There are tiny jewels decorating the ocean in every part of the world. The jewels are nudibranchs, gastropod mollusks that can be found from shallow reefs to deep seas, from the warmth of the tropics to the frigid waters of Antarctica. The variety of species in the clade Nudibranchia is staggering, as are the colors and shapes of these tiny, soft-bodied beauties. 

Nudibranchs are different from other sea slugs, which belong to other taxonomic groups; these flamboyant creatures are their own brand of special. There are two main types of nudibranchs: dorid nudibranchs and aeolid nudibranchs. The former have a branchial plume on their rumps, which is actually the gills through which they breathe. The latter have cerata covering their backs, which look a bit like spikes or horns, and which also aid in respiration. In fact, the scientific name nudibranch means "naked gills," which aptly describes the anatomy of both types. Below is an example of a dorid nudibranch and an aeolid nudibranch, respectively.

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Photo: Rogerio Paulo Silva/Shutterstock

nudibranch

Photo: Joe Belanger /Shutterstock

But breathing isn't the only use for the cerata on the backs of aeolid nudibranchs. These species can eat stinging-celled prey such as anemones and Portuguese man-of-war jellies, passing the stinging cells through their digestive system and up into the cerata, so that the nudibranch can then use the ability to sting as a defense against its own predators.

Some species of nudibranch that eat toxic sponges can save the toxins in their own bodies so that they in turn become toxic to predators.

Kind of amazing already, right? But that's just the tip, literally, of what makes these creatures fascinating, and scientists have their hands full. There are more than 3,000 species of nudibranch worldwide and more species are being discovered all the time. Nudibranch species vary wildly in coloration and size. Some species live only a month, while other species live for a year. Some species grow to only about one-quarter inch long, while other species can grow to as long as 12 inches.

And their selection of prey varies as well; some graze on algae while others dine on coral or barnacles, and some go for sea slugs other nudibranchs. But each species or group of species is specialized in what they eat and sticks to a niche. The coloration of each species comes from the prey they eat: the more colorful the prey, the more colorful the nudibranch. The colors can be used either to stand out from their surroundings and act as a warning to predators that they're toxic, or the colors can help them blend into the substrate on which they live, becoming invisible.

Nudibranchs can identify their prey through smell, using the two specialized chemical-sensing tentacles, or rhinophores, on the top of their heads:

nudibranch

Photo: littlesam/Shutterstock

As for their other senses, Advanced Aquarist explains, "Their small eyes are located below the tissue of the animal on the head near the brain, and they cannot see actual images but only take measures of light intensity." It's a bit of a sad irony: nudibranchs are gorgeous to look at, investing energy in becoming flamboyant and vibrant (granted, it's for protection and camouflage rather than vanity), and yet they can't see each other well enough to appreciate their own beauty.

nudibranch

Photo: Chanwit Polpakdee /Shutterstock

Advanced Aquarist also explains how they eat: "[N]udibranchs have a file like organ on the mouth called radula, used to scrap off their food. This organ is covered on top with rows of chitinous teeth, which are produced by the radula sac as they are warned down." This is a feature that is unique to mollusks and the shape and capabilities of the radula vary among mollusk species.

And as for locomotion, nudibranchs crawl across a surface using its foot, a broad and flat muscle that helps them cling to rocks and corals. If you've watched a snail or slug crawl across rocks or plants in your garden, then you have a good idea how nudibranchs get around.

nudibranch

Photo: Ekkapan Poddamrong8 /Shutterstock

Nudibranchs also have figured out a way to make mating about as easy as possible. All nudibranchs are simultaneous hermaphrodites — possessing both male and female organs, but still needing a mate to reproduce — and can mate with any other mature member of their same species, so there's no time wasted wandering around looking for a partner of the opposite sex. After all, it's not like they can go speeding over vast distances to find a mate.

Once they mate, each nudibranch lays a spiral-shaped coil of eggs (sort of the shape of a cinnamon roll) that later hatch into larvae that are free-swimming until they mature into adults. While most adult nudibranchs are found on the ocean floor, some species can swim short distances.

nudibranch

Photo: KKG Photo/Shutterstock

Nudibranchs are also a benefit to medical science. According to California Academy of Sciences, "Scientists have studied sea slugs' simple nervous systems for clues to learning and memory, and have raided their suite of defensive toxins in search of pharmaceuticals. They are also isolating chemicals that may help treat a variety of heart, bone, and brain conditions. A nudibranch relative known as a sea hare offered up a cancer-fighting compound that made it into clinical trials."

nudibranch

Photo: Soren Egeberg Photography /Shutterstock

The world of nudibranchs is fascinating and because there is so much more about them to study, who knows what we will learn about these incredible sea creatures. But for those of us who aren't diving under the ocean to check them out in person, we have gorgeous underwater photographs that help us appreciate their diversity and beauty.

Here are a few more of these stunning species:

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Dondice banyulensis (Photo: Adnan Buyuk/Shutterstock)

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Chromodoris joshi (Photo: Dray van Beeck/Shutterstock)

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Hermissenda crassicornis (Photo: Tory Kallman /Shutterstock)

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Nembrotha cristata (Photo: cbpix /Shutterstock)

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Hypselodoris apolegma (Photo: Rich Carey/Shutterstock)

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Flabellina bilas (Photo: scubaluna /Shutterstock)

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Hypselodoris bullockii (Photo: Fiona Ayerst /Shutterstock)

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Goniobranchus kuniei (Photo: cbpix/Shutterstock)

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