July’s critter may be cute as a button, but don’t let the squishy sea slug face fool you. This voracious hunter strikes fear into the hearts of tiny invertebrates everywhere!
A striped nudibranch, Armina californica, collected from Nisqually Reach, Washington, March 2019.
Slug of the sea
Nudibranchs, or sea slugs, are the elegant, marine-dwelling cousins of the slimy brown slugs you find in your garden. The striped nudibranch can get fairly large (up to 8 cm) and should be easy to spot. You won’t find one on a beach walk or tidepooling session though. They prefer the sandy or muddy seafloor anywhere from the low intertidal zone to 80 meters deep. We collect them in our Puget Sound sediment samples from Nisqually Reach, Everett, and Sinclair Inlet … often co-occurring with their favorite food, sea pens.
The pen is NOT mightier than the slug
Striped nudibranchs feed primarily on colonial animals called sea pens. In Puget Sound, their two main prey species are the orange sea pen, Ptilosarcus gurneyi and the slender sea pen, Stylatula elongata.
In the southern part of their range (California to Panama), they also feed on the sea pansy, Renilla koellikeri. These species can bioluminesce, or give off light, when disturbed, so munching by sea slugs can set off a tiny underwater fireworks display!
Night terrors
Striped nudibranchs cleverly let the routine of their prey determine their activity schedule. Instead of wasting time crawling around during the day when sea pens are buried in the sand, they stay buried too, with only their rhinophores, or sensory structures, protruding.
Striped nudibranches swarm and consume an orange sea pen until only the white skeletal rod remains 
The rhinophores help the nudibranchs “smell” chemicals in the water that indicate food is nearby, and they can detect changes in light with a pair of tiny eyes. When night falls they emerge, searching for fleshy victims to devour. The nudibranchs use suction to ingest pieces of the sea pen’s tender polyps, pulling the tissue off and leaving only the hard white rachis, or skeleton, behind.
Pretty with poison
You would think plenty of animals would be happy to give the striped nudibranch a taste of its own medicine in the predation department, but even the ravenous sun star, which eats pretty much everything in its path, gives this little nudibranch a wide berth. In fact, the striped nudibranch has very few natural predators. So, what makes this slow-moving nudibranch so unappetizing? It can incorporate chemicals from the tissue of its sea pen prey into its own body, making it toxic to consume.
In the groove
While the striped nudibranch may be the stuff of nightmares to a sea pen, it’s a daydream for a mollusk taxonomist. Most nudibranchs are difficult to identify, but the distinct long white ridges and dark background of this species are very distinct. It has no appendages on its top surface except for the rhinophores, which project out of a notch on its head; everything else is tucked away in a groove running down the side of the animal’s body. Hidden away in these folds are flap-like gills, the anus, and the reproductive opening, called the gonopore.
This side view shows the groove on the right side of the animal, with the anus to the left and the reproductive opening to the right. Image courtesy of Dave Cowles, wallawalla.edu.
Doing it all
Like other sea slugs, striped nudibranchs are hermaphroditic (possessing both male and female sex organs). Any two individuals can mate by connecting the reproductive “ports” on the right sides of their bodies. Eggs are laid in brownish spirals, and larvae hatch out into the water column to eventually settle and become tiny striped terrors in their own right!
Critter of the Month
Our benthic taxonomists, Dany and Angela, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of the Marine Sediment Monitoring Program. We track the numbers and types of species we see in order to understand the health of Puget Sound and detect changes over time.
Dany and Angela share their discoveries by bringing us a Benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants. We’ll share details on identification, habitat, life history, and the role each critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.