Sap-sucking slugsSacoglossa

Sacoglossa are small marine slugs that feed by piercing an algal cell with a radula reduced to a single tooth and sucking out the contents - the habit behind both the English name and the Latin one, which means "sac tongue" and refers to the pouch where used teeth are stored. Most are 1-5 cm (0.4-2 in), a few reach 10 cm (4 in), and body form varies far more than in other slugs: broad lateral flaps in Elysia and Plakobranchus, leaf-shaped cerata in Cyerce and Polybranchia, a coiled shell in Oxynoe, and in Julia a genuine two-valved shell that makes the animal look like a small clam.

Many species keep the chloroplasts of the algae they drain alive inside their own tissues, where the borrowed machinery goes on photosynthesising for days or weeks. It is the only case in the animal kingdom of stolen organelles running at that scale, and it lets a well-fed slug survive long spells without eating. Green colouring in the group is usually the algae rather than the animal. Almost every species is tied to one or two kinds of green alga, so finding them means searching the weed - Halimeda, Caulerpa, Bryopsis - rather than scanning the reef.