Divers and marine biologists near San Diego recently came across an unusual sight of tuna crabs piling up on the seabed. These crabs were swept near shore by ocean currents, creating a red carpet effect on the ocean floor. Underwater videographer Anna Sagatov witnessed this phenomenon while diving at night off La Jolla Shores beach in San Diego in late April. The crabs were seen stretching as far as the diving lights could reach, appearing as a massive and unexpected gathering.
Tuna crabs are typically found around Baja California, Mexico, and not in the shallow waters near southern California. This marks the second time in six years that they have appeared in the San Diego area. The crabs may have been pushed to this location by nutrient-rich ocean currents associated with El Nino, a period of warmer oceans that create changing currents and air pressure fluctuations. The presence of tuna crabs in San Diego could signal shifts in the local climate and marine environment.
Despite normally feeding on plankton, tuna crabs can also engage in collective cannibalism at the end of their life cycle. They are attracted to the area just above the continental crust where they become bottom feeders. Local predators benefit from the abundance of tuna crabs, with the event providing scientists and divers like Sagatov with unique opportunities to observe marine life up close.
Research into the appearance of tuna crabs in the San Diego area remains a scientific mystery. While previous sightings were linked to strong ocean currents originating in Baja, the exact reasons for their presence remain unclear. Climate change could be a factor in creating more variable ocean conditions, leading to unexpected occurrences like the gathering of tuna crabs near San Diego.
Due to the cold water conditions in Scripps Canyon, the tuna crabs are not likely to survive long after arriving in San Diego. This can lead to mass stranding events where their bodies wash ashore, coloring the sand and surrounding sea red. However, the same ocean currents that brought them to shore could also push them back out to sea, creating a cyclical pattern of movement for the tuna crabs.