decapoden

Decapoden, or decapod crustaceans, is an order of the class Malacostraca within the subphylum Crustacea. They comprise a diverse group of about 15,000 described species including crabs, lobsters, crayfish, and shrimp. Members occupy a wide range of habitats, from the deep sea to freshwater rivers and, in some cases, terrestrial environments.

Anatomy and characteristics: Decapods are defined by ten legs (the name derives from deca, ten, and poda, feet). They typically possess a cephalothorax covered by a carapace, two pairs of antennae, and five pairs of walking legs, the first pair often modified into claws (chelae). The abdomen is segmented and ends in a tail fan formed by the telson and uropods. Most have gills and undergo molting as they grow. The first thoracic appendages and overall body plan are highly variable, reflecting adaptation to diverse lifestyles.

Classification: Within Decapoda, living groups are divided into two major lineages: Dendrobranchiata (prawns) and Pleocyemata (which includes true shrimps, crabs, lobsters, and crayfish). Pleocyemata is further subdivided into Caridea (shrimp), Astacidea (crayfish and lobsters), Anomura (hermit crabs and kin), and Achelata (spiny and slipper lobsters).

Reproduction and development: Females typically brood eggs attached to the swimming legs until hatching. Developmental modes vary; many decapods have complex larval stages such as nauplius, zoea, and mysis, though some groups show direct development.

Ecology and economic importance: Decapods play key roles as scavengers, detritivores, and predators in many ecosystems. They are of considerable commercial importance in fisheries and aquaculture, but face threats from overfishing, bycatch, habitat loss, and climate change.