Scientists are perplexed by the massive group of crustaceans, but they suspect the animals were pushed north by strong ocean currents originating near Mexico

Usually, when scuba divers explore the waters off San Diego, they see lots of fish—and maybe the occasional shark or octopus. But, in recent weeks, they’ve found the seafloor completely covered in small, red crustaceans.

The plentiful creatures are called tuna crabs (Pleuroncodes planipes)—they’re related to hermit crabs but are actually a type of squat lobster. Measuring one to three inches long, these bright animals can fit in the palm of a human hand.

Tuna crabs typically live in the waters off Baja California, Mexico. But, for at least the third time in ten years, they’ve been swarming farther north, near Southern California.