Biology and relationships of the South African sacoglossan mollusk Volvatella laguncula



The oxynoacean Volvatella and the less well known Ascobulla and Cylindrobulla are the most primitive living members of the gastropod order Sacoglossa. Volvatella is a world-wide genus characterized by a capacious shell into which complete retraction is possible, a posterior exhalant spout-like pallial siphon, and a sacoglossan radula.

At Strandfontein near the Cape of Good Hope, large numbers of these small molluscs were found living among and feeding upon the roots of the alga Caulerpa filiformis, which forms sublittoral meadows on the coasts of southern Africa. They were identified as Volvatella laguncula Sowerby, 1894, originally described (from the shell alone) in material from Port Elizabeth. The mollusc lives and feeds in the muddy sand around Caulerpa roots at a depth of 5 cm or more below the surface. The captured material proved to be hermaphrodite, sexually mature and to have mated, but no spawn was found. An interesting feature of the reproductive organs was an epidermal ciliated gutter extending along the right flank from the female opening past the male opening on to the side of the head; this is interpreted as an oviducal groove conducting the egg-stream during oviposition (as has been suggested for the Japanese juliaceans). A similar groove was found on the left side but its function is unknown.

The mantle cavity contained a plicate gill; irrigation was brought about by adduction movements which rhythmically deformed the outer lip of the shell. A strong transverse muscle was present and was responsible for these movements. It is evidently homologous with the adductor muscle of the bivalved juliaceans. It is likely that what is pumped through the mantle cavity of Volvatella is an ooze of fine mud.

Volvatella, together with Cylindrobulla and Ascobulla, constitutes a link between the infaunal proto-opisthobranchs and the present-day epifaunal herbivorous Sacoglossa.

An annotated list of the world's species of Volvatella concludes the paper.