Etymology: "nudus" (naked) and "brankia" (gills) from Latin.
Nudibranchs belong to phylum "Mollusca" (meaning soft), order "Opistobranchia" (meaning with hind gills) and it encompasses around 6000 known species.
Big Horn nembrotha (Nembrotha megalocera)
Pyjama nudibranch (Chromodoris quadricolor)
Although they have a small shell during the larval planktonic stage, once adults they completely lack a skeleton and they normally have a very large powerful foot used for locomotion. Almost all the nudibranchs are benthic, which means they live connected to the bottom of the sea. There are a very few exceptions to this rule. The Spanish dancer (Hexabrachus sanguineus) is a nudibranch capable of swimming by moving the edge of its body. The Glauco atlanticus has a pelagic life (in the water mass, not on the bottom) floating near the surface thanks to an air bubble trapped in the lower part of the body. Each and every one of these creatures is equipped with chemoreceptors called rhinophores placed on the nape. These antennae are the scent or taste centres and are used as sensory organs.
All nudibranchs are hermaphrodites (have both sexes), but they are not capable of auto fecundation, so they have to meet another one. The advantage is that one can fertilize the other one. Eggs are positioned in gelatinous spirals near or on top of their prey. The individual is developed inside the egg and when the eggs hatch, the veliger (this is what the larva is called) has a short pelagic and shelled life prior to starting the rest of its life on the bottom.
They are all carnivorous and in this case it means that they eat other animals. The diet varies from sponges, to hydrozoans (sort of anemones), bryozoans (half a millimetre creatures) and sometimes other nudibranchs as well. Others eat crustaceans or dead fish. The mouth is equipped with a special set of teeth called the radula. These teeth are attached to a retractable chitinous "ribbon" attached to a cartilaginous base. The radula is used for scraping food from the bottom or from prey and it has had a paramount role during identification procedures since it differs from species to species.
Since they lose their shell after the larval stage, evolution gave them other defence mechanisms. Camouflaging by means of colours can render them practically invisible to predators; special colorations can identify them as non edible or poisonous; retained nettle cells from their prey make them again poisonous. Another defence system, which is not very spread but quite effective, is the skin production of toxic substances like sulphuric acid, used by the Spanish dancer to defend the eggs.
The big foot is used to attach the animal to the substrate and for locomotion. It contains a pair of statocysts that can be compared to our balance system in our inner ear.
You can find nudibranchs almost everywhere in the world and at any depth even though the majority live in tropical warm shallow water.
Now you know a bit more about these fantastic organisms, I guess it's time to go diving!