Dive Dry with Dr. Bill

#644: Tuna Crabs... A Closer Look

I rarely write about the same critter in the same year... heaven knows there are thousands of them in our region to write about, so I don't need to repeat myself too often. However, tuna or pelagic red crabs (Pleuroncodes planipes) continue to inundate southern California beaches, including our own here on Catalina, and therefore are of ongoing interest to many.

Now to call these "crabs" isn't scientifically accurate. True crabs belong in the group Brachyura while the tuna "crabs" are in the group Anomura. They are all crustaceans in the phylum Arthropoda (jointed foot) but the tuna "crabs" are related to the squat lobster and hermit "crabs." Of course if you asked a tuna crab, hermit crab, lobster or striped shore crab; they wouldn't have a clue about these classifications since they are primarily a system devised by humans for our benefit, and based on features we think are important. Perhaps the critters could care less!

A week ago lifeguard Paul (aka "Rambo") came over to tell me the tuna crabs were once again floundering onto our beaches where they were quickly dying due to desiccation and/or seagulls ("Mine... mine... mine"). I waded out into the water and snagged four live ones as they were about to body surf onto South Beach. I placed them in a handy Bud Lite cup, brought them up to the house and put them in the refrigerator alongside my fajita makings. I was careful not to confuse the two when I went to prepare dinner that night, as the meat in the tail of a tuna "crab" would barely fill a flea.

Several days later the four "crabs" are still alive and kicking. The cool water undoubtedly was responsible, in part because it contains more oxygen, slows down body metabolism and is also more like the "crabs" deep water habitat. I noted previously that these crustaceans are found in colder depths for part of their life, but at some point they migrate vertically through the water column into surface waters where we observe them, especially when currents or winds take them toward SoCal beaches.

The normal range for this species is off the Pacific coast of Baja California. However, during warm water periods like we are experiencing now, they often extend their range into southern and even central California. This is especially true when a north flowing counter current establishes itself flowing along the coast from Baja into our region. Thus they are often associated with El Niño events.

I took advantage of the four tuna "crabs" I had collected by bringing them out into the sun to study and film them. They were too big to accomplish much with any of my microscopes, so I simply used my camcorder in macro mode to take close-ups of their anatomy. They were still active enough to try to pinch me with their claws, so I handled them carefully.

The big eyes are prominent features, probably a result of having to find food in the darker depths or midwater where they may feed at night. Notice the sharp, pointed structure known as the rostrum between the eyes. It affords some protection for these critical sensors.

Some reports on the Internet state that this species feeds mostly on plankton. Of course you can't believe everything you read on the Internet... not even my columns. I wrote about the encounter we had on the Catallac flying fish boat a few weeks ago where the tuna "crabs" converged on anchovy dying after attacks on their school by seagulls, bonito and yellowtail. While feeding on them, the "crabs" used their claws to tear pieces of flesh off to munch on. The poor anchovy were being eaten alive (but that's "ecology" at work). Tuna "crabs" are definitely omnivores just like Dr. Bill... I'll eat anything (almost).

Looking closely at the other legs, one can see many fine "hairs" referred to by biologists as setae. The function of these has been a question of interest to scientists. Since they are extensions of the legs, they are believed to assist in swimming and also act as "parachutes" to slow the descent of the animal in the water column. Others have observed them being used on the bottom to transport food-bearing detritus to the mouth. It is also possible they may be used to filter plankton out of the water since tuna "crabs" are known to feed even on small plant plankton (phytoplankton).

Of course we hope that the appearance of these critters in SoCal waters is a harbinger of things to come... namely the heavy rains often associated with El Niños. I really need more than one or two showers a week even though I'm often submerged in the ocean. Unfortunately, the flip side of an El Niño is the probability that our giant kelp forests will suffer another year of destruction. We need the rain to break the drought and the kelp forests will ultimately come back. I'll just have to spend time learning about coral reef ecology as I travel to the Egyptian Red Sea, and back to the Philippines and Palau over the next nine months.

© 2015 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Tuna "crab" in petri dish, its big eyes and antenna; the cheliped or claw and the fine hairs or setae on the other legs.

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia