On a recent rainy weekday, I took advantage of the afternoon’s low tide and visited our intertidal friends at the north-end ferry dock.

Hoping to find some critters celebrating the lengthening days, I was happy to be greeted by numerous gelatinous egg masses adhered to the rocks. Barnacle-eating dorids, Onchidoris bilamellata, laid these jelly-like sacs, each containing thousands of tiny white eggs. Dorids are nudibranchs (pronounced noodie-brank), or sea slugs, and part of the gastropod class of mollusks. Literally meaning “nude gills,” nudibranchs have frilly feather-like gills on their backs and can be quite beautiful. They also have a pair of special chemical-sensing antennae called rhinophores projecting from their head that help them smell the environment and other animals.

Nudibranchs reproduce in an altogether curious way, because every individual is a “simultaneous hermaphrodite,” having both female and male reproductive organs at the same time. However, they cannot fertilize themselves but instead require a partner to come alongside so that they may pass packets of sperm to each other through tubes that extend from behind their heads. I told you it was curious. Once mating is successful, each individual can go on to lay millions of eggs — a distinct advantage in the numbers game.

All nudibranchs are carnivores, preying on sponges, anemones, corals and some bivalves. As exemplified by our barnacle-eating dorid friend, many prefer a diet consisting of only a few species. Like all gastropods, this dorid slowly grinds and drills its way into the shell of its prey with a file-like mouth called a radula so it can suck up the soft flesh of the barnacle. If the nudibranch in question prefers sponges or anemones, the radula helps scrape the prey off of rocks.

In a shallow, rocky pool I found a shaggy mouse nudibranch, Aeolidia papillosa, a larger species of sea slug that can be compared not unfavorably to a piece of shag carpeting. Along with the gills and rhinophores, it has fleshy cerata all over its back that move like hair in the water.

These particular critters eat anemones, whose predator defense includes stinging cells in their appendages — which of course doesn’t protect against the Aeolidia.  Amazingly, these stinging cells pass unharmed through the digestive systems of the nudibranch and are eventually incorporated onto the surface of the cerata, having been thus assimilated by the Aeolidia for its own defenses. Some species even eat and use the stinging cells from jellyfish tentacles. In fact, the color of the cerata can vary depending on the specific species that have been assimilated. They truly are what they eat.

The nudibranchs have other chemical mechanisms of protection as well, and can produce acidic mucus or toxins that taste awful or incapacitate their predators. Who eats sea slugs?  That’s a good question, and for the most part we don’t know. There is evidence that crab, fish, and a few marine worms will sometimes attack and eat nudibranchs, and many sea slugs will actually cannibalize their own kind.

The Puget Sound is home to more than 200 species of nudibranchs, and their beauty has gained them a reputation for being the “butterflies of the sea.” While some of the most colorful species are found in warmer waters, Vashon’s populations of soft-bodied sea slugs have their own particular charms.

While on walks with the Vashon Beach Naturalists, I’ve seen colorful and translucent nudibranchs that seemed to glow from within. The next Vashon Beach Naturalist event isn’t until June, but you can explore the nearby beaches and visit the creatures that live there anytime.

— Adria Magrath is a biologist, teacher and nature photographer, as well as a Vashon Beach Naturalist.