Nudibranchs are a type of snail that has lost its shell. By losing its shell it is able to swim and squeeze into smaller spaces when it wants to hide. They are also called sea slugs.
Many nudibranchs have vivid colour patterns. This is to warn predators that they have a nasty taste or are venomous. Others are very well camouflaged.
On the back of these animals waving about in the water are the gills that are grouped in a bunch.
They can flex their bodies in a wave, helping them to swim and they can use their muscular foot to crawl. Nudibranchs eat other animals such as sponges, sea anemones, barnacles and even other nudibranchs.