An Introduced Sabellid Polychaete Pest Infesting Cultured Abalones and Its Potential Spread to Other California Gastropods

Abstract

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. Additional key words: molluscan mantle, shell deposition, fouling, South Africa In October 1993, growers at some commercial abalone mariculture facilities in southern California brought an interesting sabellid polychaete to our attention. The worm infested shells of cultured red abalones, Haliotis rufescens. Heavily infested abalones ceased growth. Their shells exhibited an abnormal, beaked shape and often lacked respiratory holes. The worms appeared to bore in the shell. Because this worm had not previously been reported from California and only a few species of shell-boring sabellid in the genus Caobangia were known (Jones 1969), it was of obvious systematic and biological interest. It has been described as a new genus and species, Terebrasabella heterouncinata FITZHUGH & ROUSE 1999. It is evidently native to at least South Africa, as recognized by Ruck & Cook (1998). Several types of polychaetes infest mollusc shells. a Author for correspondence. E-mail: kuris@lifesci.ucsb.edu Spirorbids are sometimes abundant epibionts, often settling on the outer surfaces of molluscan shells (Gee & Knight-Jones 1962; Zuschin & Piller 1997). They also are sometimes very abundant on the shells of cultured abalones in California. Some serpulids are also epibionts. Spionid polychaetes, particularly species of Polydora and Boccardia, known as mudworms, infest molluscan shells. They settle on the outer surface and bore into the shell, often forming a U-shaped or flasklike burrow. The mechanisms by which spionids bore have received considerable attention (Dorsett 1961; Haigler 1969; Zottoli & Carriker 1974; Sato-Okoshi & Okoshi 1993), and these worms can cause damage to both cultured and wild populations of molluscs by weakening shells and increasing their susceptibility to predation, or by loss of energy expended in shell repair (Kent 1979; Bergman et al. 1982; Kojima & Imajima 1982; Mori et al. 1985; Sato-Okoshi and Nomura 1990; Ambariyanto & Seed 1991; Wargo & Ford This content downloaded from 207.46.13.129 on Sun, 26 Jun 2016 07:01:20 UTC All use subject to http://about.jstor.org/terms