Diseases of Echinodermata. IV. Structural abnormalities and general considerations on biotic diseases

Reported structural abnormalities mostly concern the echinoid test; these are not uncommon and indicate great plasticity of the echinoids. More than 400 animal agents have been reported from echinoderms. Most external agents either have processes that penetrate the body wall or live in cysts or galls on or in the body wall; intradigestive agents usually occur free in the digestive cavity; internal agents are mostly intracoelomic and live either free in the coelomic cavity or attached to the coelomic wall. Host reactions to invading organisms are either inflammatory-like reactions (i.e. intratissular migration of some coelomocytes towards the site of infection), connective-tissue reactions (i e. formation of a thick fibrous sheet isolating the agent from the host's connective tissue) or coelomocyte reactions (i.e. endocytosis of microorganisms or walling off of anlmal agents entering the host's coelomic cavity). All known mass mortalities due to biobc disease agents are caused by microorganisms. With very few exceptions, animal agents do not kill their ech~noderm host: most of them are well tolerated and do not appear to result in major consequences for the hosts concerned. Echinozoa (Holothuroidea and Echinoidea) and Asterozoa (Asteroidea and Ophiuroidea) exhibit a different degree of sensitivity to animal pathogens, the asterozoans' defensive mechanisms being apparently more efficient than those of echinozoans. Echinoderms play a key role in many benthic communities and their diseases especially lethal or castrating deseases should have major effects on the biological environment (this has been demonstrated in a few cases of disease-related mass rnortalities). Echinoids and ophiuroids represent a prominent part of the diet of many fishes; their role as vector of fish diseases caused by trematodes or nematodes requires investigation.