Tuna Crab Invasion!

Updated: Apr 17, 2019


Red Pelagic Crab or Tuna Crab invade La Jolla Shores, California USA

It's April. Spring has sprung, and many changes have come to our Southern California Waters. With the return of the sunny weather comes, you guessed it; warmer waters. And warmer waters literally bring with them a slew of pelagic (open ocean) creatures; none more noticeable than the appearance of the Tuna Crab, or Red Pelagic Crab. What makes them so noticeable? Well, for starters, they show up with a party.. a big party.


In the past, the Tuna Crab invasion coincided with El Nino years, however with recent water temperatures warming, the Tuna Crab invasion has hit Southern California every year since 2015 for no known reason. Perhaps the warmer water is allowing them to breed and flourish further north than in the past. In 2017 they were found in Oregon for the first time.


Turns out, the Tuna Crab is not a Crab at all.... As a matter of fact, this "Red Pelagic Crab" is actually a type of squat lobster. Most of the year they are in Baja California in the open ocean, where they filter feed on phytoplankton with tiny hairs on their legs as they drift through the ocean's water column.


Red Pelagic Crab (Pleuroncodes planipes)

I had heard for awhile of the Tuna Crab swarms that invaded La Jolla this time of year. I had friends that had warned me that they are somewhat of a nuisance, and that their presence would get old quickly. Never having seen the spectacle for myself, I was intrigued and found myself eagerly inviting the experience. When I got a text from a buddy that the invasion had begun, I was thrilled to head to the Shores and witness the swarm on a night dive.


The truth is nothing anyone would have told me would have prepared me for what I was about to experience. I have seen swarms on television shows of locusts and bird flocks, but being physically present among this amount of sheer numbers of these little crustaceans was incredible. I was stunned and in awe of how many Tuna Crabs were down there.


At about 15 feet the swarm began, and it only got denser the deeper we went. At about 60 feet, the group was piling on top of itself and blanketed the ocean floor for as far as my visibility allowed. It was amazing to see the amount of biomass in one location. It made me think how incredible the ocean is to even sustain this amount of one species. It made me consider the implications on the permanent residents of the area.


Tuna Crab invasion of the La Jolla Shores

For the sharks, rays, sea lions, dolphins, birds and whales I am confident that the invasion makes a welcome meal. To the point that by the end of their stay in a few months, the animals seem to be bored of the meal, or maybe there are just way too many Tuna Crab to eat. For whatever the reason it is common for a large amount of the population to end up washed ashore dead.


For the other bottom dwellers and competitors of the same resources to the massive swarm, perhaps the massive gathering is not such a welcomed homecoming? However the local inhabitants view the swarm, one thing is for certain. The Tuna Crabs will be here for another few weeks, after which many of them will be washed ashore dead, not to be seen again until next year.


As for the invasion being a bore or redundant, I will have to make a few more dives with them before I feel that way. For now I am thrilled to have had the experience, and I think it is worth the time to see. I have a feeling they will come to represent the spring season; the season of the Red Pelagic Tuna Crab Invasion.


If you are interested in getting in the action, we offer guided night dives at the Shores and would be more than happy to take you out on a tour to see the spectacle for yourself.



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