There are over 3000 known species of Nudibranch (naked gill). It is believed there are many more yet to be discovered. They are found all over the world, with the largest population and different types in tropical waters. Many sea slugs are quite small, but the ones found in the Great Barrier Reef off the Australian Coast can grow up to 12 in.
While they gave up the protection of carrying a shell, (hence the name Nudibranch, which I rather like) many have created different kinds of protection. They can become toxic. These guys absorb, for example, the stinging cells from sea anemones, and store them own defence or they absorb toxins from their food and incorporate that into their own defense systems.
This ability has to absorb cells has led to something I find most fascinating about one particular group of Nudibranch. This slug, Elysia chlorotica, basically eats a plant and becomes photosynthetic, in other words, solar powered. It steals the genes from the algae that it eats and becomes a plant-animal. Scientists studying this phenonemon are still attempting to understand just how the slug makes this work.