An introduced sabellid polychaete pest infesting cultured abalones and its potential spread to other California gastropods

In 1993 an unusual sabellid polychaete was brought to our attention. It grossly deformed the shells of cultured abalones in some of the California abalone mariculture facilities. This worm is not native to California and was subsequently found in rocky intertidal and subtidal habitats in southern Africa, where it had not previously been recognized. The worm is hermaphroditic and has benthic larvae that are competent to settle within 12 h and soon secrete a mucous sheath. Development of the tentacular crown occurs within a week and generation time can be short, about one month. The worm has a unique association with host gastropods. Unlike all other known shell-fouling organisms, the sabellid routinely settles inside the aperture at the growing edge of the shell. The host responds by secreting a layer of nacre over the mucous sheath to form a tube enclosing the worm, whose crown of tentacles extends through the opening of the tube to the outer surface of the shell. Heavy infestations cause the cessation of linear growth of the host as prismatic shell deposition cannot be resumed after repeated settlement of larvae. The sabellid is not very host specific; many other California native gastropods are readily infested. Bivalves do not appear to be susceptible. Efforts to find a native California predator of the adult worms were not successful. The sabellid has caused great economic damage to some facilities commercially culturing abalones. An established population of this worm has been detected in California, and further risk of establishment and spread of this worm is great. Its unique biology:suggests that it may be a useful experimental probe for studies of molluscan shell deposition and may also serve to reveal how molluscs defend themselves against organisms attempting to settle in and foul their apertures.