Ruditapes philippinarum (Japanese carpet shell)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Water Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Impact Summary
- Economic Impact
- Environmental Impact
- Impact: Biodiversity
- Social Impact
- Risk and Impact Factors
- Similarities to Other Species/Conditions
- Gaps in Knowledge/Research Needs
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Ruditapes philippinarum Adams and Reeve, 1850
Preferred Common Name
- Japanese carpet shell
Other Scientific Names
- Amygdala ducalis
- Amygdala philippinarum
- Amygdala semidecussata
- Paphia philippinarum
- Papia bifurcata
- Ruditapes semidecussata
- Ruditapes semidecussatus
- Tapes bifurcata Quayle, 1938
- Tapes decussatta
- Tapes decussatus
- Tapes denticulata Sowerby, 1852
- Tapes denticulatus Sowerby, 1852
- Tapes ducalis Römer, 1870
- Tapes indica Sowerby, 1852
- Tapes indicus Sowerby, 1852
- Tapes japonica Deshayes, 1853
- Tapes philippinarum Adams and Reeve, 1850
- Tapes quadriradiatus Deshayes, 1853
- Tapes semidecussata Reeve, 1864
- Tapes semidecussatum Reeve, 1864
- Tapes variegata
- Tapes viola Deshayes, 1853
- Tapes violascens Deshayes, 1854
- Venerupis japonica Deshayes, 1853
- Venerupis philippinarum Adams and Reeve, 1850
- Venerupis semidecussata
- Venus decussata
- Venus japonica
- Venus philippinarum Adams and Reeve, 1850
- Venus tesselata
International Common Names
- English: carpet shell clam; Japanese carpet shell; Japanese littleneck clam; littleneck clam; Manila clam; mud clam; Pacific palourde; short necked clam; short-necked clam
- Spanish: almeja japonesa; almeja japónica
- French: palourde japonaise
Local Common Names
- Germany: Japanische muschel
- Japan: asari
Summary of InvasivenessTop of page
R. philippinarum is a marine bivalve mollusc with a solid broadly oval shell on which radiating ribs and concentric grooves make the posterior area markedly decussate. The shell is variable in external colour from white to yellow or brown, often with curved radiating darker bands or dark blotches (see Pictures).
The Manila clam is indigenous to sub-tropical and temperate coastal seas of the western Pacific from the South China Sea north to the Sea of Okhotsk. The clam is high value seafood. Since the early twentieth century, due to human activity related to aquaculture and fishing industries, the Manila clam has become established along the Pacific coast of North America, the Atlantic coast of Europe, the Mediterranean Sea and elsewhere. Due to relatively high fecundity and growth rates the clam has come to dominate the most suitable habitats such as coastal lagoons. A planktonic larval stage enables local spread once naturalised.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Mollusca
- Class: Bivalvia
- Family: Veneridae
- Genus: Ruditapes
- Species: Ruditapes philippinarum
Notes on Taxonomy and NomenclatureTop of page
The type specimen of the Manila clam Ruditapes philippinarum (Adams & Reeve, 1850) was collected at the island of Mindanao in the Philippines during the voyage of H.M.S Samarang (1843-1846). The new species received the binomen Venus philippinarum, Adams & Reeve, 1850, and was described and illustrated in the report on the zoology of the voyage.
Since the 1850s both its generic and specific names have been volatile. Goulletquer (1997) lists 28 scientific and 17 common names that have been applied to the species. It has been located at one time or another in seven different genera including Protothaca, Venerupis and Tapes (Howson and Picton, 1997). A still commonly used synonym is Tapes semidecussata but in a review by Fischer-Piette and Metizier (1971) this binomen was classified as a junior synonym of T. philippinarum. However the genusTapes is defined around the type speciesTapes literata Linnaeus, 1758, which is different in shape, sculpture and genetics from the Manila clam. In contrast Venerupis is considered a legitimate synonym as the type species for the genus (V. saxatilis) is similar in molecular and shell characteristics (J Taylor, Natural History Museum, London, UK, personal communication, 2009).
Whether the distinction between Venerupis spp. and the Manila clam is sufficient to justify the separate genus Ruditapes has not yet been resolved at the molecular level. The binomen Ruditapes philippinarum is adopted for the present text.
DescriptionTop of page
R. philippinarum is a bivalve mollusc with a solid equivalve broadly oval shell and anterior beaks. External shell sculpture consists of radiating ribs and concentric grooves with the latter becoming deeper towards the posterior and anterior regions making the shell surface decussate. The shell is variable in external colour from white to yellow or brown, often with radiating darker bands expanding from the beaks or dark blotches (see Pictures). The inside of the shell is white with an orange tint and sometimes with a posterior purple area typically including the pallial sinus, which extends towards but not beyond the centre line. The holotype is located in the Natural History Museum, London, UK.
Shells are large and solid. The height of the shell is 19-31 mm; length is commonly 28-46 (25-57 mm, in naturalized populations in Europe; CIESM, 2003). Maximum length is 80 mm (FIGIS, 2004), and width is 13-22 mm (Xie, 1998). The sizes of the two shell valves are equal (equivalve) (Qi, 1998; Xie, 1998); valves are inequilateral, beaks in the anterior half are somewhat broadly oval in outline, pointing towards the front. The distance from the umbo to the front edge is about one-third of the total length (Xie, 1998). Ligament is inset, not concealed. Lunule is narrowly heart-shaped, clear though not particularly well defined, with light and dark brown fine radiating ridges. Escutcheon is reduced to a mere border of the posterior region of the ligament (FIGIS, 2004). The number of radial ribs is about 90-107 (Xie, 1998). The radial ribs are slim at the umbo; but gradually thicken towards the outer margin. Growth stages are clear. Three cardinal teeth are present in each valve; the centre tooth in the left valve and the centre and posterior teeth in the right valve are bifurcated. Pallial sinus is relatively deep and wedge-shaped. Internal margin is smooth. The exhalent siphon is long. Tentacles at the edge of the inhalant siphon are not bifurcated (Qi, 1998; FIGIS, 2004).
Shells are extremely variable in colour and pattern: white, cream, yellow or light brown, sometimes with rays, streaks, blotches or zig-zags of a darker brown/black. Shell slightly polished; inside of shell polished, white to pinkish-white with an orange or pale yellow tint, sometimes with purple/violet tint, especially in individuals from the top of the intertidal zone (Qi, 1998; FIGIS, 2004).
Similar to Ruditapes decussatus but has a more pronounced decussate structure and angulated shell (CIESM, 2003). The foot of live R. philippinarum is orange whereas in R. decussatus, the foot is white. The shell of R. philippinarum often has distinctive black and white markings. When observed feeding underwater, the siphon of R. philippinarum is joined whereas it is separate in R. decussatus (Carter, 2004).
DistributionTop of page
R. philippinarum originates from South-East Asia (Indo-Pacific) where it naturally occurs on sheltered sand, sand-silt, sand-pebble, muddy gravel and stiff clay sites at 1-10 m of water, usually below mid-tide level and in littoral lagoons and estuaries (FIGIS, 2004). Distributed from the Zhuanghe River in Liaoning to the southern part of the Leizhou Peninsula in Guangdong in China; and from the south of the Okhotsk Sea, Sakhalin, Kuril Islands, through Japan, the Korean Peninsula, Philippines, Pakistan, India, Sri Lanka and Indonesia (Qi, 1998).
Successful cultivation has led to a considerably expanded distribution range. It has been introduced to Hawaii, USA and the Pacific seacoast of North America (USA and Canada). Also introduced in the Mediterranean (Italy, France, Sardinia, Romania) and Brittany, France, where it lives in the same habitat as R. decussatus (Qi, 1998; FIGIS, 2004). In the Mediterranean and on the Atlantic coast of Europe it may form well-established natural populations, limiting or replacing R. decussatus (CIESM, 2004). Cultivation is expanding on the Atlantic coast of Europe (Spain, UK, France, Norway). Natural populations may even develop in colder parts of its introduced range, in Norway and the UK (Mortensen, 1993; Morgan and Hendry, 2002).
Distribution TableTop of page
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.
History of Introduction and SpreadTop of page
Largely as the result of human activity the Manila clam is now established along the Pacific coast of North America, the Atlantic coast of Europe and in the Mediterranean Sea. In the first such introduction Japanese clams were taken to the Hawaiian Islands in the early twentieth century (Bryan, 1919; Yap, 1977). Other Japanese clams reached the North American Pacific coast in the 1930s, apparently as an accidental introduction with stocks of Pacific oyster (Quayle, 1949), were they now extend from California to British Columbia (Magoon and Vining, 1981). In the 1960s clams were introduced from the eastern Pacific to France were they are now cultivated on both Mediterranean and Atlantic coasts (IFREMER, 1988; Flassch and LeBorgne, 1992). In 1980, clams from Oregon were introduced in the United Kingdom (Jensen et al., 2004, 2005) from where they have subsequently been taken to Ireland and Spain (Mann, 1983) They have also been imported to Tahiti, the Italian Adriatic, Germany and the UK.
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|Canada||Pacific, Northwest||1936||Unknown||Yes||Yes||DIAS (2004)|
|USA||Pacific, Northwest||1949||Unknown||Yes||Yes||DIAS (2004)|
Risk of IntroductionTop of page
The pathways for introduction are both anthropogenic and natural with the first category being of most significance in explaining the world-wide spread of the clam. While anthropogenic dispersal has included accidental release, the commoner pathway involves purposeful introduction related to the economic potential of the species as a fishery species and for mariculture. Such economically motivated activities range from government sanctioned introductions such as was the case in the UK to the informal activities of fishermen who deposit adults or spat in new locations in the hope of establishing new fishing grounds. Spat are available by post from a number of suppliers.
A planktonic larval phase facilitates natural spread of established populations to suitable adjacent locations such as has happened in Poole Harbour, UK (Jensen et al., 2005). However the extent that this mechanism might lead to “leap-froging” on tidal currents along a coast from one suitable habitat to the next is not yet clear, not least due to the difficulties of distinguishing natural from informal anthropogenic spread.
There may be considerable potential for further spread of the clam. Outside its indigenous area there are many parts of the world where it is not yet established, but which contain habitats with physico-chemical environments within the tolerance range of the species. The extent to which this potential is fulfilled will depend on its ability to succeed within the context of particular established community structures (competition, predation etc), the existence and extent of fisheries-based local or national economic development initiatives and, following initial introduction, the presence of established fishermen (or distributors) inclined to be proactive in enhancing their livelihoods.
HabitatTop of page
R. philippinarum occurs in coastal sediments from the inter-tidal zone to the shallow sub-littoral. The clam is reported as being typically found from the lower shore down (Zhuang et al., 1981) but can occur on the upper shore in certain tidal regimes with prolonged high water stands such as is found in Poole Harbour, UK (Humphreys, 2005; Humphreys et al., 2007). It can succeed in both muddy and sandy substrates and is a shallow burrower, commonly found between 3 and 5 cm below the sediment surface.
R. philippinarum is euryhaline. In China, Manila spat are capable of growth from salinities of 14-33.5, with 20.5 as the optimum. However some will survive and even grow down to 7.5 and up to 40 (Lin et al., 1983). Broadly similar figures have been reported for introduced Adriatic clams which can tolerate salinities of 15-50, with larval growth occurring from 12-32 and optimal range 20-28 (Breber, 1996) This salinity tolerance range is the basis of the clam’s successful establishment in estuarine conditions.
It has been suggested that in the Italian Adriatic there is a causal correlation between the spatial distribution of adult clams and that of benthic pinnate diatoms (Breber, 2002). Such a flora develops as a film on sediments where light is strong and wave action weak. High nutrient levels encourage the growth of such films and population density data suggests that the clams do best in somewhat eutrophic conditions such as can occur in coastal lagoons and similar environments (Humphreys et al., 2007). However the species is not restricted to such places.
Habitat ListTop of page
|Mud flats||Principal habitat||Natural|
|Mud flats||Principal habitat||Productive/non-natural|
|Intertidal zone||Principal habitat||Natural|
|Intertidal zone||Principal habitat||Productive/non-natural|
|Salt marshes||Principal habitat||Natural|
|Estuaries||Principal habitat||Harmful (pest or invasive)|
|Lagoons||Principal habitat||Harmful (pest or invasive)|
|Inshore marine||Principal habitat||Natural|
|Inshore marine||Principal habitat||Productive/non-natural|
|Benthic zone||Principal habitat||Natural|
|Benthic zone||Principal habitat||Productive/non-natural|
Biology and EcologyTop of page
Wild Manila clam individuals are diploid with a chromosome complement of 2n=38 (Gosling and Nolan, 1989). Experiments on polyploidy with a view to impeding gametogenesis (so as to retain meat quality by preventing the diversion of food reserves into gonad and gamete development) have involved inducing triploidy (3n=57). Inducing triploidy by heat shock has also been considered as a possible method of sterilization allowing farming without the risk of naturalisation. However it appears that achieving 100% induced triploidy is difficult and in any event some reproductive viability can be retained in triploid clams. Tetraploid embryos and larvae have been produced but never survive (Utting, 1995).
