Nudibranch: Species, Behavior, and Diet

Enchanting to both divers and scientists, colorful nudibranchs (pronounced "nooda-bronk" and including Nudibranchia, suborders Aeolidida and Doridacea) inhabit the sea floors of oceans all over the world. The unattractively named sea slug comes in a fantastic array of shapes and neon-bright colors that they themselves cannot see.

Description

Nudibranchs are mollusks in the class Gastropoda, which includes snails, slugs, limpets, and sea hairs. Many gastropods have a shell. Nudibranchs have a shell in their larval stage, but it disappears in the adult form. Gastropods also have a foot and all young gastropods undergo a process called ​​torsion in their larval stage. In this process, the entire top of their body twists 180 degrees on their foot. This results in the placement of the gills and anus above the head, and adults that are asymmetrical in form.

The word nudibranch comes from the Latin word nudus (naked) and Greek brankhia (gills), in reference to the gills or gill-like appendages which protrude from the backs of many nudibranchs. They also may have tentacles on their heads that help them smell, taste, and get around. A pair of tentacles called rhinophores on the nudibranch's head have scent receptors that allow the nudibranch to smell its food or other nudibranchs. Because the rhinophores stick out and can be a target for hungry fish, most nudibranchs have the ability to withdraw the rhinophores and hide them in a pocket in their skin if the nudibranch senses danger.

Species

There are over 3,000 species of nudibranchs, and new species are still being discovered. They range in size from microscopic to over a foot and a half long and can weigh up to just over 3 pounds. If you've seen one nudibranch, you haven't seen them all. They come in an astonishingly wide variety of colors and shapes—many have brightly colored stripes or spots and flamboyant appendages on their head and back. Some species are transparent and/or bio-luminescent, like the Phylliroe.

Nudibranchs thrive in an enormous variety of underwater environments, from shallow, temperate, and tropic reefs to Antarctica and even hydrothermal vents.

Suborders

Two main suborders of nudibranchs are dorid nudibranchs (Doridacea) and aeolid nudibranchs (Aeolidida). Dorid nudibranchs, like the Limacia cockerelli, breathe through gills that are on their posterior (back) end. Aeolid nudibranchs have cerata or finger-like appendages that cover their back. The cerata can be a variety of shapes—thread-like, club-shaped, clustered, or branched. They have multiple functions, including breathing, digestion, and defense.

Habitat and Distribution

Nudibranchs are found in all the world's oceans, from cold water to warm water. You might find nudibranchs in your local tide pool, while snorkeling or diving on a tropical coral reef, or even in some of the coldest parts of the ocean or in thermal vents.

They live on or near the sea floor and have been identified at depths between 30 and 6,500 feet below the ocean surface.

Diet

Most Nudibranchs eat using a radula, a toothed structure that they use to scrape off prey from the rocks they cling to; some suck out the prey after predigesting its tissue with selected enzymes, rather like a wasp. They are carnivorous, so that prey includes sponges, coral, anemones, hydroids, barnacles, fish eggs, sea slugs, and other nudibranchs. Nudibranchs are picky eaters—individual species or families of nudibranchs may eat only one kind of prey. Nudibranchs get their bright colors from the food they eat. These colors may be used for camouflage or to warn predators of the poison that lies within.

The Spanish shawl nudibranch (Flabellina iodinea) feeds on a species of hydroid called Eudendrium ramosum, which possesses a pigment called astaxanthin that gives the nudibranch its brilliant purple, orange, and red coloration.

Some nudibranchs, like the Blue Dragon, create their own food by eating coral with algae. The nudibranch absorbs the algae's chloroplasts (zooxanthellae) into the cerata, which acquire nutrients by photosynthesis using the sun to sustain the nudibranch for months. Others have evolved other ways of farming zooxanthellae, housing them in their digestive gland.

Behavior

The sea slugs can see light and dark, but not their own brilliant coloration, so the colors are not intended to attract mates. With their limited vision, their sense of the world is obtained through their rhinophores (on top of the head) and oral tentacles (near the mouth). Not all nudibranchs are colorful; some use defensive camouflage to match the vegetation and hide, some can change their colors to fit, some hide their bright colors only to bring them out to warn off predators.

Nudibranchs move on a flat, broad muscle called a foot, which leaves a slimy trail. While most are found on the ocean floor, some can swim short distances in the water column by flexing their muscles. Some even swim upside down.

Aeolid nudibranchs can use their cerata for defense. Some of their prey such as Portuguese man-of-wars have a specialized cell in their tentacles called nematocysts that contain a barbed or venomous coiled thread. Nudibranchs eat the nematocysts and store them in the nudibranch's cerata where they can be used late to sting predators. Dorid nudibranchs make their own toxins or absorb toxins them from their food and release those into the water when needed.

Despite the unsavory or toxic taste they can present to their non-human predators, most nudibranchs are harmless to humans, except those like Glaucus atlanticus which consumes nematocytes and so may consider you a predator and sting.

Reproduction and Offspring

Nudibranchs are hermaphrodites, meaning that they have reproductive organs of both sexes. Because they can't move too far, too fast and are solitary in nature, it's important for them to be able to reproduce if the situation presents itself. Having both sexes means that they can mate with any adult that happens to pass by.

Nudibranchs lay masses of spiral-shaped or coiled eggs, which are for the most part left on their own. The eggs hatch into free-swimming larvae which eventually settle onto the ocean bottom as adults. Only one species of nudibranch, the Pteraeolidia ianthina, exhibits parental care by guarding the newly-laid egg masses.

Nudibranchs and Humans

Scientists study nudibranchs because of their complex chemical makeup and adaptations. They have rare or novel chemical compounds which possess anti-microbial and anti-parasitic traits which may aid in the fight against cancer. 

Studies of nudibranch DNA also offer assistance in tracking ocean conditions relative to climate change.

Threats

These beautiful animals don't live very long; some live up to a year, but some only for a few weeks. The global population of nudibranchs is currently unassessed—researchers are still discovering new ones each year—but field observations such as that conducted by Endangered Species International suggest that many species are becoming rare, due to water pollution, degradation, habitat loss, and biodiversity decline associated with global warming. 

Sources

• Bertsch, Hans. Nudibranchs: Sea Slugs With Verve. The Slug Site, 2004.
• Cheney, Karen L. and Nerida G. Wilson. "Quick Guide: Nudibranchs." Current Biology Magazine 28.R4–R5, January 8, 2018.
• Epstein, Hannah E, et al. "Reading between the Lines: Revealing Cryptic Species Diversity and Colour Patterns in Hypselodoris Nudibranchs (Mollusca: Heterobranchia: Chromodorididae)." Zoological Journal of the Linnean Society.zly048 (2018).
• King, Rachael. Is It a Worm? A Snail? No...It's a Nudibranch!. Southeast Regional Taxonomic Center, Marine Resources Research Institute, South Carolina Department of Natural Resources.
• Knowlton, Nancy. Citizens of the Sea: Wondrous Creatures From the Census of Marine Life. Washington, DC: National Geographic Society, 2010.
• Lewis, Ricki. Celebrating National Sea Slug Day. PLOS Blogs: Diverse Perspectives on Science and Medicine, November 1, 2018.
• "Nudibranchs and other sea slugs." New Heaven Reef Conservation Program, 2016.