Ovulid snail

The Ovulidae are the allied cowries: shells shaped broadly like a cowrie, glossy, often with drawn-out ends, and in life almost entirely hidden under a mantle that the snail folds up over the shell from both sides. The mantle is the animal as a diver sees it, and it matches the host — red on a red gorgonian, pink and translucent on Dendronephthya, white and papillate on a white sea whip — with the papillae repeating the shape of the host’s polyps. Common Indo-Pacific genera include Phenacovolva, Primovula, Crenavolva and Dentiovula, and the largest of the group in this region is the shuttlecock volva Volva volva, whose shell is drawn out into long spines at both ends. Identification stops at the family or the genus in almost every case: the characters that separate species are on the shell, the shell is covered by the mantle in a living animal, and the mantle pattern is a poor guide because it tracks the host. This entry is pitched accordingly. The egg cowrie Ovula ovum belongs here too and is large and plain enough to name.

They are obligate octocoral feeders. A snail rasps the polyps and tissue of the gorgonian, sea whip or soft coral it sits on, and it does not leave; the eggs are laid in flat capsules on the same colony, usually on a patch the animal has already grazed bare, and the female often remains over them. The practical consequence is that the search is for the host and then for the damage rather than for the snail. Bare pale sections on an otherwise intact sea fan, a stripped length of sea whip, or a lump on a branch that is out of line with the polyps around it will each usually resolve into an ovulid, and a fan with one snail on it frequently has several.