Rebecca Johnson: Bright pink electric blue undersea slug

Nudibranchs – better known as sea slugs – are small creatures. But their color is pretty splashy.

Image Credit: Terrence M. Gosliner

The garden slug – a very small and humble creature – is probably familiar to you. But you might not know that the sea has slugs, too – they’re called nudibranchs. Nudibranchs (pronounced nudi-branks) are shaped a lot like garden slugs, and they have the same kind of antennae, better known as rhinofores. But their coloration is different – in many cases, sea slugs are as colorful as tropical fish. That’s according to biologist Rebecca Johnson of the California Academy of Sciences, who just received a coveted Rubenstein Fellowship for her study of sea slugs. Dr. Johnson studies the evolution of color in the ocean. She said not all colorful sea creatures are colorful for the same reason.

Rebecca Johnson: Nudibranchs are a little bit different than something like tropical fish because … a lot of tropical fish, their colors are communication between males and females, because they can see each other, and their colors tell them something. But nudibranchs can’t see … their have eyes but their eyes just kind of see shadows, dark and light, and their colors are just for communication with potential predators.

Image Credit: Terrence M. Gosliner

In other words, their bright colors tell predators they’re toxic and taste really bad. Dr. Johnson told us that sea slugs can be studded with bright pinks, and electric blues and oranges. Hundreds of species of nudibranchs have unique colors and patterns. Through genetic analysis, Johnson’s trying to figure out if there’s an underlying connection between them, color-wise. She described one of the most interesting things she’s learned so far.

Rebecca Johnson: All of the nudibranchs in the group that I study, all the species that I study off the coast of the eastern Pacific and the Atlantic … almost all of those have a blue and yellow color pattern, even if they’re not each other’s closest relatives …. There’s something about that environment that being blue or yellow or having a certain kind of color pattern helps you be protected from predators … and one of the theories is that … if color patterns protect nudibranchs from predators by warning … that they taste bad, evolving one one kind of similar color pattern makes the predator only have to learn one thing. So, over time, the blue and yellow color pattern has evolved in things that aren’t really closely related, but have the same predators.

In this way, she said, small, colorful creatures like nudibranchs can influence the palette of a really wide scope of ocean life. Other ocean creatures – worms, for example – sometimes mimic the colors of nudibranchs. They do it to share in protection from predators, once those predators – thanks to beautiful sea slugs – have learned to associate certain colors with a very bad lunch.

Rebecca Johnson: A downfall of that is, let’s say, a predators tries to eat one of those [worms] accidentally and thinks “Oh, you didn’t taste so bad.” And then, that hurts everyone.

Image Credit: Mary Jane Adams

Dr. Johnson described her favorite nudibranch for EarthSky – the one she finds most beautiful:

Rebecca Johnson: It’s really hard to choose the most beautiful, but there’s one species called Hypselodoris iacula, and iacula literally means “fishing net”. It’s bright white, and it has an orange border around the entire body, and it then it has an even brighter, shiny-white fishnet pattern in the center of its body … It is almost glittery. It almost glows. You see kind of an orange-oval-y shape with a bright-white net-like pattern. And this guy has bright orange rhinofores, but they look kind of like antennae, and bright orange gills – the gills kind of look like an orange flower on the back of this animal. When you see the patterns and the colors … it’s almost impossible to believe they exist.

She explained that not only is the color of nudibranchs engaging for her, she’s also intrigued by the way they obtain their toxins – the ones that taste so disgusting to predators.

Rebecca Johnson: So I’ve talked about how they’re really brightly colored and how they’re poisonous but what’s really amazing is that they get those toxins from their food. They eat sponges – ocean plants – and take the chemicals from the sponge and put it in their body and use that as their poison …. In some of them it’s exactly like a remix, they change the shape of the molecule as it moves through their body, so it’s not toxic inside them, but it’s toxic by the time it moves to their special poison-holding cells.

Dr. Johnson told us that, in the next year, she’s going to be busy as an Encyclopedia of Life Rubenstein Fellow:

Rebecca Johnson: One of the great things about the fellowship with the Encyclopedia of Life (EOL) is that because these animals are so beautiful, there are pictures of them all over the internet, there are thousands of pictures on flickr. But what I’m hoping to do is consolidate all this information … to share all this information via one portal. The goal [with EOL] is to have a page for every species living.

Image Credit: Terrence M. Gosliner

EarthSky thanks Dr. Johnson, Terrence Gosliner, and Mary Jane Adams for allowing the use of their images.

Beth Lebwohl