Tuna Crabs, Neither Tuna Nor Crabs, Are Swarming Near San Diego

When Anna Sagatov, an underwater cinematographer, goes on her usual night dives off La Jolla Shores in San Diego, she’s used to spotting the “occasional octopus, nudibranch and horn shark.” But what she witnessed on a late April plunge was shocking: a seafloor turned red by what she described as an “overlapping carpet of crabs.” Swirling and shifting in the current, the creatures stretched “as far as my dive…