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.

[5] 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.

[14] 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.

[17][18][19] 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.

[7] 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").

[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.

[4][28][29] 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).

[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.

These include macroalgae like Ulva lactuca, Ceramium spp., Plocamium spp., Cladophora spp., Chorda filum, Fucus vesiculosus, Pylaiella 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.

[1][4][30][32][33] In both their native and introduced ranges, C. mutica are also synanthropic, being found abundantly in fouling communities in artificial structures like submerged ropes, fishing nets, pilings, docks, buoys, aquaculture equipment, oil rig platforms, ship hulls, and even offshore wind farms.

[7][19][36] Populations reach peak numbers during the late summer (August to September) before experiencing a sharp decline in the winter months.

They are predominantly detritivores,[32] but have the remarkable ability of adjusting feeding methods from being grazers, scavengers, filter feeders, and even predators depending on the conditions of their environments.

[1][19] C. mutica sieve food particles or small organisms from the water by waving their bodies back and forth, with the comb-like setae on their second pair of antennae extended.

[4][21] Known prey organisms of C. mutica include algae (both planktonic and macroalgae), dinoflagellates, hydrozoans, bryozoans, diatoms, copepods, brine shrimps, and other amphipods.

[5] In their native habitats, the predators of Caprella mutica include the shore crab Carcinus maenas and the goldsinny wrasse (Ctenolabrus rupestris) which consume them in large numbers.

They constitute a valuable food source for these organisms due to their high levels of polyunsaturated fatty acids and carotenoids.

[19] This, in addition to their relative abundance and fast growth rates, make them a potentially important resource for marine fish feed in aquaculture.

[19] The larger sizes and greater visibility of males also make them more vulnerable targets for predators that rely on eyesight like fish.

[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.

They were redescribed by the Japanese marine biologist Ishitaro Arimoto in 1976 who noted that they were also present in the island of Hokkaido and surrounding regions.

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.

[43][44][45] C. mutica were also discovered in Ketchikan, Sitka, Juneau, Cordova, Kodiak, Kachemak Bay, Prince William Sound, and Unalaska in Alaska between 2000 and 2003.

[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.

During a species inventory, several specimens of an unknown caprellid were recovered by Platvoet et al. from artificial structures in and around the Neeltje-Jans and the Eastern Scheldt storm surge barrier in Burghsluis, Zeeland.

[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).

[54] C. mutica fouling populations may also incur minor economic effects through the cost of their removal from submerged aquaculture equipment and ship hulls.

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.