Mussel-Bound Crab

A naturalist plays hide-and-seek with a tiny crustacean.

My search for the elusive pea crab began almost a year ago on the rocky coast of northern California. It was a matter of simple curiosity at first--chasing down yet another fascinating story of animal diversity in the intertidal zone--but the longer it took to find the crab, the more obsessed I became. Before my quest ended, I had crisscrossed the United States twice, talked with fishermen on Cape Cod, recruited the assistance of a diver on Maui, engaged the services of oystermen and a marine biologist at Chesapeake Bay, and even had a few New England chefs looking on my behalf. I was in Rhode Island when I finally laid eyes on a pea crab--on the tailgate of my truck.

Not that the pea crab is rare. One species or another in the genus Pinnotheres can be found on both sides of the Atlantic and Pacific Oceans. And they've been the subject of study from the British Isles to Brazil and from California to Taiwan. My difficulty in finding them had to do with their unusual habits. The male, impossibly small and cryptically colored, is skittish; the female lives most of her life out of sight--inside the shells of live mussels, oysters, scallops, and clams and in the tubes of the marine worms known as polychaetes. And like many small animals in ever changing environments, their numbers rise and fall. You might find a crab in eight out of ten shellfish, or you might find none. Everywhere I went, there had been "plenty the week before" according to locals, but none at the moment I was inquiring. I just couldn't hit it right until David Beutel, of the University of Rhode Island's Fisheries Center, put on his boots to help. Wading into the submerged tide pools at Black Point on Rhode Island Sound, he probed among the rocks, plunging his arms into the chilly water to grope for dense clusters of mussels held together with intertwined strands of algae. Soon he emerged with a half-full bucket containing several dozen medium-sized mollusks. I carried the shells over to the back of my truck and went to work with a shucking knife. Within a few minutes I sighted my first pea crab, hunkering in the shadowy interior of a living mussel.

Pea crabs are appropriately named, not just for their small size but for the female's resemblance in shape and texture to a slightly flattened pea or bean. Soft and succulent looking, this crab was a female with a caramel brown coloring that matched rather closely the shade of the mussel's flesh (females occupying other shellfish range from white to salmon pink, but whether this coloration is adaptive is unknown). Although the mussel I found it in was less than two inches long, the crab had grown to nearly half an inch, exclusive of its legs, and occupied almost a quarter of the mussel's mantle cavity--not bad, considering the crab's inauspicious start.

All pea crabs begin their lives as free-swimming larvae less than a millimeter long. Males remain mobile throughout their two- to three-year life span and, like most other crabs, eventually develop into hard-shelled adults with prominent eyes and relatively stout chelipeds, or claws. But the males grow to no more than about 3/16 inch wide. Bristles on their second and third pairs of legs serve as an aid in swimming, and their flattened body, considerably thinner than that of the female, enables them to slip freely in and out of the narrow gape of mollusk shells as they shuttle from one temporary home to another. Females, on the other hand, generally find their way into the mantle cavity of a bivalve long before development is complete. There they mature, eventually attaining two or three times the width and thickness of the male but without the male's hard shell or bulbous claws, and with their eyes nearly concealed by a soft carapace. Poor swimmers, they remain within an individual mollusk, picking food particles off the gills or mucus strands of their filter-feeding host.

The freeloading habits of pea crabs are not news. Guillaume Rondelet's 1554 work Libri de Piscibus Marinis includes an illustration of a tiny crab, with somewhat exaggerated chelipeds, emerging from a mussel. (This drawing is reproduced at the top of the opposite page.) For most of the four and a half centuries since the publication of this treatise (and even today in some literature), the relationship between crab and mollusk has been considered commensal, with the crab benefiting by the association but neither hurting nor helping its host. The validity of this assumption is being reevaluated, however, and newer evidence suggests a less benign relationship. Compared with its nine-inch, clam-crushing relative the blue crab, this diminutive scavenger would hardly appear threatening, but its constant picking at the host's gills can result in considerable wear and tear over the years, and studies now show that the gill lesions frequently observed in occupied mollusks result in less efficient water filtration by the host. This compromised filtration, coupled with the crab's interception of food, can add up to significant metabolic costs for the host: both lower respiration rates and slower growth. Thus, while the crabs cause no immediately life-threatening damage, the female pea crab--unable to survive outside its host once it becomes established--is by all other measures a true parasite.