Ascothoracida

Ascothoracida is a small, enigmatic group of marine crustaceans that constitutes a subclass within Thecostraca, a major lineage of Crustacea. They are obligate parasites or commensals, living on echinoderms—especially echinoids and asteroids—and, less commonly, on cnidarians or sponges. They are found in oceans worldwide, from shallow reefs to deep-sea habitats.

Morphology and lifestyle: Ascothoracids are typically minute, most a few millimeters in length, with a compact body protected by a shield-like covering. The body is divided into a cephalic region and a trunk bearing thoracic appendages; the abdomen is reduced or absent. Males are often much smaller than females. The lifestyle is largely cryptic, with adults living attached to their host.

Life history: The life cycle includes free-swimming larval stages characteristic of crustaceans, followed by permanent settlement on a suitable host. Reproduction is sexual, with eggs released as developing larvae in many species and, in others, brooded by the female.

Ecology and evolution: Ascothoracida play a role as parasites in marine ecosystems and are used in studies of host–parasite interactions and crustacean phylogeny. Phylogenetically, they are placed within Thecostraca, and molecular analyses have helped clarify relationships to other crustaceans such as barnacles.