”. Bishop (1954) suggested that the large size on the Brittany coast was due to highly favourable environmental conditions, rather than age. Anger (1978) reported that on subtidal panels in Helgoland Harbour (Binnenhafen), Germany, within a month of settling, when water temperatures were 13-16 °C and there was plenty of food available, this species reached 3 mm in RCD, with animals reaching up to 9.8 mm RCD in one season. Four months after settling, some of them were large enough to reproduce. Tighe-Ford et al. (1970) found that in the laboratory after 15 weeks newly settled young of A. modestus reached a RCD of 6 to 8 mm, which is within the normal range of 5 to 10 mm for adults of the species (Southward and Crisp, 1963). Other data on growth rates can be found in Bishop (1947), Kühl (1954; 1963) and Barnes and Barnes (1962a). Crisp and Davies (1955) noted that on plates those animals at the edges of plates showed greater growth than those in the centre. Barnes and Barnes (1962a) found that temperatures of 15-20 °C favour growth. In Scotland, they found that growth takes place in the summer and autumn following settlement but little occurs in the winter. Growth was faster in subtidal conditions that in the high intertidal, but it occurred equally well in open and turbid waters.
Bourget (1977) examined the shell structure of sessile barnacles and found that A. modestus had a two-layered shell, unlike that of other Elminiinae. He noted that there did not appear to be any correlation between shell thickness and the ability to live in wave-exposed environments, but Foster (1978) noted that A. modestus is especially characteristic of shores where wave action is not too severe and this correlates with the weak construction of the shell (N.B. See also below under salinity and calcium deposition). Bubel (1975) examined the mantle epithelial cells, which are involved in the excretion of the base, wall plates etc. Bocquet-Védrine (1964, 1965) had previously examined shell growth and the moulting cycle in A. modestus. Golléty et al. (2008) found that the organic and CaCO3 production of A. modestus was much higher than that of C. montagui.
Davies (1967) noted that between 1959 and 1965, A. modestus was one of the most important organisms on the sea walls in the intertidal of the Blackwater estuary and adjacent waters in Essex, as well as on stones and shells amongst the muddy sand. He found it to be very common in the Blackwater and Colne estuaries, mostly on the lower and middle shore but also subtidally. It was the dominant barnacle species in the Blackwater, densities reaching a maximum of 3750 individuals m-2. Subsequently much higher densities have been reported, e.g. 40,000 individuals m-2 in the Arcachon Basin (Barnes, 1971) and 84,960 individuals m-2 at Roscoff (Golléty et al., 2008), which is comparable to the densities seen for the other commonly occurring intertidal barnacles, C. montagui, C. stellatus and S. balanoides, on Atlantic European shores.
Southward (1991) examined how the intertidal abundance of A. modestus had changed over 40 years in south Devon (Cellar Beach, River Yealm). Its first record in south Devon was in 1948. Although it increased in abundance in the 1950s, it then stabilised at a low level of abundance, which showed large interannual variations, which were not directly related to temperature. He suggested that large fluctuations in density reflected the number of larvae available for settlement, which could originate from breeding populations in the sublittoral or in the Tamar and Plym estuaries where lowered salinity encourages greater densities than in the River Yealm.
According to Crisp (1958), in Great Britain, the combination of its long breeding season, high intensity of settlement and recruitment and a fast growth rate (see below) allows A. modestus occupy any space left bare in winter, making it difficult for settling Semibalanus balanoides cyprids in spring to find space.
