Caprella mutica, commonly known as the Japanese skeleton shrimp, is a species of skeleton shrimp. They are relatively large caprellids, reaching a maximum length of . They are sexually dimorphic, with the males usually being much larger than the females. They are characterized by their "hairy" first and second thoracic segments and the rows of spines on their bodies. Body color ranges from green to red to blue, depending on the environment. They are omnivorous highly adaptable opportunistic feeders. In turn, they provide a valuable food source for fish, crabs, and other larger predators. They are usually found in dense colonies attached to submerged man-made structures, floating seaweed, and other organisms.
C. mutica are native to shallow protected bodies of water in the Sea of Japan. In as little as 40 years, they have become an invasive species in the North Atlantic, North Pacific, and along the coasts of New Zealand. They are believed to have been accidentally introduced to these areas through the global maritime traffic and aquaculture. Outside of their native range, C. mutica are often exclusively 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 caprellid amphipods, Caprella mutica are characterized by slender bodies and elongated appendages. Their skeletal appearance gives rise to the common names of "skeleton shrimp" or "ghost shrimp", and, coupled with their distinctive upright feeding posture, give them a striking resemblance to stick insects and "starved praying mantises". C. mutica vary in coloration from translucent pale green, brown, cream, orange, deep red, purple, and even turquoise, depending on the substrate they are found in. The brood pouches of the females are speckled with red spots. A relatively large amphipod, C. mutica are sexually dimorphic with males considerably larger than females. Males average at a length of , though specimens have been recorded to reach in length. Females, on the other hand, average at only long.
The body can be divided into three parts – the cephalon (head), the pereon (thorax), and the abdomen. The pereon comprises most of the length of the body. It is divided into seven segments known as pereonites. The rounded and smooth cephalon is fused to the first pereonite; while the highly reduced and almost invisible abdomen is attached to the posterior of the seventh pereonite. In males the first two pereonites are elongated, with the second pereonite being the longest of all the pereonites. They are densely covered with long setae (bristles), giving them a hairy appearance. 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. The remaining pereonites (third to seventh) lack the dense setae of the first two pereonites. 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. 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.
Like other crustaceans, C. mutica possess two pairs of antennae, with the first (outer) pair more than half the total length of the body. The segments of the peduncles (base) are three times as long as the flagellae ("whips" at the ends of the antennae). The flagellae have 22 segments each. The second (inner) pair of antennae are less than half the length of the first. They possess two rows of long setae on the ventral surfaces of the segments of their peduncles. Mandibles and maxillae are present at the anterior ventral surface of the cephalon. Maxillipeds, a modified pair of appendages, also serve as accessory mouthparts.
The appendages that arise from pereonites are known as pereopods. The first two pairs of pereopods are highly modified raptorial grasping appendages known as gnathopods. They somewhat resemble the arms of praying mantises. The segments of the gnathopods can be divided into two parts which fold into each other: the propodus (plural: propodi, "forelimb") and, at the tip, the dactylus (plural: dactyli, "finger"). The first pair of gnathopods are considerably smaller than the second pair and arise close to the maxillipeds. The inside margins of the propodi possess two spines. Both the propodi and dactyli have serrated inner edges. The second pair of gnathopods are very large with two large spines on the middle and upper edges of the inside margin of the palm of the propodi. The upper spine is known as the "poison spine" or "poison tooth" and may be of the same size or much larger than the lower spine (the "closing spine"). Despite the name, it remains unclear if the poison spine is indeed 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.
Their dactyli are powerful and curved into a scimitar-like shape. The second pair of gnathopods are densely covered in hair-like setae while the first pair only has setae on the posterior margins. The third and fourth pereopods are absent. In their place are two pairs of elongated oval gills arising from the third and fourth pereonites, respectively. In mature females, two brood pouches also develop in the third and fourth pereonites. These are formed by oostegites – platelike expansions from the basal segments (coxae) of the appendages. 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.
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).
Taxonomy and nomenclature Caprella mutica were first described in 1935 by A. Schurin from specimens collected from the Peter the Great Gulf in the Sea of Japan. It belongs to the genus Caprella in the subfamily Caprellinae of the family Caprellidae, a group of highly specialized amphipods commonly known as skeleton shrimp. Caprellids are classified under the superfamily Caprelloidea of the infraorder Caprellida.
C. mutica are known as koshitoge-warekara ("spine-waist skeleton shrimp") in Japanese. In the Netherlands and Belgium, where the first invasive populations of C. mutica in Europe were discovered, they are known as machospookkreeftje in Flemish (literally "macho ghost shrimp"). The name is derived from the 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. 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. 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.
Ecology and biology Caprella mutica inhabit shallow protected marine bodies of water. They can often be found in dense colonies attached to submerged artificial structures, marine macroalgae, and other organisms. They are primarily omnivorous detritivores, but can adapt to other feeding methods depending on food availability. They are preyed upon by fish, crabs, and several other predators.
C. mutica are generally found in temperate and subarctic regions. They can not tolerate water temperatures higher than . They also die within five minutes if exposed to water temperatures of . On the lower end, they can survive temperatures lower than , but are rendered immobile if not altogether in a state of suspended animation. Salinity tolerance of C. mutica does not go below 15 psu, and they are unable to survive in freshwater habitats. However, in their native habitats, it has been observed that they can survive salinities as low as 11 psu. They are also sensitive to exposure to air, and will die within an hour if taken out of the water.
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,Ito A, Aoki MN, Yahata K, Wada H. Embryonic development and expression analysis of Distal-less in Caprella scaura (Crustacea, Amphipoda, Caprellidea). Biol Bull. 2011 Oct;221(2):206-14.Boos K (2009) Mechanisms of a successful immigration from north-east Asia: population dynamics, life history traits and interspecific interactions in the caprellid amphipod Caprella mutica Schurin 1935 (Crustacea, Amphipoda) in European coastal waters. Ph.D thesis, Freie Universität Berlin Breton G, Faasse M, are incubated for about 5 days at 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.
Habitat
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 . They may spend their entire lives clinging to a substrate in an upright position. 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. C. mutica are poor swimmers and move around predominantly in an undulating inchworm-like fashion, using their posterior pereopods and gnathopods. They are generally reluctant to let go of their substrates and will only do so if agitated. 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.
Substrates they are most commonly found on in their native habitats include beds and floating clumps of macroalgae like Sargassum muticum, Sargassum miyabei, Sargassum pallidum, Neorhodomela larix, Polysiphonia morrowii, Cystoseira crassipes, Laminaria japonica, Chondrus spp., and Desmarestia viridis; as well as in marine plants (like eelgrass of the genus Zostera), hydrozoans, and bryozoans.
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 Ulva lactuca, Ceramium spp., Plocamium spp., Cladophora spp., Chorda filum, Fucus vesiculosus, Pylaiella spp. and the introduced Sargassum muticum; hydrozoans like Obelia spp. and Tubularia indivisa; bryozoans; tube-building amphipods like Monocorophium acherusicum and Jassa marmorata; and even soft-bodied tunicates like Ascidiella aspersa and Ciona intestinalis.