A giant purple blob from the sea — a slug — is invading East Bay beaches and waterways this summer, and some experts say it may be caused by warmer temperatures near coastal waters.

 

These California sea hares are harmless plant eaters. But their big size and unusual abundance this year is turning heads at the shorelines in the cities of Crab Cove in Alameda and Miller Knox Regional Park in Richmond, as well as Lake Merritt in Oakland and Tomales Bay in Marin County.

 

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.

 

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 or a heart. They are called sea hares because their thick antennae resemble rabbit ears.

 

The boom in 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.