Reproductive periodicity and host-specific settlement and growth of a deep-water symbiotic sea anemone

Abstract

Sea anemones Allantactis parasitica Danielssen, 1890 (Actiniaria: Hormathiidae) living as epibionts on various gastropods were found at depths of ~725–1100 m off Labrador (eastern Canada). Live specimens collected with their hosts were studied in the laboratory to elucidate the role of the gastropod host in the reproductive and colonizing success of its actinian symbiont. Broadcast spawning occurred twice a year, in spring and late fall, in correlation with maximum phytoplankton or phytodetritus abundance, often during copulation of the gastropod hosts. The fully developed planula stage was reached after ~22 d. Settlement on the host’s shell generally occurred 40–44 d postfertilization, although some larvae delayed settlement for up to 22 weeks in the absence of a host. Independent and pairwise settlement trials showed that shells of live bathyal gastropods were highly favoured compared with shells of shallow-water gastropods, empty shells, pebbles, or sand. Juveniles developed 24 tentacles and reached ~10 mm in basal disk diameter and ~12 mm in stalk height after 21 months of growth. The estimated growth rates of A. parasitica suggest that symbiotic individuals would require 6–7 years to reach the mean maximum adult size (~35 mm, basal disk diameter), whereas asymbiotic individuals would need 11–12 years.