They are the rock stars of marine invertebrates. Fabulous and flamboyant, they live fast, reproduce furiously and die young. But they are also evolutionary marvels: chemical wizards, crafty thieves, sly poisoners—and the sexiest of intertidal residents.
Beachcombers call them sea slugs. Marine biologists categorize them as nudibranchs (rhymes with swanks and translates as “naked gills”), relatives of snails that shed their shells early in life. While many aquatic creatures rely on camouflage, nudibranchs flaunt eye-popping colors—regal purple, hot pink, electric blue, neon yellow—and exuberant patterns.
The bold fashion statements aren’t meant to impress one another. With tiny eyes that can only distinguish light from dark, sea slugs navigate by smell, waving rhinophores—“nose horns”–atop their heads like little chemical antennae. Their gaudy bodies broadcast a simple message to would-be predators: Don’t mess with me!
Much of their rainbow wardrobe comes from their daily diet. Extremely picky eaters, most sea slugs devour just one or a few prey species, such as hydroids, anemones and even other nudibranchs. These meals supply not only nutrients but also the compounds that create their vivid colors—and often their defensive artillery.
Aeolid nudibranchs that feast on anemones and other cnidarians steal stinging cells and shuttle them intact to the tips of the finger-like cerata on their backs. When threatened, they unleash these weapons like pirates firing captured cannons against an enemy.
Their life goals are basic: Eat, avoid being eaten and reproduce. As hermaphrodites with both male and female organs, any two individuals can—and usually do—mate. Biologists demurely refer to this as “maximizing reproductive efficiency”—an existential imperative for creatures that live only from a few weeks to about a year.
When chemical signals draw two compatible sea slugs together, they slowly circle until the reproductive openings on the right sides of their bodies align. In most species, each transfers sperm to the other. Although these encounters seem like casual hook-ups, sea slug courtship may be more sophisticated than biologists once believed. Some species nuzzle, stroke, bite, and fight in prolonged foreplay. In the recently described Doto urak, partners repeatedly crawl over and around one another before finally connecting.
After their union, new parents release delicate streamers of thousands—sometimes tens of thousands—of fertilized eggs. Coiled into rose-shaped spirals or gossamer ribbons, they look more like exquisite works of art than the beginning of the next generation.
With more than 3,000 identified species–and possibly many more awaiting discovery in oceans around the world—nudibranchs display a seemingly infinite variety of forms and colors. During a recent Sea Slug Search on the Sonoma Coast, volunteers counted 182 nudibranchs representing seventeen species in just two hours.
My favorite nudies include:
Opalescent Nudibranch, a shimmering blue dragon cloaked in white-tipped cerata.
Clown Nudibranch, studded with orange knobs and branching gills that burst like miniature fireworks.
Hopkins Rose, pretty in pink, with petal-like gills that mirror the pink encrusting bryozoan it feeds upon.
Spanish Shawl, a show-stopper that undulates like a flamenco dancer’s silk scarf.
These soft, squishy creatures, rarely more than a few inches long, lack shells, bones, and a centralized brain. Yet they endure in a sea of dangers, thanks to an unlikely combination of jewel-like beauty, clever defenses, potent chemistry—and a love life as extravagant as their colors.
That’s more than a mere rock star could claim.
Photo credits: Gary Cobb published at https://nudibranch.com.au, Amber Dorica, Surfrider Foundation