Attached barnacles filter feed their food from the water, using cirri (see above regarding larval food preferences). In the laboratory, Crisp (1964a) found that for maximum growth A. modestus needs a high concentration of suspended matter, characteristic of rich inshore waters, and is less successful than Semibalanus balanoides in gathering food except in relatively still water. In these relatively still, rich, inshore waters, where there is little wave exposure or tidal currents, it is best at filtering food and shows maximum growth (Crisp, 1964). Crisp (1958) had suggested that since A. modestus commonly occurs in muddy estuarine conditions, it may be specially adapted for feeding on detritus, but Barnes and Barnes (1962a) noted that it grows equally well on open coast plankton. Raymont (1976) noted that it could use almost any type of phytoplankton food available, and Crisp (1964) reported that A. modestus has a greater potential to collect sparsely distributed food particles than S. balanoides. Crisp (1964) noted that A. modestus usually has a slower growth rate than S. balanoides, but has a faster cirral beat (Southward, 1955a) and a greater relative feeding efficiency than indigenous barnacle species in the British Isles, e.g. S. balanoides or Perforatus perforatus and ‘C. stellatus’ (Crisp and Southward, 1961). Although at settlement orientation to the current is neglible, as it grows A. modestus orientates itself to the water current such that the carina usually points away from the current source, while the cirral net faces the current (Crisp and Stubbings, 1957). Crisp and Maclean (1990) described the relation between the dimensions of the cirral net, the beat frequency and the size and age of the animal in S. balanoides and A. modestus. The latter has relatively longer cirri with a few more segments. Crisp and Southward (1956) described the circulation of water within the mantle cavity.
In the laboratory, Southward (1955a) found that cirral beating ceased at temperatures below 2 ºC (while S. balanoides continued down to 1.8 ºC), reached a maximum of 22 beats/10 sec at 24 ºC, declined sharply at 30 ºC and ceased completely at 33-35 ºC. Southward (1955a) noted that this species started to show adverse effects at between 20 and 24 ºC, which is similar to the highest temperatures in its native habitats, but he suggested that the ability to carry on beating at temperatures down to 2 ºC indicated that it had acclimatised to European conditions, since in its native range the monthly mean sea temperatures do not drop below 7 ºC. He suggested that this ability to withstand lower temperatures may explain why it was able to colonise the Low Countries and southern England. In Britain it is active at lower temperatures than the native southern species and higher temperatures than the native northern species and showed a much greater frequency of cirral beat (17-18 beats per 10 sec) at 20 °C (Southward, 1955b; 1957) than any of the native species examined (S. balanoides, Amphibalanus improvisus, Balanus crenatus, P. perforatus and ‘C. stellatus’). Southward (1955a) suggested that this ability, combined with its great fecundity, might explain the success of A. modestus in competition with native species. Southward (1955b) found that specimens collected from the Mean Low Water Neap level had a significantly more rapid cirral beating than those from the high water level (MHWN). Crisp and Ritz (1967) reported that the cirral activity of adults is markedly affected by temperature acclimation and suggested that the ‘racial’ differences noted by Southward (1964; 1965) for a number of barnacle species may lie within the limits to be expected as a result of acclimation to the local temperature.
According to Barnes and Barnes (1966), A. modestus is an estuarine or quiet water species, distinctly euryhaline and euythermal in its tolerances.
It is a euryhaline species, with adults active between 19 and 40 psu (Barnes and Barnes, 1974; Davenport, 1976; Cawthorne, 1979). Foster (1970) noted that after experimental or natural acclimation, it is tolerant of salinities down to 14 to 17 psu. It survived indefinitely in a laboratory salinity regime that repeatedly fell to 20% seawater (Davenport, 1976) while the same author reported it encountering virtually fresh water in the field.
In its native range, Jones (1990) noted that it tolerates reduced salinities, while Moore (1944) found that in the laboratory adults would open in freshwater conditions. Bishop (1947) pointed out that barnacles found on the hull of a ship travelling at 17 knots from south Australia and New Zealand to Liverpool had survived for 30 days, including travelling via the freshwater lakes of the Panama Canal.
