Mucus as a means of defense

The shell is a very important defensive structure for snails. Slugs (essentially snails which have lost their shell) have had to evolve ways to protect their soft, tempting flesh from fish, crustacea and other hungry animals that might eat them. One way of doing this is to have glands in the skin which secrete noxious, distasteful, and in some cases poisonous chemicals. Even the most primitive 'slugs' such as the Bubble Shells and shelled sacoglossans produce milky-white acidic mucous secretions from glands around the edge of the mantle. In other opisthobranchs, like the pleurobranch, Berthellina citrina illustrated here, clusters of white glands can be seen as part of the colour pattern. These glands are often called "repugnatorial glands". Many nudibranchs take distasteful chemicals from the animals they feed on and store then in mantle glands. One good example occurs in the chromodorids which have glands around the mantle edge containing antifeedant chemicals which they take from the sponges they eat. Mexichromis macropus is a good example. In some geographical areas, groups of chromodorids have evolved similar colour patterns so that fish learn to leave that distasteful colour pattern alone. In southeastern Australia many species have a red-spotted colour pattern. The phenomenon, which we call mimicry, is well illustrated by red-spotted chromodorids in southeastern Australia. There is also a picture of another chromodorid, Chromodoris woodwardae, with prominent mantle glands.
Bill Rudman.