Caprella mutica

C. mutica was first described from sub-boreal areas of north-east Asia in 1935 (Schurin, 1935). The first reports of C. mutica outside its native habitat were from the Pacific and Atlantic coasts of North America in the 1970s (Carlton, 1979) and 1980s (Marelli, 1981; Cohen and Carlton, 1995) and it has since spread to both northern and southern hemispheres (Ashton et al., 2007a). C. mutica is one of the largest caprellid amphipods, mature males attain body lengths of up to 50 mm (Nishimura, 1995) and populations can attain densities > 300,000 individuals/m² (Ashton, 2006). C. mutica is an aggressive species, out-competing native caprellid amphipods for space, even at low densities (Shucksmith, 2008).C. mutica is frequently associated with man-made structures and is found in abundance on boat hulls, navigation/ offshore buoys, floating pontoons and aquaculture infrastructure. It is highly likely that its dispersal is associated with vessel movements whilst attached to hull fouling. Whilst the wider environmental implications of C. mutica have not yet been confirmed, it is likely that it has an important impact on benthic and plankton communities (Cook et al., 2007b). This species is currently not present on an alert list (e.g. IUCN, ISSG) or listed as a regulated pest etc.

It is highly likely that C. mutica may be accidentally introduced to South America, southern Australia, and South Africa (Ashton et al., 2007a). All these regions are within the temperature boundaries for C. mutica and they are also key areas of shipping activity. South America and Australia are particularly well linked to south-east Asia as dispersal routes for introduced species (Carlton, 1987). Within Europe, this species is likely to continue to extend its current range, but is unlikely to survive in the central and eastern Baltic Sea due to low salinities (FIMR, 2006), and based on current knowledge it is not expected to become established in the Mediterranean Sea on account of the high summer seawater temperatures (Cook et al., 2006a). Long-range distribution will most probably depend on the movement of floating artificial structures, such as commercial and recreational vessels, whilst attachment to floating marine algae may account for more localised movements (Cook et al., 2007b). It is unlikely that C. mutica will be deliberately introduced to a region.

In the native range, C. mutica is typically associated with either attached or drifting macroalgae, including Sargassum spp. or aquaculture structures, such as the ropes used for the culture of the macroalga Undaria spp. in Otsuchi Bay (Kawashima et al., 1999). In regions outside its native range, it has been found associated with areas of anthropogenic activity, such as harbours, marinas, navigation buoys, oil rigs and aquaculture sites (Willis et al., 2004; Ashton, 2006; Kerckhof et al., 2007).

Direct sequencing of mitochondrial DNA (cytochrome c oxidase subunit I gene) has been used to investigate genetic variation between native and non-native populations of C. mutica. Three hundred individuals were PCR-amplified, which included 51 from the native area in Japan and 249 from non-native populations. Overall, base frequencies were biased towards A and T (A = 26%, T = 36%, C = 20%, G = 18%). All unique genetic sequences for C. mutica have been deposited with GenBank (accession numbers: DQ466220-466523). The genetic divergence between non-native populations of C. mutica in the northern hemisphere indicated significant differentiation between populations in different oceanic provinces and among populations in geographic regions. A total of seven haplotypes were found in the non-native populations, indicating considerably reduced genetic diversity relative to the native populations in Japan, where 31 unique haplotypes were observed. Collectively, the Atlantic populations were shown to have a comparatively high genetic diversity and most of the populations from this oceanic province share haplotypes. The unique haplotypes present on the Pacific coast of North America indicate strong genetic differentiation and no gene flow between the Atlantic and Pacific populations (Ashton, 2006).

Populations of C. mutica showed an increase in spring to a peak abundance in late summer (August – September), followed by a decline in population abundance during the winter in coastal waters on the west coast of Scotland. This cycle typically follows annual seawater temperatures for the region and is similar to that observed in the native habitat in the Sea of Japan (Fedotov, 1991). Juvenile C. mutica were present throughout the year on the west coast of Scotland in fish farm cages, which may indicate either continuous reproduction in this species or delayed growth of over-wintering juveniles. Females tend to dominate the population and this is most marked in autumn and winter (Ashton, 2006). Ovigerous females are found throughout the year and the number of eggs is positively related to the length of the female (Ashton, 2006). The greatest number of ovigerous females are found in the spring and summer months (April to October) (Ashton, 2006). The number of eggs per female is highly variable (min = 3, max = 363, average = 74, SD = 47.7) (Ashton, 2006). Females produce their first brood approximately 53 days post-hatching at an average body length of 8.5mm and at a seawater temperature of 13–14^oC (Cook et al., 2007b). Each female has an average of two sequential broods released at approximately 20 day intervals. The maximum number of recorded hatchlings produced by a single female at a seawater temperature of 13.0 ± 0.5°C is 82. Juveniles typically emerge from the brood pouch at a body length of approximately 1.3 mm (Cook et al., 2007b).

C. mutica is able to survive for up to 20 days without additional food under laboratory conditions (Cook et al., 2007b) andis tolerant of a broad range of temperature and salinity conditions, with 100% mortality at 30°C (48 h LT50, 28.3 ± 0.41°C), and salinities lower than 16‰ (48 h LC50, 18.7 ± 0.24‰) over a short (48 h) exposure period (Ashton et al., 2007b). Although lethargic at low temperatures (2°C), no mortality was observed, and the species is known to survive at temperatures as low as -1.8°C. The upper LC₅₀ was greater than the highest salinity tested (40‰), thus it is unlikely that salinity will limit the distribution of C. mutica in open coastal waters. However, the species will be excluded from brackish water environments such as the heads of sea lochs and estuaries. The physiological tolerances of C. mutica are beyond the physical conditions experienced in its native or introduced range and are thus unlikely to be the primary factors limiting its present distribution and future spread (Ashton et al., 2007a).

Nutrition

Caprellids are known to be opportunist feeders and to consume any readily available organic material (Keith, 1969). C. california and C. equilibria were found to consume organic detritus, diatoms, dinoflagellates and crustaceans, including other caprellids, amphipods, isopods, harpacticoid copepods and ostracods (Keith, 1969). In laboratory studies, C. mutica has been observed grazing upon the diatom Cylindrotheca fusiformis Reumann and Lewin and the macroalgae, Fucus vesiculosus L. They have also been observed actively filtering particles out of the water column using their second pair of setose antennae as filtering structures. Feeding trials with Artemia- nauplii revealed feeding rates of approximately 20 Artemia /hour for males and 13 Artemia /hour for females. Feeding rates remain constant over a period of 24 hours indicating continuous feeding throughout the day and night. Low temperatures led to a significant decline (about 60 and 50% for males and females, respectively) in feeding rates. When offered high food quantities, the average feeding rates were higher compared to when smaller quantities of food were offered (K Boos, Biologische Anstalt Helgoland, Alfred Wegener Institut for Polar- and Marine Research, Germany, personal communication, 2008).

C. mutica is typically found associated with attached macroalgae and drifting seaweeds in its native habitat, including Sargassum spp and on aquaculture structures, such as ropes for Undaria culture in Otsuchi Bay (Kawashima et al., 1999). In non-native habitats, this species typically clings to a wide range of artificial structures (aquaculture nets, ropes, pontoons, submerged lines and boat hulls) and on fouling organisms associated with these structures (including macroalgae - particularly red filamentous algae, such as Ceramium spp., hydroids, ascidians, mussels and barnacles) (Ashton, 2006; Cook et al., 2006a). In addition, studies have found that C. mutica, prefers to attach to filamentous (e.g. filamentous and branching macroalgae) and soft structures (e.g. solitary tunicates) compared with hard structures, such as bivalve shells (Shucksmith, 2008).

See Water Tolerances.

Very little information is known about the natural enemies of C. mutica. A recent study, however, has found that the European shore crab Carcinus maenas will consume large quantities of C. mutica in laboratory experiments (Shucksmith, 2008).

General

• Laboratory use
• Pet/aquarium trade
• Research model

All unique genetic sequences for C. mutica have been deposited with GenBank (accession numbers: DQ466220-466523).

The main difficulty in the detection of C. mutica has been with the identification of this species prior to 2002, as little taxonomic information was available outside its native region. This problem, however, should be rectified with the inclusion of a detailed identification notes on C. mutica in Willis et al. (2004) and the inclusion of this species in the new edition of Hayward and Ryland’s ‘Handbook of the Marine Fauna of North-West Europe’ later in 2007/08. 

It is relatively easy to mistakenly identify C. mutica as the closely related Asian species Caprella acanthogaster. Detailed descriptions of the morphology of C. acanthogaster can be found in Arimoto (1976) and Guerra-Garcia and Takeuchi (2004).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Early warning systems

No known early warning system is in place specifically for C. mutica, but as this species becomes more widely known, it is likely that it will be identified more regularly in routine surveys.

Rapid response

No known rapid response system is available to manage C. mutica.

Public awareness

In the UK, public awareness has been largely funded by a charitable trust, the Esmée Fairbairn Foundation, with support from the UK government environment agencies which has enabled the establishment of a marine non-native species website including C. mutica, production of leaflets, posters, splash-proof ID guides and popular articles and public lectures throughout the UK. Public awareness of C.mutica in other invaded regions, however, is unknown.

Eradication

No known eradication attempts have been made for C. mutica.

No known containment/ zoning attempts have been made for C. mutica.

Control

No known control programmes have been undertaken for C. mutica.

Potential physical methods for the control of C. mutica may include:

Biological control

Potential use of benthic predators.

Chemical control

No known chemical control.

Monitoring and Surveillance

Monitoring for C. mutica is conducted on a regular basis in the UK, Belgium and the Netherlands. A monthly monitoring programme for C. mutica at a fish farm and marina in the Lynne of Lorne, west coast of Scotland has been conducted since 2004.

Mitigation

No mitigation measures have been developed for C. mutica.

Limited factual information is available on the economic and environmental impacts and control methods for C. mutica and these areas would greatly benefit from future research.

UK: Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll, PA37 1QA, Oban, Scotland, UK, http://www.sams.ac.uk/

16/11/07 Original text by:

Elizabeth Cook, Scottish Association of Marine Science, Ecology Department, Dunstaffnage Marine Laboratory, Oban Argyll, Scotland, UK