malacostracans

Malacostraca is the largest class of crustaceans, comprising the majority of living species, including crabs, lobsters, crayfish, shrimps, prawns, krill, isopods, amphipods, and mantis shrimps. With tens of thousands of described species, malacostracans occupy a wide range of marine, freshwater, and terrestrial habitats.

Typical malacostracans have a body divided into a head (cephalon) and a trunk. The trunk usually consists of eight thoracic segments and six abdominal segments, with a telson and uropods forming a tail fan in many species. The head bears two pairs of antennae, and most appendages are biramous, meaning they branch into two rami. In several groups a protective carapace covers part of the thorax. The group exhibits considerable variation in form, from flattened isopods to elongated shrimp-like decapods.

Ecologically, malacostracans are ubiquitous in aquatic systems and in some terrestrial environments. Krill and other pelagic crustaceans are key prey for many larger animals; decapods include top predators as well as scavengers; isopods and amphipods contribute to detrital food webs and nutrient cycling.

Most species are dioecious, with internal or external fertilization in many groups. Larval development ranges from free-swimming nauplius or zoea stages in some decapods and other orders to direct development in others.

The name Malacostraca derives from Greek malakos, soft, and ostrakon, shell. The class is diverse and includes economically important groups such as lobsters, crabs, shrimp, and krill, as well as ecologically significant isopods and amphipods.