It was a night dive off La Jolla Shores Beach in late April. Anna Sagatov and a friend got in the water and headed out to sea toward an underwater canyon. An ocean photographer, Sagatov carried her camera and lights.
“As we got closer to the canyon, we started to descend and it was a carpet of tuna crabs. Red, as far as my dive lights could illuminate. It was surreal,” Sagatov said.
Tuna crabs are crabs, but they look more like lobsters. They’re members of the Anomura group, which includes lobsters and hermit crabs. Their other common name is squat lobster.
Their visit near the shore of San Diego is one of several local appearances of the subtropical sea creatures in recent times. They arrived last month and divers say some are still out there in La Jolla Canyon.
The tuna crabs are a tasty snack for many sea predators, like tuna or rockfish.
“I did actually see a predation event. When I was down in the canyon on April 21, I caught some footage of a rock fish eating one,” Sagatov said.
The tuna crabs eat krill in the open ocean, and Sagatov found out that they sometimes eat each other. She saw a group of them feasting on a single one. She saw another, finishing off a fellow squat lobster’s arm and claw like it was a French fry. It was kind of dark, she said.
Seeing tuna crabs this close to the shore in San Diego is not unheard of, but it is remarkable. Sagatov remembers once operating cameras on an ocean research vessel when she caught glimpses of them.
“I have seen these squat lobsters that live in the deep sea — in these really extreme environments. So to go out here and see thousands of them is pretty cool because in the deep sea environment, you maybe see one or two at a time,” Sagatov said.
Where tuna crabs come from
Tuna crabs are most often found in subtropical waters. They have been seen as far south as Panama and as far north as San Francisco bay. Warm water currents that come with El Niño have moved a lot of the creatures northward.
“Well they came up from their center of distribution off Baja California, accompanied with the warm water that happened about 10 years ago. But they really haven’t left in the deeper water,” said Ed Parnell, a research oceanographer with Scripps Institution of Oceanography.
And he said that deeper water is where these San Diego crabs are likely headed.
“As the water cools and we have this onset of La Niña they’ll probably distribute themselves back deeper into the water column down on the bottom, down to the shelf break which is at about 100 meters,” Parnell said.
There are times when tuna crabs wander into shallow water and are stranded on shore, turning beaches red with their color. That typically gets lots of media attention.
Anna Sagatov said seeing so many tuna crabs in the sea off La Jolla Shores was another special moment for her.
“It’s really special to discover something,” she said. “Honestly, every dive I go on, I feel a sense of wonder. And even if we don’t see anything extraordinarily unique, I love being in the ocean.”
