Halosydna brevisetosa   Kinberg, 1855

Halosydna brevisetosa is a thick, robust scaleworm with 18 pairs of elytra (scales) along its mottled brown dorsum and thick spiny setae (bristles) projecting laterally from underneath the scales. This native species is commonly found in rocky intertidal and subtidal environments, in fouling communities and algal holdfasts, and can also be commensal in mollusk shells and the tubes of large polychaetes. It ranges from Alaska to Baja California, and prefers clean water of relatively high salinity and dissolved oxygen. The elytra of scaleworms help to direct the flow of oxygenated water along the body surface when the worm is wedged into spaces where oxygen is limited (adapted from Morris et al 1980).

Halosydna are scavengers and detritus feeders. Their feeding apparatus consists of an eversible proboscis armed with powerful jaws at its inverted tip.

Eggs of a gravid (egg-bearing) female H. brevisetosa are held within the body cavity and are released for external fertilization.