Barnes et al. (1970) showed that the shells of Austrominius modestus and Amphibalanusimprovisus, which occur in estuarine and quiet waters, are less resistant to damage than more open water species. Barnes and Barnes (1962b) suggested that, under conditions of low salinity, calcium deposition is reduced and shells of many animals with a calcareous exoskeleton are consequently thinner. According to Barnes et al. (1970), in “estuaries where the lowest salinities are associated with their inner and most protected parts, a more fragile shell will be less hazardous to survival than if it were developed in the more exposed parts. In areas such as the Baltic where there is a permanent low salinity regime which is not associated with estuarine conditions the production of a more fragile shell would be expected to restrict the species to even more protected situations than usual”. Crisp (1958) found A. modestus to be the least resistant to mechanical damage in comparison to native barnacles in Britain (S. balanoides, A. improvisus, Balanus crenatus, P. perforatus and ‘C. stellatus’. Knight-Jones and Stevenson (1950) noted that it was intolerant to wave action.
According to Harms (1986), its larvae tolerate high salinities, which may occur through evaporation in intertidal areas, while Tooke and Holland (1985) reported that its cold tolerance could be increased by acclimation to high salinities, which raises the levels of intracellular solutes, such as amino acids.
A.modestus is an eurythermal species, being able to tolerate higher temperatures than Semibalanus balanoides (Southward, 1958) and lower temperatures than‘C. stellatus’ although breeding is most rapid at moderately high temperatures (Barnes and Barnes, 1962a). Southward (1965) found that in the laboratory its adults could survive over 5 h at 40 ºC, but ‘C. stellatus’ could last 29-30 h at this temperature and 30 mins at 50 ºC. Harms and Anger (1989) point out that in its native range, in the subtropical and temperate zones of New Zealand, water temperatures range between 4 and 24 °C, and air temperatures in the southern part seldom fall below 0 °C (Batham, 1958; Slinn, 1968; Rainer, 1981; Ballantine, 1983). Crisp (1958) found that it had a better tolerance to desiccation and high temperatures (20 °C) than the arctic-boreal S. balanoides and Low Water Neap to subtidal species, but was not as tolerant as ‘C. stellatus’ (see also Ritz and Foster, 1968). Foster (1969) noted that intertidal temperatures on New Zealand shores can reach up to 40 °C, while those in Plymouth (UK) may be over 30 °C (Southward, 1958). Although A. modestus can tolerate high temperatures and has considerable resistance to desiccation, for more than 10 days (Moore, 1944), Foster (1971a) reported that the median lethal times of A. modestus spat and adults desiccated at 0% relative humidity were 7 and 46 hours respectively, which was longer than Balanus crenatus but much shorter than ‘C. stellatus’. Foster (1971b) noted that at high shore levels desiccation death can, and does, occur in A. modestus. Moore (1944) and Knight-Jones and Stevenson (1950) noted that it was intolerant to insolation.
With respect to cold temperatures, Kühl (1963) suggested that “In the tidal zone Elminius seems to be more sensitive to low winter temperatures than the other indigenous barnacles; this is especially true for the Elbe estuary. If the air temperature is not too low, like at Helgoland or Borkum, the Elminius population will survive in the tidal zone during winter”. Harms and Anger (1989) reported that extremely low winter air temperatures (<-20 °C) in some winters in Helgoland, Germany, caused mortality of intertidal and subtidal populations. According to Theisen (1980), a similar situation occurred in Danish waters in the late 1970s. Although dense populations of A. modestus (with 73% A. modestus versus 22% S. balanoides and Amphibalanus improvisus) had been found in the intertidal zone in December 1978, by the following March, after a severe winter, no live A. modestus could be found there. They suggested that perhaps some might have been able to survive subtidally where they would have escaped the severe frost in the intertidal zone, and be able to repopulate it, but they could still find none in the intertidal zone in October 1979. According to Jensen and Knudsen (2005), A. modestus has not been able to establish in the Danish Wadden Sea, because of borderline low temperatures. They die out in cold winters, repopulating there in mild winters. Tooke and Holland (1985) and Tooke et al. (1985) have compared cold tolerance in A. modestus and S. balanoides. Tooke and Holland (1985) and Ritz (1967) found that the cold tolerance of S. balanoides increased from summer to winter, but that of A. modestus did not change. A. modestus’ lower median lethal temperature varies between about -5°C and -6.6°C. A. modestus had a low tolerance to freezing in both winter and summer (Tooke and Holland, 1985). Tooke et al. (1985) suggested that the differences between the two species with respect to their freezing tolerances may be related to seasonal changes in total phospholipid fatty acid composition of the plasma membrane.
