You see them on every dive, but how much do you know about them beside that they are colorful?

Within the family of sea slugs, the clade is the largest, with more than 3,000 described species.

The word "nudibranch" comes from the Latin nudus, naked, and the Greek brankhia, gills. The name nudibranch is appropriate, since the dorids breathe through a "naked gill" like plumes of bushy extremities on their back, near their tail rather than using gills. Nudibranchs have head tentacles, which are sensitive to touch, taste, and smell while the club-shaped rhinophores detect odors. Their eyes are simple and able to discern little more than light and dark. The eyes are set into the body, are about a quarter of a millimeter in diameter.

An Aeolidiella stephanieae with a typical body: 1. Head tentacles, 2. foot tentacles, 3 rhinophores and 4. the eyes.

A Hypselodoris bullockii where you easily can see the branchial plumes on its back which it is using to breath.

The top 5 countries to see nudibranchs are: Indonesia (547 species), Australia (425 species), Philippines (413 species), Kenya (297 species), Malaysia (257 species). And the top 3 places in the Philippines to find nudibranchs are Puerto Galera, Dauin and Anilao. Just in the Puerto Galera area there are more than 120 registered species.

But what do they eat?

Their eating habits varies a lot from species to species, but most nudibranchs are carnivorous = meat eater. Some others feed on sponges while others again feed on hydroids = jellyfish. On the other hand, some eat other sea slugs and on some occasions even get cannibals and eat members of their own species. Other groups feed on sea squirts or anemones.

2 Nembrotha kubaryana eating on a sea squirts colony.

The surface dwelling nudibranch, atlanticus is a specialist predator of jellyfish, such as the Portuguese Man O' War. This predatory mollusk sucks air into its stomach to keep it afloat and using its muscular foot it clings to the surface film. If it finds a small victim, it simply envelopes it with its capacious mouth, but if the prey is a larger jellyfish the mollusk nibbles off its fishing tentacles, the ones carrying the most potent poisonous. Like some others of its kind does not digest the poison; instead, it uses them to defend itself by passing them from its gut to the surface of its skin.

Sex on the reef.

Nudibranchs are hermaphroditic, and thus have a set of reproductive organs for both sexes, but they can rarely fertilize themselves. Their eggs are typically deposit within a gelatinous spiral.

2 Nembrotha rutilans mating on a coral bed.

Eggs of a nudibranch.

How do they defend themselves?

Among the nudibranchs can be found the most colorful creatures on earth. In the course of evolution, sea slugs have lost their shell because they have developed other defense mechanisms. Their anatomy may resemble the texture and color of the surrounding plants, allowing them to camouflage. Others, as seen especially well on chromodorids, have an intense and bright coloring, which warns that they are distasteful or poisonous.

A Glossodoris cincta is very well camouflaged and hard to see.

Some sponge-eating nudibranch concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators. Another method of protection is the release of an acid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the slime automatically.

So next time you see a nudibranch during your dive… give it a thought about what a unique creature you are looking at.