All About Nudibranchs

Colorful Sea Slugs

You may never have heard of them, but once you've seen a nudibranch (pronounced nood-i-brank), you'll never forget these beautiful, fascinating sea slugs. Here is some information about these interesting ocean organisms, with links to content that feature nudibranchs.

Trinchesia sibogae
Fotografia de Naturaleza/Moment Open/Getty Images

Nudibranchs live in oceans all over the world. These often brilliantly-colored animals are related to snails and slugs, and there are thousands of species of nudibranchs. There are two main types of nudibranchs - dorid nudibranchs, which have gills on their posterior (back) end, and eolid (aeolid) nudibranchs, which have obvious cerata (finger-like appendages) on their back.

Nudibranchs move on a foot, have poor vision, may be toxic to their prey, and some are even solar-powered. Despite their fascinating characteristics, finding nudibranchs is often not hard - there may be one in your local tide pool.

Glaucus atlanticus / GregTheBusker, Flickr
Glaucus atlanticus Nudibranch. This nudibranch eats Portuguese man-of-wars and stores its venom for its own use. This is one nudibranch that may sting humans. Courtesy GregTheBusker, Flickr

There are about 3,000 nudibranch species, and more are being discovered all the time. It may take awhile to discover nudibranch species because of their small size - some are only a few millimeters long, although some can grow longer than a foot. They can also easily disguise themselves by blending in with their prey.

Here you can learn even more about nudibranchs - how are they classified? What do they eat, and how do they reproduce? You can also learn about the unique defense mechanisms of these mostly small creatures, and how they are used by humans.

Octopus in the Red Sea / Silke Baron, Flickr
Octopus in the Red Sea. Courtesy Silke Baron, Flickr

Nudibranchs are in the Phylum Mollusca. Organisms in this phylum are called mollusks. This group of animals includes not only nudibranchs, but a diverse array of other animals, such as snails, sea slugs, octopus, squid, and bivalves such as clams, mussels, and oysters.

Mollusks have a soft body, a muscular foot, usually recognizable 'head' and 'foot' regions, and an exoskeleton, which is a hard covering (although this hard covering is not present in adult nudibranchs). They also have a heart, digestive system, and nervous system.

Lightning Whelks / Bob Richmond, Flickr
Lightning Whelks, Busycon sp. Courtesy Bob Richmond, Flickr

To further narrow down their classification, nudibranchs are in the Class Gastropoda, which includes snails, sea slugs, and sea hares. There are over 40,000 species of gastropods. While many have shells, nudibranchs do not.

Gastropods move using a muscular structure called a foot. Most feed using a radula, which has tiny teeth and can be used for scraping prey off a substrate.

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What Is a Rhinophore?

Striped Pajama Nudibranch / Courtesy www.redseaexplorer.com
Striped Pajama Nudibranch (Chromodoris quadricolor), showing yellow rhinophores at top. Courtesy www.redseaexplorer.com, Flickr

The word rhinophore refers to body parts of a nudibranch. Rhinophores are two hornlike tentacles on the head of a nudibranch. They may be in the shape of horns, feathers, or filaments and are used to help the nudibranch sense its environment.

The Spanish shawl nudibranch has a purple to a bluish body, red rhinophores, and orange cerata. These nudibranchs grow to about 2.75 inches in length and can swim in the water column by flexing their bodies from side to side.

Spanish shawl nudibranchs are found in the Pacific Ocean from British Columbia, Canada to the Galapagos Islands. They may be found in relatively shallow water but can live in water depths up to about 130 feet.