The current northern limits of the distribution of A. modestus in Europe are probably set directly and indirectly by the effect of temperature on reproduction and resultant competition for space. Eno et al. (1997) noted that low water temperature is likely to restrict the northwards spread of this species. In comparison to further south, low winter temperatures result in the production of fewer broods (of a small size) and the fertilised embryos may not be released until temperatures warm up. In comparison the arctic-boreal S. balanoides, which it can outcompete further south, releases a single larger brood in spring, which can occupy any available space. Barnes and Barnes (1969) noted that in northern France, in areas where it had not become well established yet in the late 1960s, small populations appeared each summer only to be destroyed in the winter. Crisp (1964b) reported mortalities of 50-80% of A. modestus in the intertidal zone in south and southeast England following the very cold winter of 1962/3. A similar winter mortality was reported for Cuxhaven, Germany (Kühl, 1963). Barnes and Barnes (1966) pointed out that it only increased in abundance in the Clyde estuary in Scotland following the warm summer of 1959.
Further south, where temperatures are warmer, A. modestus can release larvae and settle during winter months and thus pre-empt S. balanoides, by occupying any available space before S. balanoides’ young are released. Barnes et al. (1972) suggested that it may be as tolerant to desiccation as Chthamalus stellatus, but its restriction to lower intertidal levels on some Atlantic shores in the southwest of France may be because “temperature conditions per se are becoming too high to favour” it, which is what Barnes and Barnes (1966) suggested for a number of rias in Spain. These same temperature conditions also allow C. stellatus to produce multiple broods, which may compete for space with Austrominius settlers. At Petit Nice, France, Barnes et al. (1972) also suggested that Austrominius may be restricted to lower levels “due to the ‘depression’ of the cyprid at the time of settlement to lower levels because of the greater clarity of the water compared with…truly estuarine conditions usually favoured”.
A. modestus has been found to become infected by a number of species of parasites in Europe, such as Hemioniscus balani which prevents it from breeding, but in general prevalences are quite low. Crisp and Davies (1955) reported high prevalences of the isopod parasite H. balani in subtidal A. modestus. This parasite occupies the same position in the mantle cavity where fertilised eggs are normally found (Crisp and Davies, 1955). Crisp and Davies (1955) noted that individuals bearing the parasite were never found to be fertile, but did not show other adverse effects and many tolerated the parasite for up to a year. O’Riordan and Murphy (2000) reported low levels of H. balani, but with the highest level of infection (12%) in the samples taken from the warm-water outfall of a power station. Barnes and Barnes (1968a) noted that only 2% of the Austrominius at Arcachon, France had a parasite resembling Hemioniscus, whereas Crisp and Molesworth (1951) reported much higher levels (>50%) in some areas of England and Wales. Crisp (1950) found it in A. modestus on the southwest coast of England and south Wales. Crisp (1968) has also recorded them in Austrominius.
A. modestus has been found, in southwest Ireland and Isle of Cumbrae, Scotland, to be infected with trematode cysts (R. (O’Riordan) Ramsay and S.C. Culloty, University College Cork, Ireland, in prep., personal communication, 2009). Another parasite that occurs in A. modestus is the eugragarine Nipyxioides elminii, which occurs in the intestine (Ormières, 1983).
In its native range, north of Auckland, New Zealand, Luckens (1975) reported the mussel Xenostrobus pulex and the rock oyster Crassostrea glomerata smothering A. modestus. Furthermore, while A. modestus settled before the alga Corallina officinalis, the latter smothered and outcompeted it on both vertical and horizontal surfaces. On subtidal panels in Helgoland Harbour (Binnenhafen), Germany, Anger (1978) reported high mortality of A. modestus in October 1977, probably due to being overgrown by colonies of the tunicate Botryllus schlosseri.
