Nudibranchs, although often given the unglamorous name 'sea slug', are amongst the most beautiful of sea creatures. (Incidentally, not all sea slugs are nudibranchs.) There are 3000 species worldwide with 350 species known from the Great Barrier Reef. The average size is about 50mm.
The name nudibranch, meaning 'naked-gill', is derived from the plume-like external gills on the backs of most species. These molluscs have done away with shells, which leaves us to wonder how, with their marvellously bright and colourful patterns they manage to protect themselves. Methods vary. Some are quite well camouflaged in the colourful surroundings of the reef.
Many are nocturnal and some are able to swim off vigorously when attacked. But the bright colours of many are designed to tell potential predators that they taste terrible. Some have toxic glands just below the surface of the mantle.
One group is particularly clever. These are distinguished by long projections on their back called cerata. They feed on corals and sea anemones, managing to ingest the stinging cells (nematocysts) from these animals and pass them, unfired, through their bodies to their cerata where they are used for defence. When threatened some nudibranchs shed their cerata which stick to the predator firing nematocysts as the nudibranch makes its escape. Although the cerata are used for respiration and digestion, the nudibranch survives this loss and simply grows them back again. One species removes algal cells (zooxanthellae) from soft corals, giving them a new home in the enlarged and flattened cerata on its back. Here the zooxanthellae continue to produce food from sunlight, as they did in the coral, giving the nudibranch a free feed.