Nudibranchs are the adorable, charismatic, and voracious slugs of the sea. The Salish Sea is home to the shag-rug sea slug, a type of nudibranch (sea slug) covered in cylindrical gills (cerata) that make the creature look like a very dirty, moving shag rug.

Also called a shaggy mouse sea slug, it usually grows to about an inch long (although they can reach up to four inches). This nudibranch is typically brownish cream colored with brown and grey spots. Its favorite prey is sea anemones — a tricky dish to consume, kind of like eating a plate of spaghetti infested with hornets.

While sea anemones look like harmless underwater flowers, they have stinging cells in their arms, called cnidocytes, that help the anemone stun and capture its prey. These cnidocytes contain a spring-loaded barb that shoots out and delivers venom, stunning its prey and allowing the anemone to transport its dinner to its mouth, located in the center of the creature like the middle of a flower.

Close-up of  nudibranch and egg ribbon. photo by John F. Williams

So how does the shag-rug sea slug get around the cnidocyte problem when it wants to eat a sea anemone? First, it has to find anemones in the vastness of the Salish Sea, using rhinophores which are scent and taste receptors on its head. Once an anemone is located, the sea slug spreads mucus that may help protect it from the anemone’s stinging cnidocytes. Having a tough skin in its mouth and esophagus also protects the sea slug.

Not only can the shag-rug sea slug deal with consuming the anemone’s stinging cnidocytes, it goes further by stealing them, a behavior known as kleptocnidy. The sea slug consumes the cnidocytes, without triggering some of them to discharge their barbs and venom, then transports the cnidocytes inside its own body and deposits them to the cerata (cylindrical gills) on its back. Now those cnidocytes are a defensive weapon for the shag-rug sea slug, stolen from its anemone prey. Unlike its cousin sea snails, the shag-rug sea slug lacks a protective shell, and so having an array of stinging barbs on its back is vital to its own defense.

postscript

With all the talk about Artificial Intelligence, I was curious to see what Microsoft Bing Image Creator would come up with if I asked it to produce an image of a shaggy mouse nudibranch or Mousey Sea Slug. These are the interesting images it came up with. The AI definitely got the rhinophores and cerata correct, albeit a little too hair-like in some depictions, but the tiny black mouse eyes are a little too mammalian and definitly not found on nudibranch. Still a fun use of AI technology.

Shaggy mouse nudibranch technical description, references, and more photographs from the Rosario Beach Marine Laboratory, a campus of Walla Walla University

Video of shaggy mouse nudibranchs feeding – and defensive techniques of their sea anemone prey (2.5 minutes)

Video about a variety of nudibranchs and their defenses, by KQED, PBS (4 inutes)

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