Photo of pink tritonia by Jan Kocian.
What’s pink, white, and frilly all over? Here’s a hint: it’s not a ballet tutu, or a doily crocheted by your grandma, or that mushy Valentine’s card you sent to your crush — you know, the one that seemed like a really good idea at the time. I’m talking about the pink tritonia: the walking (or in this case, crawling) embodiment of the excitement — and terror — of blossoming love.
Taxonomy heartaches
Before we delve into this fascinating critter, I want to make a disclaimer: I got taxonomically catfished by this beast, and investigating its true identity has been a roller coaster ride. As a result, I am sticking with the common and genus names here. If you’re interested in taxonomy, please see the footnote at the end of this article.1 Okay, let’s get to it, shall we? 
Getting to know you
Like that person you just met online, the pink tritonia is a multifaceted creature with both delightful and daunting attributes. No one can argue that its white racing stripes and rosy hue are charming, and like many other nudibranchs (a.k.a., sea slugs), it has external gills in delicate tufts down its back. But something about the wide, fringed oral veil, which sweeps the sand and conceals a gaping mouth, reminds me of a Jim Henson creation: loveable, but with a bit of an unpredictable streak. “Don’t look at me, I’m hideous!” Photo of pink tritonia waving its oral veil by Dave Cowles, wallawalla.edu.
Pink panther
Pink tritonia with oral veil lifted to reveal a mildly terrifying mouth. Photo and illustration by Jan Kocian.
If you’re a sea pen like Ptilosarcus gurneyi or Stylatula elongata, there’s no question about it — the pink tritonia is the stuff of nightmares. Tritonias are ravenous predators, scouring the sandy bottoms of protected bays and coastal areas for juicy sea pens to chomp. I've seen footage of their strike attacks on unsuspecting victims, and watching this slow-motion dance of death always leaves me feeling a bit... unsettled.
Blind date
Tritonias aren't always so fearsome — they show their softer sides by spending three times more of their lives engaged in romance than they do in pursuit of food! They have no eyes, but can “smell” potential mates in the water around them using their rhinophores, antennae-like stalks which can retract into protective sheaths.
Pink tritonias are hermaphrodites, possessing both male and female reproductive organs. Although they can’t self-fertilize, being able to mate with any other individual they meet greatly increases the number of “fish in the sea.” Later, both partners lay fertilized eggs in lacy pink clusters.
Left: A pink tritonia with an egg string emerging from the reproductive port, or gonopore, on its right side. Right: A lacy mass of tritonia eggs. Photos by Dave Cowles, wallawalla.edu.
Pink slip
All this roaming around, albeit at slug speed, means sometimes encountering foes instead of friends. When the pink tritonia needs to give a predator the slip, it shifts from crawling-on-mucus mode into high gear. The mere presence of a sunflower star, Pycnopodia helianthoides, can trigger an escape swimming response, in which the tritonia compresses its body into a flat shape and thrashes around until it lifts off the sediment (and with a little luck, drifts to safety with the water currents).
Global positioning slugstem
If the slug ends up getting carried away with its swimming (literally), it has tools to help it get home. Not only can it orient its body with the flow of water (a sensory response called rheotaxis) but it also has a built-in navigation system. The pink tritonia’s entire body acts as a compass, sensing the Earth’s magnetic fields and helping guide the animal towards shore. This sensory ability, called magnetoreception, has been the study of much research.
Tickled pink
This unfortunate pink tritonia, washed up at Tolmie State Park in south Puget Sound, reminds me of a piece of sashimi. Photo courtesy of Paul Anderson, Ecology.
Scientists at UW’s Friday Harbor Labs have been enamored with pink tritonias since the 1960s, when a researcher inadvertently produced dramatic movements in a dissected (and very deceased) tritonia while poking around in its head. The round structures he accidentally stimulated were discovered to be individually identifiable brain cells called giant neurons. Amazingly, the pink tritonia has some of the largest giant neurons in the animal kingdom.
Giant neurons may help explain some of this slug’s remarkable escape and orientation behaviors, and have inspired many groundbreaking studies on brain structure and function. Now, who can resist a critter with beauty AND brains?
Critter of the Month
Dany is a benthic taxonomist, a scientist who identifies and counts the sediment-dwelling organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see to detect changes over time and understand the health of Puget Sound.
Dany shares her 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.
1This particular nudibranch has been known by the species name Tritonia diomedea in this region for many years, and as T. diomedea, has been the subject of much research. In 2006, T. diomedea was found to be synonymous with another Pacific species, Tritonia tetraquetra, and the name was changed to that latter species (Martynov, 2006). This change was widely accepted by the taxonomic community, although T. diomedea is still frequently used in broader circles. In 2020, a study using phylogenetic techniques to examine the classification of the Tritoniidae confirmed this synonymy. According to this paper, Tritonia tetraquetra does indeed occur here on the west coast – but it is described as an orange sea slug with no white on it. The pink sea slug with white lines is classified as the co-occurring Tritonia exsulans (Korshunova and Martynov, 2020). To make matters even more confusing, a third, completely different species called the orange peel nudibranch was also once called Tritonia tetraquetra, but is now called Tochiuna gigantea. If you google the three Tritonia species above, you will find search results for each matching the pink tritonia pictured in this article. Some have theorized that the color variations in the pink tritonia arise from the different types of sea pens they eat, but it’s hard to argue with genetics. I believe that this latest name might be the correct one and that the pink tritonia should be called T. exsulans. I’d love to hear your opinions!