David Doubilet

A nudibranch [noo-duh-brangk] is a marine mollusk which sheds its shell after the larval stage. They are noted for their often extraordinary colors and striking forms. More than 3,000 different species have been identified. The word ‘nudibranch’ comes from the Latin ‘nudus’ (‘naked’) and the Greek ‘brankhia’ (‘gills’).

Nudibranchs are often casually called sea slugs, but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs (such as the colorful Aglajidae) are often confused with nudibranchs. Nudibranchs are found worldwide, at virtually all depths of salt water, but reach their greatest size and variation in warm, shallow waters.

Some species have venomous appendages (cerata) on their sides which are used to deter predators. 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, and consist of a lens and five photoreceptors. They have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odors. They vary in adult size from under an inch to over two feet. Nudibranchs are hermaphroditic, and thus have a set of reproductive organs for both sexes, but they cannot fertilize themselves.

All known nudibranchs are carnivorous. Some feed on sponges or other filter feeders, and some eat other sea slugs or their eggs (e.g. Favorinus) or, on some occasions, are cannibals and prey on members of their own species. The surface dwelling nudibranch, Glaucus atlanticus is a specialist predator of siphonophores (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 Glaucus simply envelops it with its capacious mouth, but if the prey is larger the mollusk nibbles off its fishing tentacles, the ones carrying the most potent nematocysts (stinging cells). Like some others of its kind Glaucus does not digest the nematocysts; instead, it uses them to defend itself by passing them from its gut to the surface of its skin.

Nudibranchs includes some of the most colorful creatures on earth. In the course of evolution, sea slugs have lost their shell because they have developed other defense mechanisms. Some species evolved an external anatomy with textures and colors that mimicked surrounding plants to avoid predators. Other nudibranchs, as seen especially well on chromodorids, have an intensely bright and contrasting color pattern that makes them especially conspicuous in their surroundings. This is believed to be an example of aposematic coloration; the shocking coloration warns potential predators that the slugs are distasteful or poisonous. They can also take in plant cells and reuse the chloroplasts (plant cell organelles used for photosynthesis) to make food for themselves.

Nudibranchs use a variety of chemical defenses to aid in protection. Some sponge-eating nudibranchs concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators. Certain species are able to produce their own chemicals de novo without dietary influence. 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 mucus automatically.


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