Abyssal sea anemones (Cnidaria: Actiniaria) of the northeast Pacific symbiotic with molluscs: *Anthosactis nomados*, a new species, and *Monactis vestita* (Gravier, 1918)

Abstract

We describe *Anthosactis nomados*, new species, which belongs to family Actinostolidae, and redescribe *Monactis vestita* (Gravier, 1918), a species belonging to family Hormathiidae. Anemones of both species live attached to molluscs on the soft-sediment abyssal plain of the northeastern Pacific Ocean. *Anthosactis nomados* is known from 260 specimens, most of which were still attached to scaphopod shells when studied, and the others of which show evidence of having been attached to one. By altering the definition of genus *Anthosactis* to include all tentacles being of equal length, this species can be accommodated in it. The species also differs from the other six species previously ascribed to the genus in the size of its nematocysts. We examined about 600 specimens of *Monactis vestita*, more than 350 of which show evidence of having been attached to a gastropod shell but none of which was still associated with the host when studied. This is the first report of *Monactis vestita* in such a symbiosis and from the Pacific Ocean. We hypothesize that both anemone-mollusc symbioses are mutualistic. In that between *Anthosactis nomados* and the scaphopod, which is the third anemone-scaphopod symbiosis to be documented, the protection provided by the sea anemone may allow the scaphopod to avoid the corrosive environment of sub-surface sediments and to forage at the surface where it can obtain energy and calcium carbonate. For both symbioses, the mollusc may resuspend sediments and carry the actinian to food-rich areas, and the anemone may protect its host from predators.