Electrophoretic studies suggest that natural hybridization with the European native T. decussatus is not possible due to the extent of genetic dissimilarity (Fava and Meggiato, 1995).
Reproductive activity in the Manila clam is dependent on exogenous factors such as temperature, which interact with endogenous factors such as condition, which is in turn related to food supply. In temperate latitudes pronounced seasonal variations determine the onset of early gonad development subsequent gametogenesis and spawning. Evidence from various parts of the world are reasonably consistent, indicating lower temperature limits on gonad activity, gamete ripening and spawning to be 8, 12 and 14°C, respectively (Ohba, 1959; Holland and Chew, 1974; Mann, 1979; Xie and Burnell, 1994; Drummond et al., 2006).
R. philippinarumis dioecious and gonochoric, although hermaphroditic individuals are found very occasionally. The sex ratio in wild populations is approximately equal (Holland and Chew, 1974). Much of the information on the life-cycle of this economically significant species has been achieved under hatchery culture conditions. Under cultivation at 25°C, fertilized eggs take about 24 hours to develop through a free swimming trochophore stage into an early veliger called the D-larva. This stage, named after the characteristic capital D shape of the shell, is normally of mean length 90-95 mm. These larvae may remain in their pelagic phase for around 8 days after which metamorphosis commences. Time in the plankton will be a factor in the natural dispersal of the Manila clam. In the wild it is likely that the duration of the pelagic larval stages will vary from the figures given above in line with habitat conditions and therefore location. The characteristic adult shape of the pediveliger larva (mean length 215 mm) is achieved in optimal cultivation conditions by day 12.
After settlement, growth in cultivation is dependent on stocking density, with mean length of 1000 mm at 30-40 days being achievable in good conditions (Utting and Spencer, 1991).
Concerns over the possible effects of Manila clam invasion on indigenous biodiversity along with interest in its economic potential in Europe have stimulated comparisons between it and the indigenous sympatric Tapes decussatus. In general R. philippinarum is considered to be hardier, faster growing and with higher fecundity than the native T. decussata (FAO, 2009).
When grown by Beninger and Lucas (1984) in a common habitat on the French Atlantic, coast R. philippinarum metrics revealed markedly greater shell and flesh growth and suggested a quicker recovery, in terms of body condition, from periods of physiological stress in autumn and winter. Moreover at temperatures above 15°C, the Manila clam is markedly better able to achieve organic weight growth than T. decussata given the same diet (Laing et al., 1987).
Analysis of gross biochemical composition in these clams indicated that while protein constituted the main reserve for both species, lipids contributed more to reserves in R. philippinarum.
Laing (1993) has shown that after periods of nutritive stress juvenile Manila clams do not immediately respond to the sudden restoration of food supply in the laboratory with a resumption of normal feeding rates: a strategy he considers adaptive in the context of the gradual development of natural algal blooms: A delayed response making it more likely that metabolic rate remains related to the reliable availability of food.
Holland and Chew (1974) reported the existence of two ecologically distinct forms of clam in which shell thickness varied with sediment type but there is as yet no reported evidence of genetic variants within the species.
Fecundity and population density
Despite intensive predation of spat, the Manila clam is capable of achieving high densities in its preferred habitat. Reported densities of wild populations range from 259-5744 m2 (Ohba, 1959; Ponurovski and Selin, 1988; Breber, 2002), although the northern most naturalised European population density is lower at up to 156 m2 (Humphreys et al., 2007).
Using carbon-14 labelled food species Sorokin and Giovanardi (1995) have shown the Manila clam to have a wide feeding spectrum ranging across bacteria, algae and rotifers. Optimal concentrations of the algae Nitzschia and Chlorella are around 8-9 mg (wet weight) per litre and for bacteria 4.5-5 mg per litre.
Diatoms are thought to provide the main component of the Manila clam diet in the wild. In culture the diatoms Skeletonema costatum and Chaetoceros calcitrans have been shown to have a relatively high nutritional value for juvenile manila clams. Other diatoms and some flagellates, such as Choomonas salina, although less valuable can also deliver growth.
