The superorder Peracarida is very successful, having members in marine, freshwater, and even terrestrial habitats. They have been found in hot springs living at temperatures up to 40 degrees C, and in the deepest parts of the ocean. The common pill bug, the isopod Amradillidium vulgaris, has been transported all around the world in the soil of house plants, while it's deepsea relative, Bathynomus gigas is the largest peracarid, growing to a length of 44cm. The group can be recognized by a several shared characters. In females, flaps (endites) termed oostegites, originating off of the first segment (coxa) of the thoracic appendages, form a brood pouch (marsupium)(except in Thermosbaenaceans, which brood in the carapace). Development is direct in that the young hatch out a manca stage, which resembles the adult but lacks the last pair of thoracic appendages. The carapace is generally reduced, and if developed, is not fused to the posterior thoracic segments. Of the nine orders, only five have been encountered in this study.
Cumaceans are odd looking and little seen crustaceans. There are less than 900 species, generally at size range of 0.5-2cm long. The have a bulbous head and thorax mostly covered by a carapace, followed by a long thin abdomen. The body is often crooked, and resembles comma. Cumaceans live in soft sediments, and males swim up into the water at night to seek females. These males are common in shallow water light trap samples from rubble and mangroves in the BVI.
The mysids or opossum shrimp, are easily confused with decapod shrimps, and euphausiaceans (krill). Unlike the latter two groups, the carapace of a mysid is never fused to more than the first four thoracic segments, eggs may be seen in the marsupium, and a statocyst (organ for orienting to gravity) can usually be seen on either side of the telson (in each uropodal endite). Mysids are present in the plankton and on the bottom. There are nearly 700 described species at present. in the BVI, the most readily seen mysids are those of the genus Heteromysis which are associated with corals or anemones. These are sometimes seen swarming near their host.
In general, tanaids resemble thin amphipods with large chelipeds. Unlike amphipods, tanaids have a carapace, which is fused to the first two thoracic segments. The chelipeds arise from the second thoracic segment, and are much larger in males. Tanaids are benthic and are found at all depths, they range into brackish waters and often live in burrows, or tubes attached to solid surfaces. Most of the 900 or so species are much less than 2 cm in length.
Isopods are closely related to amphipods, both lack a carapace, have the first thoracic segment fused to the head, and bear expanded lateral side plates off of the coxa of the thoracic appendages. Isopods are generally easily distinguished by their wide depressed body and flattened abdominal appendages (pleopods) used as gills. There are over 4000 species, ranging is size from 0.5 mm to the 440 mm of the giant Bathynomus. They are common in freshwater, marine, and terrestrial habitats, and some are parasites in the branchial cavity of decapods.
Amphipods are common in marine and fresh waters, and a few can even be found in moist terrestrial environments. For the most part the most common species are laterally compressed (except for the stick-like caprellids, the skeleton shrimp), thus readily differentiated from isopods. They reange in size from about 1mm to deepsea giants of 250 mm. Most of the 6000 or so species are less than 10 mm. Together with polychaete worms amphipods are usually the most abundant organisms in benthic soft bottom samples, their hydrophobic integumenst often causing them to get caught in the surface tension of the sorting dish.