In laboratory experiments, Barnett (1979) found that the common dogwhelk Nucella lapillus preferred to predate on Semibalanus balanoides rather than A. modestus, unless the whelks had been starved for extensive periods. Nucella usually used a prizing technique to attack A. modestus but drilled S. balanoides. Crisp (1958) and Barnes and Barnes (1962b) noted that competition between S. balanoides and A. modestus is affected by the selective predatory activity of N. lapillus and may explain some of the initial colonisation success of A. modestus in northwest Europe (Crisp, 1958), whereby space becomes available on the shore due to N. lapillus’ predation on S. balanoides. Potts (1970) reported that the sea slug Onchidoris also showed a preference for S. balanoides over A. modestus.
The species has spread by a combination of remote and marginal dispersal. Vectors involved include shipping (fouling on hulls, in ballast water and other parts of the ship), flying boats, and movement of organisms for aquaculture. Its spread has been facilitated by its ability to attach to a range of substrates, animate and inanimate. See History of Introduction/Spread section for further information. Its introduction to all areas has been accidental.
In the years just after its introduction into Great Britain, Bassindale (1947) and Knight-Jones (1948) suggested that A. modestus was a potential threat to the periwinkle and oyster industries. With respect to periwinkles, Anon. (1948 in Crisp, 1948) wrote that there were “complaints about barnacles on winkles from sources from which the trade is accustomed to receive clean winkles”. Anon. (1948 in Crisp, 1948) noted that fouling on oysters had to be removed before sale. According to Knight-Jones (1948), it became a pest on oyster beds off Essex, causing fouling (on periwinkles, mussels and oysters) but it was the most serious competitor for space with oyster spat. This was exacerbated by the fact that A. modestus began settling before the oysters, but also continued on afterwards. Although oyster spat could overgrow and smother A. modestus, they became misshapen and stunted, and so less valuable for the oyster industry. It was suggested that the species could become a problem to oyster farmers in France and the Netherlands where, at that time, they used artificial spat collectors which were uncovered at low tide. Harding (1948) pointed out that A. modestus adults may compete with adult oysters and mussels for food, while its cyprids may compete with their larvae for space for settlement. Albrecht and Reise (1994) and Görlitz (2005) have long-term data of it fouling on mussel and oyster beds (Crassostrea gigas) in the List tidal basin, northern Wadden Sea. As mentioned above, one of the ways this species spread was by movement of shellfish and boats associated with aquaculture. It also caused a problem fouling coastal craft in Essex (Knight-Jones and Waugh, 1949) and in favourable conditions can cause fouling on a range of substrates that may effect commercial interest (Eno et al., 1997).
A. modestus has a number of competitive advantages over native species in Europe, including being able to breed and hence recruit throughout the year; it can live intertidally and subtidally and it is both eurythermal and euryhaline (Barnes and Barnes, 1966). In quiet waters, Barnes and Barnes (1966) suggest that its eurythermal and euryhaline tolerances give it a considerable advantage over Semibalanus balanoides. In Britain, Southward and Crisp (1952, 1954) reported that its spread in the rivers Plym and Tamar, southwest Britain, may have been facilitated by a reduction in numbers of S. balanoides. Barnes and Barnes (1961) even reported A. modestus settling on top of S. balanoides. It has the ability to settle higher on the shore in the intertidal zone than S. balanoides and also in the sublittoral zone where S. balanoides does not (Foster, 1971b). Bennell (1981) reported that during sampling from 1974-1978 at the Menai Bridge, the island of Anglesey, North Wales, peaks of abundance of A. modestus occurred only when S. balanoides cover was low, which may have been due to competition for space.