Breber (2002) observed that while larval and juvenile Manila clams take their diatom diet from the plankton, adults appear to be dependent on benthic diatoms which grow as a film on the sediment. Nevertheless clams can be maintained in the laboratory on the basis of suspension feeding.
ClimateTop of page
|A - Tropical/Megathermal climate||Preferred||Average temp. of coolest month > 18°C, > 1500mm precipitation annually|
|C - Temperate/Mesothermal climate||Preferred||Average temp. of coldest month > 0°C and < 18°C, mean warmest month > 10°C|
|D - Continental/Microthermal climate||Preferred||Continental/Microthermal climate (Average temp. of coldest month < 0°C, mean warmest month > 10°C)|
Water TolerancesTop of page
|Parameter||Minimum Value||Maximum Value||Typical Value||Status||Life Stage||Notes|
|Depth (m b.s.l.)||Optimum||Shallow enough for benthic diatom growth|
|Dissolved oxygen (mg/l)||>4.5||Optimum||Adult||Xie, 1998|
|Dissolved oxygen (mg/l)||>4.5||Optimum||Broodstock||Xie, 1998|
|Dissolved oxygen (mg/l)||>4.5||Optimum||Egg||Xie, 1998|
|Dissolved oxygen (mg/l)||>4.5||Optimum||Larval||Xie, 1998|
|Dissolved oxygen (mg/l)||>4.5||Optimum||Fry||Xie, 1998|
|Salinity (part per thousand)||14||33.5||Optimum||7.5-40 tolerated, preferred range allows growth of spat|
|Spawning temperature (ºC temperature)||20||24||Optimum||Broodstock||Xie, 1998|
|Water pH (pH)||8.2||Optimum||Egg||Xie, 1998|
|Water pH (pH)||7.8||8.4||Optimum||Adult||Xie, 1998|
|Water pH (pH)||7.8||8.4||Optimum||Broodstock||Xie, 1998|
|Water pH (pH)||7.8||8.4||Optimum||Larval||Xie, 1998|
|Water pH (pH)||8.20||8.35||Optimum||Fry||Xie, 1998|
|Water temperature (ºC temperature)||18||24||Optimum||Larval||Xie, 1998|
|Water temperature (ºC temperature)||18||30||Optimum||Adult||Xie, 1998|
|Water temperature (ºC temperature)||18||30||Optimum||Broodstock||Xie, 1998|
|Water temperature (ºC temperature)||21||24||Optimum||Egg||Xie, 1998|
|Water temperature (ºC temperature)||>14||Optimum||Less than 8 tolerated. 14 is lowest for complete reproductive cycle|
|Water temperature (ºC temperature)||<5||>35||Harmful||Adult||Xie, 1998|
|Water temperature (ºC temperature)||13||22||Optimum||Fry||Xie, 1998|
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Acanthopagrus schlegelii||Predator||Adult/Broodstock||Xie Kaien, 1998|
|Ariidae||Predator||Adult/Broodstock||Xie Kaien, 1998|
|Arius thalassinus||Predator||Adult/Broodstock||Xie Kaien, 1998|
|Carcinus maenas||Predator||Adult/Larval||not specific|
|Haematopus ostralegus||Predator||Adult/Fry||not specific||Crowe, 2004|
|Numenius arquata||Predator||Fry||Crowe, 2004|
|Portunus trituberculatus||Predator||Adult/Broodstock||Xie Kaien, 1998|
|Raja||Predator||Adult/Broodstock||Xie Kaien, 1998|
|Takifugu||Predator||Adult/Broodstock||Xie Kaien, 1998|
|Umbrina roncador||Predator||Adult/Broodstock||Xie Kaien, 1998|
Notes on Natural EnemiesTop of page
Non-native Manila clam populations are known to attract the attention of indigenous predators such as birds and crabs. In Poole Harbour, UK, the clam’s avian predators include the herring gull, Larus argentatus, which can be frequently observed in flight dropping adult Manila clam prey onto hard surfaces in order to break open the shell, and crows (J Humphreys, University of Greenwich, UK, personal communication, 2009). For the European oystercatcher, Haematopus ostralegus, the Manila clam represents a new food species (Caldow et al., 2007). In Washington State, USA, clams are eaten by gulls, crows and scoters (Toba et al., 1992).
