Astacidea

Astacidea is a clade within the suborder Pleocyemata of the order Decapoda. It comprises freshwater crayfish and their closest living relatives. The group is divided into two living lineages, the superfamilies Astacoidea and Parastacoidea, which together include the majority of extant crayfish species. Members of Astacoidea are distributed mainly in the Northern Hemisphere and include the familiar crayfish of North America and Europe, while Parastacoidea dominates in the Southern Hemisphere, including Australia, New Guinea, and parts of South America. The two lineages differ in a variety of morphological and developmental traits, but both show the characteristic crustacean pattern of a cephalothorax covered by a carapace, five pairs of walking legs, and a prominent tail fan.

Ecology and life history: Astacideans inhabit freshwater systems such as rivers, streams, ponds, and lakes, with some tolerance for brackish environments in a few species. They are generally omnivorous, feeding on detritus, plants, invertebrates, and carrion. Reproduction involves females carrying fertilized eggs on the underside of the abdomen attached to swimmerets; the young hatch as free-swimming juveniles and go through several molts before reaching adulthood.

Evolution and conservation: The group has a rich fossil record extending back to the Paleozoic. Today, crayfish face threats from habitat loss, pollution, climate change, and the introduction of non-native species, with conservation programs in place for several native species and regions.