Elsewhere in Europe A. modestus has been found to compete for space and food with a number of barnacle species. Barnes and Barnes (1966) described it as “a severe competitor of Chthamalus in relatively quiet waters”, with examples of shores in northwestern Spain and Portugal, where they found Chthamalus limited to the upper shore, due to dense zone of A. modestus lower down. According to Barnes and Barnes (1966) ‘Chthamalus stellatus’ may be restricted to the upper levels on a shore due to competition from Amphibalanus improvisus, Balanus amphitrite and A. modestus.
According to Crisp (1958)A. modestus competed with Amphibalanus improvisus and Balanus crenatus, on the lower shore and in the sublittoral, but Knight-Jones (1948) noted that in the infralittoral zone and subtidally it met with severe competition from A. improvisus. Crisp (1958) suggested that it would be interesting to observe the interaction between A. modestus and A. improvisus if the former spread to the Baltic, where A. improvisus is the dominant species. He noted that since A. modestus breeds in summer, its dominance has a profound effect on the composition of the summer plankton, greatly increasing the number of barnacle nauplii, presumably at the expense of other larvae. At Zeebrugge, Den Hartog (1953) noted that it competed strongly with S. balanoides between 25 and 100 cm below mean high water mark. In Germany, Kühl (1963) suggested that it competed with S. balanoides, A. improvisus and B. crenatus, but at that time these species had not yet been ousted by Austrominius. However, Witte et al. (2010) reported that at Sylt, in the North Sea, in summer 2007, abundances of A. modestus had overtaken those of S. balanoides and B. crenatus. At Arcachon, France, Barnes and Barnes (1968a) reported that it competed with A. improvisus and Perforatus perforatus at lower levels and it was found to smother ‘C. stellatus’ (Barnes et al., 1972).
Since it produces large numbers of nauplii in the summer, these may compete with other members of the zooplankton, such as the larvae of other benthic species (Crisp, 1958; Farnham, 1980).
In Lough Hyne Marine Nature Reserve, Ireland’s first and only marine nature reserve, A. modestus has started to dominate the intertidal barnacle fauna (Lawson et al., 2004). Semibalanus balanoides’ numbers have decreased, but there has been no experimental work to establish whether the species are competing there. Unfortunately, there has been very little experimental work to date to examine competition or A. modestus’ effects on ecological processes anywhere in its introduced range.
Since barnacles accumulate much higher levels of trace metals (by uptake from the water and assimilation during feeding) than other marine invertebrates (see e.g. Rainbow, 1987; 1998), they can be used as biomonitors of trace metal availabilities in coastal waters (see e.g. Phillips and Rainbow, 1988; 1994; 1995; Rainbow and Phillips, 1993). A. modestus though shows lower assimilation efficiencies from its diet than various Balanus species (Rainbow and Wang, 2001). At least one study concluded that it was unsuitable for monitoring copper and zinc (Elliott et al., 1985). Thomas and Ritz (1986) have examined the zinc phosphate granules (where zinc is stored in a metabolically unavailable form) in A. modestus. It has been used in toxicity tests for chemicals used to dissolve oil-spills: the activity of the adults’ cirri is affected by low concentrations. Acute toxicity stops larval locomotion and development is retarded or inhibited (Corner et al., 1968).
On shores in Australia and in Australian museum collections A. modestus has often been mistakenly identified as A. covertus (Foster, 1982; Jones, 1990), so there are few reliable records of A. modestus in Australia (e.g. those of Pope (1945) may have included more than one species). According to Jones (1990) “the two species are very similar and are difficult to identify if the shells are encrusted with mud or eroded away”. Although shell colour can be used to distinguish the two species in some instances, “the main morphological difference between E. covertus [A. covertus] and E. modestus [A. modestus] concerns the presence or absence of pectinate setae on the medial face of the posterior ramus of cirrus III”. According to Foster (1982) “In young, uneroded shells the reddish to buff colour of E. covertus [A. covertus] is in marked contrast to the vivid white shell of E. modestus [A. modestus]. Also the narrow contrastingly-coloured ribs of E. covertus [A. covertus] differ from the uniformly white and broadly-folded parietes of E. modestus [A. modestus]”. E. covertus does not occur in New Zealand (Foster and Anderson, 1986). Bayliss (1988) describes how A. modestus can be distinguished from Austrominius adelaidae Bayliss (formerly known as Elminius adelaidae Bayliss) which occurs in South Australia.
