Dive Dry with Dr. Bill

#640: Invasion of the Tuna Crabs

Last Friday I got a call at 6:00 am from friend and local boat captain Sandra. Normally I'd still be sawing logs at that time since my alarm is set for 6:40, but I'd gotten up at 5:30 that morning and was still nursing my first cup of coffee, hoping my eyes would begin to focus so I could get to work on my computer. Sandra said there were red crabs all over the harbor so I grabbed whatever clothes I could (without concern for whether they matched), hopped into the Dr. Bill Mobile and drove downtown with my camcorder.

From the end of our Pleasure Pier I could see what looked like rain drops falling on the water's surface. Of course I knew these were the red crabs breaking the surface. The sky was overcast, and without adequate sunlight it was difficult to get any good footage, but there were plenty of them. I'm sure many residents and visitors are curious about these occasional critters so here's my take on their appearance.

I first saw red or tuna crabs here back in the 1970s, certainly during the 1978-79 El Niño event but possibly earlier. Their appearance in our waters usually coincides with warm water events. Why? Their normal distribution is off Mexico, but when warm water develops they appear off our coast as well thanks to the north-flowing California Counter-current that develops. I've seen them in every decade I've lived here, sometimes covering the beaches after they kick the bucket.

Tuna crabs (Pleuroncodes planipes), also known as red crabs, longostilla and occasionally longostino; are normally found from Mexico south to Chile. For part of their life they are pelagic, residing out in open water, but are also found on the bottom. During the pelagic stage, they are actually members of the plankton feeding on phytoplankton and small zooplankton using the hairs on their legs to capture them. In doing so they provide a vital link in the planktonic food webs by consuming critters that rely on sunlight to produce food and transferring that energy up the food chain when they become dinner themselves for birds, marine mammals (including whales) and fish. Given their name it is probably obvious that tuna eat them, but they are joined by billfish, yellowtail and others. Even turtles and squid munch down on them.

A dive friend from San Diego replied to my Facebook post about them by pointing me to an article written in the San Diego Reader. It stated that these were suicidal baby lobster that beached themselves on the shores of Baja. One reader even suggested they were a product of Fukushima radiation. I guess this is a result of the rather poor science education too many receive. Tuna crabs existed long before nuclear power plants were even thought of. Tuna crabs are related more closely to the squat lobsters than true lobster, and certainly aren't babies of our spiny lobster. Look at those tiny tails! Not even a mouthful.

I can't write about their sex lives since I've never observed that. Some researchers believe that they leave their bottom-dwelling or benthic stage in spring and ascend into the upper water column and that these swarms are related to reproduction. Their gatherings may number as high as 200 billion individuals! Scientists were initially puzzled by the appearance of their larvae out in deeper water since they are usually found in shallower coastal water as adults. Eventually they discovered that the larvae get swept off the coast by the California Current and that after they grow up a bit, they take deeper water currents back to shore.

John and Karen from Afishinado recently invited me out on the Catallac with some of the organizers for the Bud Light Whatever USA event here. Captain John saw seagulls feeding on a baitball and slowly motored over so we could see it close-up. What we witnessed was an amazing food web all in one place. It appeared the gulls were feeding on young baitfish and possibly the tuna crabs. Those baitfish that were mortally wounded during the onslaught, but not consumed, were surrounded by the tuna crabs that began feeding on them. I'm wondering if that might be a new choice with respect to their menu selections.

Then from below came a rush of bonito feeding on the tuna crabs and baitfish followed by a school of yellowtail. To add insult to injury, sea lions appeared to polish off a few bonito and yellowtail, possibly with a tasty tuna crab cocktail as an hors d'oeuvre. It was pretty amazing to observe The Mutual Eating Society (as I referred to it when teaching marine biology back at the Catalina Island School). The only thing that might have completed the picture would have been a great white shark chowing down on one of the sea lions. Since I didn't take a camera with me, that could have been a real possibility!

© 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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Images of the tuna or red crabs courtesy of Phillip Cola and tuna crabs washed up on South Beach

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia