The scaleworm Halosydna brevisetosa
is mostly free-living, although it is occasionally found with other invertebrates, such as within the tubes of tubeworms. The elytra in this species are diagnostic, that is, they are 18 in number, and always 18. The elytra are modified scales and may function in protection, or in providing a space within which seawater moves for gas exchange. Seawater is apparently driven through the space between the elytra and the dorsal skin by beating of cilia on the skin. There is an opinion that the fluid within the large body spaces of the worm moves in a counter-current direction to the seawater flow, thus creating optimal exchange of respiratory gases (oxygen and carbon dioxide). Halosydna
has large jaws and is a predator on other bottom-dwelling invertebrates such as small crustaceans and worms. More on elytra function in Halosydna
can be found in A SNAIL'S ODYSSEY: LEARN ABOUT SANDWORMS & RELATIVES: PHYSIOLOGICAL ECOLOGY
Note Author: Tom Carefoot, Department of Zoology, University of British Columbia.