Unlike A. modestus, which it is often found with it in Australia, E. covertus has not been introduced to Europe. Foster and Anderson (1986) noted that “it seems likely that all of the European studies on the biology and ecology of Elminius refer to E. modestus” [not A. covertus], but Egan and Anderson (1985) noted that there are some inconsistentencies with respect to larval decriptions; however, the adults are very different in colouration. Similarlr to A. modestus, A. covertus occurs in the intertidal area of harbours and estuaries on sheltered shores, but the latter occurs towards the upper limit of the tidal range, whereas A. modestus occurs in the midlittoral and shallow sublittoral (to 5 m) zones and is an important fouling species (Jones, 1990). These differences in vertical zonation may explain why A. modestus has become invasive but not A. covertus. Another member of the subfamily Elminiinae, Elminius kingii, is confined to South America (Chile, below 30 °S; Cape Horn to Punta Arenas, Argentina; Falkland Islands (Newman and Ross, 1976)).
In Europe, Knight-Jones and Stevenson (1950) noted that it could be difficult to distinguish initially the newly settled A. modestus from those of Balanus improvisus (now known as Amphibalanus improvisus).
According to Southward (2008), in Britain and Ireland, “Sublittoral specimens [of A. modestus] sometimes superficially resemble Balanus crenatus, among which they grow and it is necessary to clean the shells thoroughly of epizoic growths to check identity”, while Barnes et al. (1972) noted that in areas where there is a lot of sand scour (e.g. Hossegor, France) specimens can become eroded, and may at first glance resemble Amphibalanus improvisus or Amphibalanus eburneus (Gould) (formerly known as Balanus eburneus). See section on Reproductive Biology (under 'Biology and Ecology') for information on how to distinguish the larvae from other species in the plankton.
Leloup and Lefevere (1952) and Crisp and Davies (1955) point out that, unlike Semibalanus balanoides and Balanus crenatus Bruguière, which show quite a lot of morphological variation under different ecological conditions, such as crowding, A. modestus does not show much plasticity. Instead A. modestus remains conical in shape, although there is some variation with population density; it may be taller and more cylindrical when crowded, with straight ridges, (with narrow parietes tapering towards the basis) versus the low uncrowded form (with concave ridges) (Leloup and Lefevere, 1952; Crisp and Davies, 1955).
The only control method is the scraping of ships’ hulls and buoys to remove barnacles (Eno et al., 1997).
It is unclear why A. modestus has not successfully been introduced from European Atlantic shores into North America, with other species such as the shore crab Carcinus maenas or the edible winkle, Littorina littorea. Crisp (1958) and Farnham (1979) pointed out that one of the limiting factors for the establishment and spread of a non-indigenous species is the number of reproductive units produced to constitute a large enough innoculum. One limit to the spread of A. modestus is that it is an obligate cross-fertilising hermaphrodite (Crisp, 1954; Barnes and Crisp, 1956), so sufficient densities have to be reached in an area, in order for individuals to be close enough to breed. Breeding is not possible if individuals are isolated by a distance of 5 cm (the upper limit of a fully extended penis) or more (Crisp, 1958).
There has been very little experimental work to date to examine competition or A. modestus’ effects on ecological processes anywhere in its introduced range, or potential effects of climate change on its invasive range and impacts on native species. This is becoming more relevant now, since recent publications have indicated that after 50 years of fairly stable invasive range and abundances, it has been reported to be increasing in abundance in a number of locations.
Corner EDS; Southward AJ; Southward EC, 1968. Toxicity of oil-spill removers ('detergents') to marine life: an assessment using the intertidal barnacle Elminius modestus. Journal of the Marine Biological Association of the United Kingdom, 48:29-47.