California Sea Hare Slug Found At Oregon Coast Aquarium

Aquarists cleaning out a water storage reservoir at the Oregon Coast Aquarium made an unusual discovery recently: a large sea slug, the California sea hare (Aplysia californica), apparently thriving in the 8,500-gallon concrete impoundment where seawater from nearby Yaquina Bay is pumped in and eventually filtered for use in the Aquarium’s exhibits. Anna Welsh, an aquarist intern at the Aquarium who was one of several staffers cleaning the reservoir, recognized the potato-sized mollusk immediately. “I’m originally from Monterey [CA], where sea hares are a common sight in bays, estuaries and rocky intertidal areas,” she said. “But I was definitely surprised to see it here, outside of its range.” The California sea hare is usually found off the coasts of California and northern Mexico. Only a handful of historical records show California sea hares this far north—and Yaquina Bay seems to be the absolute northernmost extend of its occurrence.

How the hare got to the Aquarium is something of a mystery. “I have been diving, snorkeling, seining, and fishing here on the Oregon Coast for 20 years and have never seen one,” said Jim Burke, the Aquarium’s Director of Animal Husbandry. “My thought is that it arrived here in its free-floating larval state, most likely as a hitchhiker inside a boat’s bilge or fish hold.” In this scenario, the larval sea hare, released into Yaquina Bay from a boat, somehow managed to be sucked up the Aquarium’s intake pump, passing through a six-millimeter-gauge filter unscathed.  Against astronomical odds, it found a haven in the predator-free reservoir, amid waving fronds of kelp and the resident clams, mussels, barnacles and other filter-feeders.

“Every three months or so we have to put on our rubber boots and hip waders and clean out the mud that’s accumulated in the holding tanks,” said Meghan Holst, an aquarist at the Aquarium. “It can be a chore, but it’s also an opportunity to collect specimens for our exhibits—animals that are essentially grown on-site from larvae imported with the seawater. Sometimes we find incredible creatures hidden away in the tanks, like the sea hare.”

The name “sea hare” refers to the tapered sensory organs on its head (called “rhinopores”) that waggle to and fro like rabbits’ ears. These sea slugs can grow to more than a foot in length and weigh up to five pounds. They’re strictly herbivores, grazing on algae much like a rabbit nibbles on tender green shoots. Algae figures so prominently in their lives, in fact, that the coloration of an individual sea hare varies depending on the hue of algae it eats. If a sea hare is disturbed, it may release a cloud of ink, much like an octopus. And like an octopus’s ink, this murky cloud doesn’t merely obscure a predator’s sight—it also chemically jams their sense receptors, effectively giving the predator a “stuffy nose” so that it can’t sniff out the escaping mollusk. Interestingly, the ink of sea hares has a scent of its own that has been described as “pleasant and cedar-like”.

Sea hares are remarkable in that they have the largest neurons in the animal kingdom (as well as relatively few of them), a trait that makes them extremely valuable subjects in neurobiological research. In 2000, Dr. Eric Kandel of Columbia University, New York, was awarded a Nobel Prize in Physiology for his work on synaptic function, using the nervous system of sea hares as his experimental model. Like octopuses, a sea hare’s life ends shortly after reproducing, typically after only a year or so. The individual at the Aquarium isn’t quite adult size, but its growth may have been stunted from living in the reservoir, so guessing its age is difficult. At any rate, visitors can now see this incredible and unusual animal in one of the “close-up” tanks of the Coastal Waters Gallery.

Information and photos by the Oregon Coast Aquarium