Field and laboratory observations on the predation of spat and juvenile clams by Carcinus maenas suggest that in Atlantic waters this crab is capable of decimating spatfalls (Spence et al., 1991, 1992). Crabs of the genus Carcinus also predate Manila clams in the Adriatic (Mistri, 2004).
On lower shores predators include the gastropod molluscs Polinices melanostomus and P. tumidis (Ansell and Morton, 1987). The Manila clam suffers from a range of viral and bacterial pathogens including the bacterium Vibrio tapetis which can stunt growth and cause a brown deposit along the mantle edge known as “brown ring” disease (Figueras et al., 1996). Protozoan parasites include Perkinsus atlanticus which is reported as impeding reproduction, and Bonamia sp.which is suspected of causing periodic high mortality (FAO, 2009).
Invertebrate parasites include the trematode Proctoeces orientalis which occupies the renal system, and Cercaria spp. which can effect both reproduction and growth (FAO, 2009).
Means of Movement and DispersalTop of page
The Manila clam has a pelagic larval phase allowing tidal currents to bring about dispersal at least to suitable adjacent habitat. The distance such larvae can travel will depend on tidal currents and period in the plankton.
While adult clams are often found in soft sediments young spat preferentially settle on solid surfaces such as are represented by gravel, pebbles and shell debris (FAO, 2009). This is consistent with Utting and Spencer’s (1991) observation that in cultivation young spat crawl up vertical surfaces and attach together in clumps using their secreted byssus threads. Adult clams, however, do not appear to be restricted to habitats with gravel or dead shell surface components. An observation that along with some demographic population data raises the question as to whether populations in fine sediments may be enhanced by secondary recruitment based on horizontal migrations of young clams (Humphreys et al., 2007).
The Manila clam was accidentally introduced into the Pacific coast of North America in the 1930s, contained within batches of Pacific oyster Crassostrea gigas imported from Japan. It subsequently spread quickly along that coast including as far north as British Columbia (Nosho and Chew, 1972; Bourne, 1982).
The current worldwide distribution of the Manila clam is based largely on the intentional introduction of the clam for economic exploitation during the twentieth century. The reported sequence and extent of introductions is outlined in the section History of Introductions.
Economic exploitation includes both fisheries and aquaculture. For example in 1983 the Manila clam was released into the Italian Adriatic (initially in the Venice lagoon but subsequently elsewhere) with the specific intention to “surrogate the local fishery of the indigenous carpet clam Tapes decussatus” (Breber, 2002). This introduction was supported by the Province of Venice. In the UK a national government supported project led to the introduction of the clam in Poole Harbour but with the purpose of promoting aquaculture on leased areas of seabed.
In addition to government supported introductions are the commercially motivated activities of seafood traders and fishermen whose activities range from formally recognized introductions to informal and sometimes illegal introductions involving the dumping of spat or even adults in the hope of a future harvest. Such informal activity is probably widespread: a possibility that can make it difficult or impossible to determine the extent of natural dispersal.
Impact SummaryTop of page
|Environment (generally)||Positive and negative|
|Fisheries / aquaculture||Positive|
Economic ImpactTop of page
The economic impact of R. philippinarum introduction is generally positive in terms of livelihoods and employment. The clam provides for economic growth in coastal communities through new or increased direct revenue streams from fishing, aquaculture and wholesaling. Indirect benefits in related industries such as equipment manufacture and transport and multiplier effects as communities raise their spending power are also positive consequences of successful Manila clam introduction. Such economic benefits explain the direct support for introduction of the clam received from local or national governments.
Environmental ImpactTop of page
Impact on Habitats
Naturalised Manila clam populations can add significantly to the total annual secondary production in appropriate habitats such as the Venice Lagoon, not least due to their growth rates and densities compared to indigenous species (Breber, 2002). Even near the northern limit of their European distribution, where population densities are modest in comparison to Adriatic lagoons, the clam can still make a significant contribution to benthic production (Humphreys et al., 2007). For the European oystercatcher, Haematopus ostralegus, newly naturalised Manila clams have been shown to have population level implications - individual-based modeling indicates a reduction in the predicted over-winter mortality in the oystercatcher population (Caldow et al., 2007).
At high densities Manila clams can affect nutrient dynamics (Bartoli et al., 2001) and alter the abundance of zooplankton, a phenomenon that has led to calls to restrict cultivation in the Venice lagoon (Sorokin et al., 1999). In the same location Pranovi et al. (2006) has estimated the total filtration capacity of the macrobenthos to have been doubled as a result of the Manila clam introduction, with a consequent altering of ecosystem function in terms of stronger benthic-pelagic coupling and reduced resilience.
Manila clam aquaculture and fisheries also have impacts on habitats. Cultivation of clams typically involves lower shore ground plots which are covered in plastic netting to protect the clams from predation by crabs (Spencer et al., 1991, 1992). Experimental plots in the Exe estuary, UK, were found to have increased sediment deposition with resulting elevation relative to surrounding ground. During the summer period growth of the epiphyte Enteromorpha on the nets was thought to interfere with the hydrographic regime reducing water flow (Spencer et al., 1996). In Canada, Munroe and Scott McKinley (2007) found such netting to increase the organic content of the sediment.
The effects of mechanized clam fishing involving the hydraulic “pump-scoop” dredge may also affect sediment granulometry although results are inconclusive (Jensen et al., 2005; Parker and Pinn, 2005).
Impact on Biodiversity
It has been argued that in Venice the Manila clam has replaced the bivalves Cerastoderma glaucum and Tapes decussata (Occhipinti-Ambrogi and Savini, 2000) However, Breber (2002) has subsequently shown that 2% of the catch from a commercial dredge in the Venice lagoon were T. decussata. In the case of the Goro lagoon, (about 50 km south), he found the presence of a naturally low and varying density of T. decussata to be “hidden” (rather than replaced) by a “prodigious abundance” of Manila clams. Pravoni et al. (2006) may have resolved this issue, reporting a sharp reduction (rather than eradication) of all other bivalves in the Venice lagoon in terms of both area of distribution and population density.
Experimental UK netted cultivation plots (with and without clams) showed increased abundance of deposit feeding polychaetes, especially Amphitrite acutifrons and Pygospio elegans, and a reduction in cirratulids (Spencer et al., 1996). Moreover the naturalisation of the clam in Poole Harbour, UK since 1980 has coincided with a decline in the abundance of the bivalves Scrobicularia plana and Macoma baltica (Caldow et al., 2005). However, an alternative cause for these declines may be tributyl tin pollution. Finally in the UK as in Italy, T. decussata can exist side by side with larger numbers of Manila clams. The native species representing about 7% of the combined Venerid total in some parts of Poole Harbour (J Humphreys, University of Greenwich, UK, personal communication, 2009).
On balance it would seem that while high density clam cultivation can impact the community composition of the seabed, the impact of Manila clams at densities typical of wild naturalised populations whether directly through competition or indirectly though fishing activity is less clear. A point that highlights the need for further work.
Impact: BiodiversityTop of page
Introduced Ruditapes philippinarum have become naturalized in many areas of Europe, competing with native R. decussatus, limiting their populations and replacing them in some cases (CIESM, 2003). The regional government of Galicia (Spain) has banned the use of Manila clam seed for semi-extensive use on beaches, and several authorities are actively promoting the use of the native clam (Varadi et al., 2000).
Social ImpactTop of page
The economic benefits that accrue from Manila clam introduction translate to social benefits such as increased standard of living in families who derive an improved livelihood or employment. Local communities benefit from the effects of economic development.
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Has a broad native range
- Abundant in its native range
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Fast growing
- Has high reproductive potential
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Modification of hydrology
- Modification of natural benthic communities
- Modification of nutrient regime
- Soil accretion
- Threat to/ loss of native species
- Antagonistic (micro-organisms)
- Competition - monopolizing resources
- Pest and disease transmission
- Rapid growth
- Highly likely to be transported internationally accidentally
- Highly likely to be transported internationally deliberately
- Highly likely to be transported internationally illegally
- Difficult/costly to control
UsesTop of page
The general economic benefits of Manila clam introductions from fishing and aquaculture have been described above. In 1990, just seven years after the initial introduction of the Manila clam to the Italian Adriatic lagoons, between 70 and 100 tons of Manila clam were being marketed daily (Breber, 1992). The market value around that time in Europe was between £4000-£6000 equivalent per tonne (Spencer et al., 1991). Near the northern limits of its wild European distribution in Poole Harbour, UK, the regulated fishery grew to approximately 500 tonnes per annum between 1988 and 2004 (Humphreys, 2010).
The economic benefits that accrue from Manila clam aquaculture and fishing translate to social benefits such as increased standard of living in families who derive an improved livelihood or employment. Local communities benefit from the effects of economic development as local production replaces imported products.
As a high value shellfish, catches from newly established Manila clam populations can be distributed through established local seafood supply chains, many of which connect with international traders. The existence of such supply chains combined with the high value of the product create strong local pressure for introduction involving formal and regulated procedures and sometimes the active support of local or national development agencies. Such formal introductions generally involve consideration of the possible negative impacts of the clam on biodiversity. The aquaculture and fishing industries of non-native areas has also led to commercial production of spat for the purposes of “seeding” areas of seabed. Trade in Manila spat is also international.
This combination of high value, existing demand, supply chain access and spat availability creates an informal and often illegal component of the industry. Once in the supply chain, the origin of clams is difficult or impossible to determine.
These circumstances encourage informal activity on the part of fishermen and traders despite the efforts of fishery patrols and inspectors, which in turn makes it difficult, if not impossible, within available resources to control informal introductions.
Once introduced it may take some time before the presence of the Manila clam is recognized as such in contrast to similar native species and in any event the arguments of even statutory conservation organizations will often be strongly contended by fishermen and their community or industry representatives.
Moreover once aquaculture or a fishery is established the planktonic larva of the clam will potentiate the spread of the species despite any zoning regulations that might apply. While the production and supply of sterile triploid clams was once thought to be a possible solution, it has proven to be ineffective as a means of controlling the spread of the species.
For these reasons efforts to discourage the spread of the clam have been politically or practically overcome.
Discovery of new populations of Manila clams is sometimes impeded by their absence from local and regional identification guides. In European waters the Manila clam is consequently often confused with the native Tapes decussata. A useful guide to distinguishing these two species has been produced by Wimbledon (2003).
Similarities to Other Species/ConditionsTop of page
Similarity with indigenous Venerids such as Tapes decussatus in Europe can lead to mis-identification (and therefore under-reporting) of newly arrived Manila clams during ecological sampling and surveys, particularly if they are not yet covered in local keys. Anatomical inspection of fresh or preserved specimens reveals the inhalant and exhalent siphons to be fused along most of their length - a feature which when combined with the above features can be useful in distinguishing them from some other Venerids.
Gaps in Knowledge/Research NeedsTop of page
Most published research on the Manila clam has been motivated from a fisheries rather than a conservation standpoint. Consequently the range of circumstances in which the Manila clam can be invasive is as yet uncertain, and the extent of biodiversity threat ambiguous. However it is clear that when naturalized in suitable habitats the clam can achieve very high densities such that in the Adriatic lagoons of Italy it is regarded as a truly invasive species. This potential for invasion combined with uncertainty of the extent of the risk indicates the need for further research on the impacts of naturalised (rather than cultivated) populations and the detailed patterns of current distribution especially at geographical limits. In the context of published climate change scenarios such information may also allow a better analysis of the long term invasive threat represented by the species.
ReferencesTop of page
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OrganizationsTop of page
France: IFREMER - Institut français de recherche pour l'exploitation de la mer, 155, rue Jean-Jacques Rousseau, 92138 Issy-les-Moulineaux Cedex, www.ifremer.fr
Italy: FAO (Food and Agriculture Organization of the United Nations), Viale delle Terme di Caracalla, 00100 Rome, http://www.fao.org/
UK: CEFAS (Centre for Environment Fisheries and Aquaculture Science), Cefas Weymouth Laboratory, Barrack Road, Weymouth, Dorset DT4 8UB, Weymouth, UK, http://www.cefas.co.uk/
ContributorsTop of page
25/03/11 Original text by:
John Humphreys, University of Greenwich, Old Royal Naval College, Park Row Greenwich Park Row, Greenwich London SE10 9LS, London SE10 9LS, UK [Invasive Species Compendium]
Ningsheng Yang, Ouyang Haiying & Yan Caiping, Chinese Academy of Fishery Sciences, No. 150, Qing Ta Cun, Yong Ding Road, Beijing 100039, China [Aquaculture Compendium]
Chinese Academy of Fishery Sciences, No. 150, Qing Ta Cun, Yong Ding Road, Beijing 100039, China
Ouyang Haiying & Yan Caiping
Distribution MapsTop of page
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