Leaf Sheep Nudibranch

When the leaf sheep feeds on its host alga, it pierces the algal cells and sucks out their contents, digesting most of the material for nutrition but selectively retaining the chloroplasts, the tiny green structures that carry out photosynthesis, and storing them intact within the cells that line the branching digestive gland running through its body. These captured chloroplasts, a phenomenon known as kleptoplasty or chloroplast theft, keep functioning for days to weeks inside the slug, continuing to convert sunlight into sugars that the leaf sheep can use as an energy source, effectively allowing the animal to supplement or replace eating for a time simply by resting in the light.

It belongs to a group of sea slugs called sacoglossans, several of which have independently evolved the ability to retain functional chloroplasts, and within this group some species, including the leaf sheep, are able to maintain the borrowed chloroplasts in working condition for a strikingly long time compared to most animals that ingest chloroplasts and lose them within hours. The exact mechanisms are still being studied, but they appear to involve the slug protecting the chloroplasts from damage and, in some cases, the chloroplasts themselves being unusually robust, though the chloroplasts cannot reproduce inside the slug and are eventually used up, requiring the slug to keep eating fresh algae to replenish them.

A typical leaf sheep is only around 5 millimetres long, roughly the size of a grain of rice, and even large individuals rarely exceed a centimetre, so it is easy to overlook entirely. Its body is covered in rows of finger-like projections called cerata, which in this species are broad, flattened, and leaf-shaped, tipped with dark and pink pigment, and which contain the branches of the digestive gland where the stolen chloroplasts are housed. These leaf-like projections both increase the surface area exposed to sunlight for photosynthesis and, together with the slug's green colour, camouflage it among the algae it lives on, while the two rounded rhinophores on its head give it its endearing, sheep-like face.

It is often called a nudibranch in casual usage, but it is technically a sacoglossan sea slug rather than a true nudibranch, as the two are separate groups within the broader category of sea slugs. Nudibranchs are typically predators or scavengers with a very different feeding biology, whereas sacoglossans like the leaf sheep are specialised herbivores adapted for piercing and sucking the contents out of algal cells, and it is this sap-sucking feeding style that made the evolution of chloroplast retention possible in the group. So while the leaf sheep shares the general soft-bodied, shell-less, marine snail body plan people associate with nudibranchs, it sits on a different branch of the sea slug family tree.