Cool Critters: Nudibranchs

Every once in a while we like to remind you that science is cool, wacky and occasionally funny. So far, we’ve introduced you to a new anglerfish and Abbott’s boobies. Well, the time has come to name these features and so we dub them “Cool Critters.” And now, for the first official cool critter, meet the nudibranch.

Nudibranchs (pronounced just the way it looks: “nudi” sounds like nudey as in naked/exposed/in one’s birthday suit + “branch,” which sounds like brank, although some argue that it sounds like the part of a tree) are found in every ocean. They are gastropods—like snails—but they don’t have shells. Hence the name nudibranch, which means “naked gill.” Nudibranchs are sometimes called “sea slugs,” but nudibranch is much more fun to say.

Nudibranchs come in every color. They range in size from 0.25 inches to 12 inches and live from a few weeks to a year. They’re the super models of the slug world: Check out all the pretty nudibranchs featured in the June issue of National Geographic here.

Blah, blah, blah. Now, here’s the good part: nudibranchs may be the coolest animals ever. They are simultaneous hermaphrodites, kleptomaniacs and they can photosynthesize. Let me explain.

Nudibranchs are loners, so meeting an attractive nudibranch for nudibranch baby-making can be a real challenge. But, if they do cross paths with an adult of the same species, they’ll be prepared—they’re simultaneous hermaphrodites, which means that every nudibranch has male and female reproductive organs. When the love “slugs” meet, they each fertilize the other’s eggs. A few days or weeks later, both nudibranchs lay eggs.

And just because I know you’re wondering: No they can’t impregnate themselves.

Nudibranchs are carnivores, but some species capture and farm tiny plants called zooxanthellae. They protect the plants by storing them in their outer tissues and morph their body shape to maximize sun exposure. In this sunny, protected environment, the zooxanthellae photosynthesize like crazy. They convert light into sugars for themselves, and pass on any excess to their nudibranch host. That way, the nudibranch has tons of fuel, even when food is scarce.

Without a protective shell, you’d think a nudibranch would be an easy snack for a predator. Don’t be so sure. Some nudibranchs feast on animals like anemones and jellyfish (cnidarians) that have stinging cells (nematocysts). These nudibranchs are called kleptocnidae (sounds like kelpto-nighty) because they steal nematocysts from the cnidarians they eat. The kleptos keep the nematocysts intact and functional and store them in their cerata (the tentacles or fringes on their backs and sides). Armed with stolen stinging cells, these nudibranchs defend themselves by stinging predators.

For the nudibranchs that can’t sting their predators, there are other options. One species feeds on soft coral that has nematocysts, but it doesn’t use them. Instead, when it is attacked, it drops a few cerata. The cerata release a sticky substance and wiggle around, distracting the predator and allowing the nudibranch to escape.

Admit it. These photosynthesizing kleptomaniac hermaphrodites are pretty cool, right?