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Dive Dry with Dr. Bill

#429: A "Pea" in a "Pod"

Some time back I was diving with Jason Manix off Moonstone Cove to film squid and fire salps. When Jason returned to the boat, he had several rock scallops in his goodie bag to share with the rest of us. I stopped taking marine life back in the mid 1970s as a strictly personal decision, but have no problem with those that do... especially when they share their take with me! Besides, it's really hard to capture anything with a camera in one hand, and I'd much rather "take" pictures and video I can share with divers and non-divers alike. Back in 1969 the first critter I tasted from our waters was a rock scallop on my very first dive off Catalina at Arrow Point. Sweet! Some of my associates in the conservation movement wonder how I can eat any of our friends from the sea, but I do it in such moderation I don't worry about it.

When Jason opened the scallops, two out of the four contained a tiny crab that I quickly identified as a pea crab. There are several species of these crustaceans, most of which live inside the shells or burrows of other marine life, and some inside the "cavities" of certain others. This is a wise choice for something so small most fish could gulp them down whole. Personally, I have a strong distaste for peas... but one of my sisters used to love them, and I could pass mine to her under the table.

Now I am no expert on pea crabs, but I believe this one may be the mussel pea crab, Fabia subquadrata. Given my aging eyes, it is hard for me to study such tiny life without my microscope, and I didn't take it out on the boat with us. Males are usually less than 1/4" and females may reach just over 1/2 inch in width. The geographic range of this species is reported as from the Aleutian Islands in Alaska to La Jolla, although they are rare south of San Pedro. In this southern transition region, another species (Fabia concharum) takes over and is found from San Pedro down into Baja. If I were a betting man, I'd put what little money I have on the former... but I'm not! I don't bet even on a sure thing!

There are many other species of pea crab, but this one frequents the shells of mussels and clams. Some refer to this as commensalism, an ecological relationship where one animal benefits and the other is not affected. However, others believe the pea crabs actually harm their hosts and this should be referred to as an example of parasitism. The crab does get protection from the hard shells of scallops, clams and mussels... at least until Jason appears. The scallop and other bivalves filter food out of the water and transfer it in a mucus sheath to their "mouth." The crab uses its claws to intercept some of this from one of the two gills, and often damages that gill in the process. It very nicely allows the mollusc to feed freely from the other gill.

Most of the pea crabs found in bivalve shells are sexually mature females. They have soft shells rather than the hard shells one normally associates with crabs. Without the protection of the scallop, clam or mussel shell, they would be extremely vulnerable. These females are very hostile towards others of their species, and will not allow another one to enter the shell. The presence of more than one crab would probably kill the host mollusc. Females may carry eggs throughout much of the year, with one individual having about 1,200 of them. When the eggs hatch, the tiny crab larvae enter the surrounding water and drift with the plankton as they pass through several different life stages. By the time they reach just under 1mm (1/25"), both males and females seek bivalve hosts, usually of smaller mollusc species.

Now, if you're really perceptive (and my readers usually are), you may be asking... how do they mate if the males and females are in separate mollusc shells? At the appropriate time, both sexes shed their exoskeletons, grow new hard ones and exit the bivalve stage left (or right) to enter the plankton. Here in open water they once again are together and, if luck has it, mate for the first... and only... time. Mated females return to larger mollusc hosts, and shed their hard shells once they are safely inside. The ladies store the sperm from their single mating for the rest of their life, a year or at most two, and may produce more than one brood using it.

At least half of you are probably wondering what happens to the males. It is believed that a few of them may return to hosts where they live a short, solitary life. Scientific reports suggest most of them are never seen again, their primary purpose in life over. Talk about women's liberation. Of course the female grows so big over the rest of her life that a tryst with one of the tiny males later in life is almost like suggesting a flea and an elephant can mate. I'm sure at least half my readers are very glad that they did not enter life as a pea crab!

© 2011 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. 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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Male rock scallop attached to, and hinged shell of one detached from the rock;
pea crab inside the shell of a rock scallop (courtesy of Jason Manix).

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia