Invasion of the blob? Giant purple sea slugs slime California beaches

© Morgan Dill
Giant sea slugs called sea hares have been washing up in some East Bay beaches in unusual numbers this summer.

Alameda -- A giant purple blob from the sea -- a slug -- is invading East Bay beaches and waterways this summer to the wonder and curiosity of beach combers and naturalists.

It's no danger to people -- unlike the tar-like in the classic 1958 science-fiction movie about an alien goo that devours everything in its path. These California sea hares are harmless plant eaters, but their big size and unusual abundance this year is turning heads at the shorelines at Crab Cove in Alameda and Miller Knox Regional Park in Richmond, and also Lake Merritt in Oakland and Tomales Bay in Marin County. One Alameda beach visitor called police to report he had spotted a human heart sprawled in the sand.

Nope, he was told, it was just a California sea hare likely washing onto East Bay beaches, tide pools and inlets more often this year because of unusually warm water hugging the California coast.

"We are getting calls from the public asking what the heck is this big weird purple blob," said Carolyn Jones, a spokeswoman for the East Bay Regional Park District. "It's native to our area. It's not endangered, but they are rarely seen other than an occasional one here or there." Officials have no precise count, but dozens have been seen on some beaches at the same time, and two dozen were spotted last month in an inlet to Lake Merritt in Oakland. The first ones were spotted last fall, but more have been seen in May and June -- including ones that captured the crowd's attention last weekend at an annual sand castle building contest at Crown Beach in Alameda.

© Ken-ichi Ueda
Giant sea slugs called sea hares have begun appearing in Lake Merritt in Oakland.

The slugs can reach 15 pounds or more and 30 inches in length, although the ones in the East Bay are smaller -- about the size of a large fist, croissant, or, for that matter, a heart. They are called sea hares because their thick antennae resemble rabbit ears.

They wash up along beaches or sand flats after laying their eggs and dying, naturalists say.

The boom of sea hares may be related to warmer temperatures near coastal waters, said Morgan Dill, a naturalist at the Crab Cove Visitor Center in Alameda.

"We can't say for sure why we're seeing so many, but the Bay temperatures are definitely warmer this year," Dill said.

The warm coastal waters also are linked to the appearance of a small pink slug called Hopkin's rose nudibranch in Northern California waters this year, said Terry Gosliner, senior curator at the California Academy of Sciences in San Francisco. The small slug is typically found much farther south of the Bay Area.

© Morgan Dill
Giant sea slugs called sea hares have been washing up in some East Bay beaches in unusual numbers this summer. This is a mass of sea hare eggs.

"We're seeing more of these kinds of warming events, and I suspect it may be part of long-term global change," Gosliner said. "These are signals."

He said the abundance of sea hares also may be related to a periodic population boom in the mollusks.

In Oakland, local resident Joel Peter was stoked last month to see about 22 sea hares moving through in a canal into Lake Merritt.

He said he believes Oakland helped open up the sea slugs path to the lake with a canal widening that increased tidal flows into the lake from San Francisco Bay.

"I had never seen one before, and then all of a sudden there were 22 of them with these brilliant colors," Peter said. "They really caught my eye."