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 50 mm (2.0 in). 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.
Like all caprellidamphipods, 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 25 to 30 mm (0.98 to 1.18 in), though specimens have been recorded to reach 50 mm (2.0 in) in length. Females, on the other hand, average at only 15 to 20 mm (0.59 to 0.79 in) long.