On the heels of Halloween, with all the creative costumes out there designed to scare other people away, I was thinking of some of the crazy “costumes” that sea creatures put on to scare their predators away. There are myriad strategies that animals use to look dangerous. Some animals have giant eyespots, many times bigger than their actual eye, in order to trick predators into thinking that they, too are much bigger than they actually are. Others have terrifying spines that stick up out of their bodies and would be a terrible mouthful to try to eat.

Other sea creatures, on the other hand, may not look so scary to us, but their coloration signals to predators that they are not good to eat – that they could, in fact, be poisonous to eat. Instead of looking ugly, these creatures are often some of the most beautifully colored ones in the sea. Their unscary coloration, however, covers up scary toxins inside their bodies. That’s because these creatures usually don’t have much else to protect them.

One of the most beautiful and varied sea creatures that often get overlooked has a name that gives away its vulnerability – nudibranch means “naked gills.” That’s because their feathery gills are exposed rather than covered up inside a shell. Their less elegant name, sea slug, may help to recognize them as essentially a snail with no shell. They belong to the same group of soft-bodied marine creatures as clams, mussels, and sea snails, but they don’t have the same armored protection. So, instead, they instead have toxins that make them either taste terrible or actually poison their would-be predator. They get these by eating other sea creatures that have toxins in them and concentrating them in their own bodies.

Nudibranchs are one of the most varied and widely found sea creatures in the world’s oceans. Perhaps that’s why there is a whole day dedicated to them. Oct. 29 was National Sea Slug Day in case you missed it. There are more than 2,000 species of nudibranchs found worldwide. They vary from less than a quarter of an inch to up to nearly two feet in length. Their color patterns can make them look strangely like other animals – like the blue dragon or the sea sheep (both of which are worth looking up). Or, they can be shockingly bright like the neon sea slug. In addition, their feathery gills often look like shaggy hairdos and their tentacles also add to their oddball personalities.

Often brightly colored creatures are relegated to warmer waters. But, nudibranchs are common right off our coast. They’re just not easy to find as they are all small and often in deeper waters. One of the most common species is Flabellina verrucosa. It isn’t the brightest, but its reddish fan-like (flabellina means fan in Latin) gills have bright white tips that make it a little easier to spot (unless it is feeding on an anemone which it perfectly resembles). There are several other Flabellina species as well, and lots of others with neat Greek and Latin derived names like Cuthona, Eubranchus and Aeolidia. And these are just the ones that we know about.

Because nudibranchs are often elusive, there are new species being discovered every day. They are important both as climate indicators and for their potential medicinal benefits. Where certain species of nudibranchs live helps scientists understand changing ocean conditions. And, the toxins they use to defend themselves have the possibility of helping to fight human diseases including certain types of cancer.

In the post-Halloween beginning to November, take a moment to appreciate some of the bizarre ways that nature can be both scary and lovely, and masters of disguise.