Slugging it Out at Crown Beach

A curious sight washes ashore along the bayside.


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If you find a sea hare on the beach, just observe it. They eject a purple ink when agitated.

Photos by Morgan Dill

In October, dozens of sea slugs washed up on the shores of Robert W. Crown Memorial State Beach and Crab Cove, catching the attention of curious beachgoers. Far bigger than your garden-variety slugs, the peculiar creatures, called California sea hares, made a strong impression for their sheer size and deep purple color.

The slugs were grouped alone or in pairs, spaced out every few feet among clumps of seaweed.

“We were originally alerted to their presence by a park visitor who had taken a picture and came into the Crab Cove Visitor Center and didn’t know what he had found,” says Morgan Dill, a naturalist aide for the East Bay Regional Park District. “The sea hares were still alive when they first washed up on the beach, but over the course of the next few days, gradually dried out and no longer were alive.”

According to iNaturalist.org, the California sea hare is likely the world’s largest gastropod, weighing up to 35 pounds. The striking marine animals live in coastal regions where they feed on algae and eelgrass, taking on the color of what they eat. They have two pairs of antennae on their head, the anterior pair being much larger and ear-like, giving them their name.

They feed with a pair of jaws and a ribbon-like radula, and their thin, see-through shells are covered by a mantle. The Monterey Bay Aquarium website states that each sea hare is both male and female and has both sexual organs. Aside from telling light from dark, sea hares can’t see.

It’s uncertain why so many sea hares chose to make Alameda their final resting place this past October. According to Dill, sea hare sightings have increased, and the fascinating aquatic gastropods are always out in the Bay, eating seaweed and swimming around with their wing-like extensions. Cyclical mass mortality events such as the one at Crab Cove are not uncommon, though it has been a long time since one has happened on the island.

“The [mortalities] are likely not due to pollution, but more likely due to a large population boom about a year ago because of favorable conditions in the water for these large sea slugs,” Dill says. “The juveniles rapidly mature at the same time, and at the dying end of their life cycle, they all tend to wash up on shore and die at the same time.”

 If you come across a California sea hare while at Crab Cove, keep in mind that putting it back in the water will not prevent it from dying. While the slugs aren’t dangerous, they will eject a purple ink if agitated. Visitors should know that that Crab Cove is a marine preserve and anything found there is protected and should be left undisturbed.

“It is best to just let these creatures be, and not disrupt the process of their death,” Dill says. “It is completely fine to look at them up close, though, and observe their beautiful skin color.”

The adult generation lives in the shallow, sheltered places, while younger sea hares choose to stay in deeper waters. Along with sea hares, several species of sea slugs thrive in the Bay. Nudibranchs are sea slugs with bright colors and frills, among them the Salted Yellow Doris, which possess a plume of gills, and the white frond aeolis, which have small projections running parallel along their backs. Sea cucumbers, somewhat similar in shape, are often mistaken for sea slugs.

The sea hares need not wash up on shore to be seen; if you’re lucky, you might find one on the mud flats when the tide is low. If the hares have struck your curiosity, and you don’t want to rely on chance, trek over to the Monterey Bay Aquarium, where they’re on exhibit at the Sandy Shore & Aviary and the Kelp Forest. The California Academy of Sciences also houses a large collection of sea slugs.

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