Caprella mutica

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Japanese skeleton shrimp
Caprella mutica
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Superorder: Peracarida
Order: Amphipoda
Family: Caprellidae
Genus: Caprella
Species:
C. mutica
Binomial name
Caprella mutica
Schurin, 1935
   Native range of C. mutica
Circles are areas where specimens have been confirmed: 1: 
Akkeshi Bay[1]
Synonyms[2][3]
  • Caprella macho
    Platvoet et al., 1995
  • Caprella acanthogaster humboldtiensis
    Martin, 1977

Caprella mutica, commonly known as the Japanese skeleton shrimp, is a

predators. They are usually found in dense colonies attached to submerged man-made structures, floating seaweed
, and other organisms.

C. mutica are

synanthropic
, being found in large numbers in and around areas of human activity. Their ecological and economic impact as an invasive species is unknown, but they pose a serious threat to native populations of skeleton shrimp in the affected areas.

Description

Like all

sexually dimorphic with males considerably larger than females. Males average at a length of 25 to 30 mm (0.98 to 1.18 in),[4] though specimens have been recorded to reach 50 mm (2.0 in) in length.[3][9][10] Females, on the other hand, average at only 15 to 20 mm (0.59 to 0.79 in) long.[4][11]

The body can be divided into three

setae (bristles), giving them a hairy appearance.[11][12] The second pereonite also has two to three pairs of spines on the back, with an additional two pairs at the sides near the base of the limbs.[3] The remaining pereonites (third to seventh) lack the dense setae of the first two pereonites.[13] The third pereonite has seven pairs of spines at the back while the fourth pereonite has eight pairs. Both have three to seven pairs of spines near the base of the gills. The fifth pereonite has five pairs of back spines and a pair of spines at the sides. The sixth and seventh pereonites each have two pairs of back spines, situated at their centers and near the posterior.[3] Females differ from males in having much shorter pereonites which lack the dense covering of setae. The cephalon and first pereonite also possess a single pair of spines each, though they can sometimes be absent.[3][6]

Morphology of Caprella mutica (male)

Like other

Maxillipeds, a modified pair of appendages, also serve as accessory mouthparts.[12][13]

The appendages that arise from pereonites are known as

venomous, though they are perfectly capable of inflicting potentially lethal injuries on small organisms. Recent studies have associated the spines with pores that lead to possible toxin-producing glands.[4][15][16]

Their dactyli are powerful and curved into a

coxae) of the appendages.[7] The fifth to seventh pereopods function as clasping appendages. They all have propodi with two spines on their inside margins. The seventh pair of pereopods are the longest of the three pairs, followed by the sixth pereopod pair and the fifth pereopod pair.[13]

C. mutica closely resemble Caprella acanthogaster, also a native of East Asian waters. It may be difficult to distinguish the two species, particularly since Caprella mutica can exhibit considerable morphological variations among males. C. mutica can only be reliably differentiated by their setose first and second pereonites (smooth in C. acanthogaster), as well as the elongated oval shape of their gills (linear in C. acanthogaster).[14]

Taxonomy and nomenclature

Caprella mutica were first described in 1935 by A. Schurin from specimens collected from the

Caprellida.[2][18][20]

C. mutica are known as koshitoge-warekara ("spine-waist skeleton shrimp") in

junior synonym (invalid name) Caprella macho, applied in 1995 to the species by Dirk Platvoet et al. who initially believed they were a different species. "Macho" is a humorous reference to the characteristic "hairy chests" of the males of C. mutica.[13][21] Caprella acanthogaster humboldtiensis, another invalid name of the species, was first applied to misidentified specimens of C. mutica recovered from Humboldt Bay, California by Donald M. Martin in 1977.[22][23] Some specimens collected from the Firth of Clyde, Scotland in 1999 were also initially misidentified as Caprella tuberculata, but have since been determined to be introduced C. mutica.[1]

Ecology and biology

Caprella mutica inhabit shallow protected marine bodies of water.

omnivorous detritivores, but can adapt to other feeding methods depending on food availability. They are preyed upon by fish, crabs, and several other predators.[5][19]

C. mutica are generally found in

freshwater habitats.[1][24] However, in their native habitats, it has been observed that they can survive salinities as low as 11 psu.[25] They are also sensitive to exposure to air, and will die within an hour if taken out of the water.[24]

C. mutica reproduce all throughout the year, with peak seasons in the summer months. Males are highly aggressive and exhibit

sexual competition over the smaller females. The eggs, which average at 40 per female,[26][27] are incubated for about 5 days at 22 °C (72 °F) in the female's brood pouch. Upon hatching, they reach sexual maturity in about 21 to 46 days. Their average lifespan in laboratory conditions is 68.8 days for males and 82 days for females.[4][28][29]

Habitat

In their native habitat around Japan, Caprella mutica are typically found clinging to floating seaweeds like this Japanese wireweed (Sargassum muticum)[30]
A well-camouflaged individual of Caprella mutica collected from the Boston Harbor Marina in Boston Harbor, Washington

In their native habitat, Caprella mutica are found in the

infralittoral (or neritic) and littoral zones of sheltered bodies of water to a depth of about 0.7 to 13 m (2.3 to 42.7 ft).[4][19] They may spend their entire lives clinging to a substrate in an upright position.[19][30] These substrates are typically floating with filamentous, leafy, branching, or turf-like structures of the same color as their body for camouflage as well as transportation.[10][30] C. mutica are poor swimmers and move around predominantly in an undulating inchworm-like fashion, using their posterior pereopods and gnathopods.[1][21] They are generally reluctant to let go of their substrates and will only do so if agitated.[24] Different populations in different substrates are known to exhibit different exoskeletal coloration, suggesting that they can change color to blend in with their environments. The exact mechanisms for this color change, however, remains unknown.[4]

Substrates they are most commonly found on in their native habitats include beds and floating clumps of

Laminaria japonica, Chondrus spp., and Desmarestia viridis; as well as in marine plants (like eelgrass of the genus Zostera), hydrozoans, and bryozoans.[19][30][31]

In their introduced ranges, they also tend to seek out organisms that exhibit structures their slender bodies can easily blend with. These include macroalgae like

Monocorophium acherusicum and Jassa marmorata; and even soft-bodied tunicates like Ascidiella aspersa and Ciona intestinalis.[1][4][30][32][33]

In both their native and introduced ranges, C. mutica are also

ship hulls, and even offshore wind farms.[19][34][35] In their introduced ranges (particularly in Europe), they are primarily and even exclusively found inhabiting artificial structures.[19]

C. mutica can reach extremely high densities in their introduced range when colonizing artificial structures. A survey of C. mutica populations in Chaleur Bay, Quebec revealed concentrations of 468,800 individuals per 1 m2 (11 sq ft); while a survey in Dunstaffnage Bay, Firth of Lorn, Scotland reported 319,000 individuals per 1 m2 (11 sq ft). In contrast, C. mutica in their native habitats reach maximum densities of only around 1,220 to 2,600 individuals per 1 m2 (11 sq ft).[7][19][36] Populations reach peak numbers during the late summer (August to September) before experiencing a sharp decline in the winter months.[1]

Diet and predators

Caprella mutica are

cannibalistic behavior on dead and dying individuals of their own species or genus.[1][4]

Like other caprellids, C. mutica are preyed upon predominantly by

polyunsaturated fatty acids and carotenoids.[5][19] They also provide an important link in the food chain between plankton and larger fish.[19] This, in addition to their relative abundance and fast growth rates, make them a potentially important resource for marine fish feed in aquaculture.[37][38] Introduced populations of C. mutica have become a major part of the diets of native wild and farmed fish.[24]

Reproduction and life history

Wild populations of Caprella mutica show a higher number of females than males. This may be related to the fact that females are aggressively defended by males from competing males, resulting in high male mortality.[19] The larger sizes and greater visibility of males also make them more vulnerable targets for predators that rely on eyesight like fish.[19] C. mutica are r-strategists. They reproduce all throughout the year, with peak seasons in the summer (March to July).[1]

gnathopods of Caprella mutica males are used in aggressive "boxing matches" that can often be lethal.[28]

Males exhibit

courting behavior. They aggressively engage in "boxing matches" using their large second pair of gnathopods in the presence of receptive females. These encounters often have lethal results, as the gnathopods and their poison teeth can be used to impale or slice an opponent in half. Males will also repeatedly touch the exoskeletons of the females with their antennae to detect signs of moulting (ecdysis). Like all crustaceans, females are only capable of mating shortly after shedding their old hardened exoskeletons.[4][5][28]

Amplexus lasts for 10 to 15 minutes. Once mated, the males will defend the females for a short period (around 15 minutes). After this period, the females begin to exhibit aggressive behavior and will drive off the males. They will then bend their fourth and fifth pereonites at a 90-degree angle. Once their genital openings (located on the fifth pereonite) are aligned with the opening of the brood pouches, they quickly deposit fertilized eggs into them. Females carrying fertilized eggs remain highly aggressive towards males throughout the brooding period, indicating maternal behavior intended to protect the developing embryo from male aggression.[4][28]

A brood pouch of a female can contain 3 to 363 eggs, averaging at 74 eggs. Larger females tend to produce more eggs. The eggs are incubated inside the brood pouch for 30 to 40 days before hatching. Like all amphipods, caprellids lack a planktonic larval stage and the hatchlings resemble miniature adults. The juveniles may cling to their mothers upon hatching and the females continue to protect their offspring that remain close. Hatchlings measure around 1.3 to 1.8 mm (0.051 to 0.071 in) and grow to an average of 0.4 to 0.9 mm (0.016 to 0.035 in) per instar.[1][4][19][28]

