Sacoglossans (commonly called “Sap Sucking Slugs”) are almost all herbivores that live in association with green algae of the order Siphonales, particularly those of the genus Caulerpa.
All sacoglossans have a single row of radula teeth that are dagger-like and which are used to pierce the plant cell wall so that the cell fluid can be sucked out and swallowed. The chloroplasts from the algae may be sequestered and “farmed” within the cells of the sacoglossans digestive gland. This ability has caused them to also be termed “solar powered slugs”. A small number of sacoglossan species which lack a functional radula feed upon the spawn of other sea slugs. The name of the order, Sacoglossa, is derived from another common feature that sacoglossans possess, a uniseriate radula, from which the older, worn and discarded teeth are stored in a special sac.
The more primitive members have an external shell into which the animal can completely withdraw e.g. the Volvatellidae. The Juliidae, remarkably, have a bivalved shell into which the animal can completely withdraw. Some have a reduced shell which covers the viscera only e.g. the Oxynoidae.
The majority of others completely lose the shell when the larvae metamorphose into the adult. In the order Sacoglossa, the shell is never wholly internalised but as the shell becomes progressively reduced the body form becomes more elaborate. A variety of body forms are represented across the order – from limaciform, to aeolidiform, or even leaf-like.