Peracarids

Peracarids are a diverse and widespread infraclass of crustaceans belonging to the class Malacostraca. This group is characterized by a unique brood pouch, the marsupium, formed by modified thoracic appendages called oostegites, which is located on the ventral side of the female's thorax. Peracarids are found in nearly all aquatic environments, from freshwater and marine habitats to terrestrial settings. They exhibit a vast array of lifestyles, including detritivores, scavengers, predators, and even parasites. Common examples of peracarids include amphipods, isopods, and mysids. Amphipods are typically laterally compressed with distinct body segments and often have two pairs of antennae, while isopods are dorsoventrally flattened and possess a more uniform body shape. Mysids, also known as opossum shrimp, are small, shrimp-like crustaceans often found in large swarms in marine and brackish waters. The development of peracarids is direct, meaning they hatch as miniature versions of the adult and do not undergo a planktonic larval stage, a significant difference from many other malacostracan groups. This direct development and the protective marsupium are considered key adaptations contributing to their evolutionary success and ecological dominance in many environments.