C. mutica mature rapidly, moulting at an average of once each week until they enter the "premature stage", becoming sexually differentiated at the fifth instar. The durations between moulting cycles then become longer in their seventh to ninth instars, averaging at once every two weeks until sexual maturity. This can occur in as early as 21 days and not later than 46 days after hatching, depending on environmental conditions.[7][19][24] In wild populations, however, this can take as much as six months when the juveniles are hatched in the late summer.[19]

Males begin to increase in size at a faster rate with each successive moult after the seventh instar. Females, on the other hand, produce their first brood at the seventh instar. They may moult several times as adults, becoming sexually receptive each time until death. The average lifespan of C. mutica in laboratory conditions is 68.8 days for males and 82 days for females.[7][29]

Distribution and invasive ecology

Global distribution of Caprella mutica:
   Native distribution
   Potential ecological range
   Introduced populations
Location and date of first records in each area are also shown, along with likely introduction pathways indicated by arrows.[1]

Caprella mutica are native to the subarctic regions of the Sea of Japan in northwestern Asia. They were first discovered in the Peter the Great Gulf in the

Crassostrea gigas).[10][39]

Genetic studies of the mitochondrial DNA (mtDNA) of the populations of C. mutica reveal high genetic diversity in the Sea of Japan region, unequivocally identifying it as their native range. In contrast, non-native populations in North America, Europe, and New Zealand had poor variation. The detection of genetic material present in non-native populations, however, also make it probable that there are unknown regions that C. mutica may also be native to; though it might also be simply that the sample groups used in the studies were too small. Comparisons of mtDNA of the different populations make it possible to trace the possible routes of introduction. The most likely of which is that the original non-native introduction was to the west coast of North America, which exhibit the highest genetic diversity in non-native populations. Multiple later introductions happened in Europe and eastern North America. From here, additional populations were transported to nearby ports.[1][24][40] Europe and eastern North America are also the possible sources for the New Zealand C. mutica population.[17]

North America

The first specimens of C. mutica outside of its native range was recovered from

oyster spat of the Pacific oyster (Crassostrea gigas) from Japan for oyster farming. Oysters are usually transported with algae as a packing material, particularly Sargassum muticum in which C. mutica are associated with.[1]

Populations of C. mutica discovered in

shipping activities and intracoastal secondary spreading of the original populations.[43][44][45]

C. mutica were also discovered in

Juneau, Cordova, Kodiak, Kachemak Bay, Prince William Sound, and Unalaska in Alaska between 2000 and 2003. This was the first instance of a non-native marine species being found in the Aleutian Islands.[46] In 2009, they were discovered to have spread into British Columbia, Canada. This indicates that C. mutica have completely expanded up the entire west coast of North America.[47]

In 2003, surveys by the Massachusetts Institute of Technology (MIT) Sea Grant along the Atlantic coast of the United States revealed multiple established populations in seaports along the coastlines of Connecticut to Maine.[48][49] In the same year, C. mutica were also reported in Passamaquoddy Bay and Chaleur Bay of New Brunswick and Quebec, Canada.[1]

Europe

C. mutica populations in Europe were first found in the

Eastern Scheldt storm surge barrier in Burghsluis, Zeeland. As with the case of the first discovery of C. mutica in North America, Platvoet et al. initially misidentified them as a new species. Remarking upon the resemblance of the caprellids to C. acanthogaster, they named it Caprella macho.[13] They were later discovered to be introduced populations of C. mutica rather than a new species.[14]

Since then, additional populations have been detected in Belgium (1998), Norway (1999), Germany (2000), Scotland (2000), England (2003), Wales (2003), Ireland (2003), France (2004), and Denmark (2005).[11][50] They exist in extremely dense populations and are all associated with areas of high human activity. They are believed to have been introduced through shipping and aquaculture equipment from the United States and Asia. As of 2011, there have been no recorded sightings of C. mutica around the Iberian Peninsula, the Baltic Sea, or the Mediterranean Sea.[33]

New Zealand

Caprella mutica were first detected in New Zealand in the port of

sea chests of international shipping.[10][17]

Impact

The direct environmental and economic impacts of introduced C. mutica populations remain unknown.

Salmo salar), consume large amounts of non-native C. mutica.[24]

However, their larger sizes and very aggressive behavior also make them a serious threat to native species of skeleton shrimp.

Helgoland region of the German Bight in the North Sea, have revealed that C. linearis have more or less vanished and have been replaced by C. mutica.[54] C. mutica fouling populations may also incur minor economic effects through the cost of their removal from submerged aquaculture equipment and ship hulls.[24]

Control

There are no known effective control measures for invasive Caprella mutica populations as of 2012. It has been suggested that the seasonal population fluctuations may be taken advantage of. Eradication efforts done during the winter months when C. mutica populations are dormant and at their lowest numbers, are potentially more effective in preventing their recovery during the summer months. Because of the great difficulty in detecting and removing them, however, control methods will likely focus on preserving native species populations rather than the eradication of established C. mutica.[1][24]

See also

  • List of invasive species
  • List of the world's 100 worst invasive species

References

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External links

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