Naked indeed, and clearly exposed as such. How do they manage to survive in a brutal world full of hungry predators?

Chemical defences
Without a protective shell, nudibranchs, and other sea slugs, had to develop a number of other defensive mechanisms against predation. These include cryptic colouration, or camouflage, and behavioural modifications, such as only being active at night. But probably the most significant development has been the use of chemicals to make them posionous, or at least extremely distasteful, to potential predators. In fact, Opisthobranchs (the subclass that nudibranchs belong to) have become subjects for research by marine products chemists who are gradually uncovering just how widespread and in what complex ways chemicals are used by sea slugs. Many store these chemicals in special glands in their skin.
 
What has caught the attention of chemists and physiologists are the many different pathways the molecules are produced. In some cases, these compounds are absorbed from ingested prey and stored unaltered by the opisthobranch, and in other cases, the compounds undergo some modifications. They may also be produced entirely by the opisthobranch itself.

All different
Usually, each species has very specialised food requirements, and often its defensive molecules are also unique and differ from even closely related species.
 
In particular, terpenoid compounds, which are derived from sponges, are concentrated around the mantle border and in the mucous secretions of the mantle. At their natural concentrations these chemicals deter crabs and reef fishes from preying on the slug.

Why all the colours?
Many animals, which are very distasteful or poisonous to eat, have bright colour patterns. The bright colours are considered to be a message to potential predators warning them to stay away. We call such warning colouration Aposematic colouration.
 
In one nudibranch family, the Chromodorididae, the colour patterns of many species are spectacular and obvious. Research in recent years has shown that these animals have specialised glands in their mantle that contain poisonous and distasteful chemicals from their sponge food. It is thought that by linking bright colour to bad tastes, these nudibranchs can teach fish and other potential predators to leave them alone.
 
In a development of this, we often find geographic areas where groups of unrelated chromodorids have evolved very similar colour patterns, so that they share the load of teaching fish to leave the colour pattern alone. One example of this mimicry in southeastern Australia are a group of about ten red spotted species, some of which are very difficult to tell apart. Most chromodorids have these mantle glands. ■