‘Sea slug’ has become something of a catch-all term for a number of different species of marine animals, though probably the most familiar is a kind of gastropod called the nudibranchs. The reason these snails are ‘nude’, so to speak, is because they lose their shells at the larval stage just a few days after they hatch. The loss of this defence has meant nudibranchs have had to evolve some ingenious ways to survive.
As well as acquiring a psychedelic range of patterns and colours to deter predators, many also gain a secret weapon from their food. By eating prey like sea anenomes, which naturally contain stinging cells called nematocysts, the sea slug can repurpose them for itself, storing them in fleshy appendages on its back called cerata.
Sea slugs typically lay their eggs and live on hydroids, which themselves are one life stage of a predatory colonial invertebrate related to jellyfish. On hatching, a tiny snail (just a few hundred micrometres long) emerges called a veliger. Via swimming and crawling, it seeks its own hydroid to settle on. It only resembles a snail for a few days. After that, it loses its larval shell in a process called detorsion, where the shell, mantle and several major organs twist around the body.
After a week, it develops the means to eat. The radula is an organ covered in thousands of tiny razor-sharp teeth, or denticles, for gnawing into the hydroid. Finally sensory organs called rhinophore buds emerge on the creature’s head, which detect chemicals. These can be retracted if it is attacked.