Pea Crabs
Chuck Louch, PTMSC Docent
If you're a midwestern flatlander like me you've probably never heard of pea crabs. I hadn't until I ran across a reference to them while reading about something else. This lacuna in my knowledge is not particularly surprising since, as their name implies, pea crabs are small creatures, usually less than half an inch across that spend most of their lives as symbiots, living on or in the bodies of other invertebrates. There are many species of pea crabs, all grouped together by taxonomists into the Family Pinnotheridae; a name that is probably derived from the Greek roots, pinn(a) for mussel and thero for wild beast. This makes some sort of sense because the most familiar pea crabs are found in certain species of edible mussels and oysters, and they are undoubtedly wild.
One of these in our area is the mussel crab (Fabia subquadrata) which infests California mussels (Mytilus californianus) and horse mussels (Modiolus modiolus). It's the female of the species that is usually seen, which is to be expected since it's large for a pea crab, being about the size of the end of your thumb. What's more, it is considered by some people to be a tasty morsel in its own right, almost in a class with the oyster pea crab (Pinnotheres ostreum) that lives in the Atlantic Coast oyster and is such a delicacy that public health codes are modified so that parasitized oysters may be marketed and served in public eateries.
The life cycle of the mussel crab involves two stages that are so dissimilar that they were at one time placed in different genera. The first stage, the familiar edible one, is composed entirely of large, adult females. They have soft, membranous shells and each one lives by itself within its mussel host. These females produce broods of larvae that mature into the second stage; small adults of both sexes. Upon reaching maturity these small crabs leave their hosts to join mating swarms in open water. In this planktonic stage they are traditional-looking crabs with a hard shell, swimming legs densely fringed with hair, and a thick fringe of hair along the front of the carapace. After mating, each gravid female enters a mussel where, over a period of 21 to 25 weeks, she goes through five molts before reaching maturity. She lives in her mussel home for about a year, producing larvae from eggs fertilized by sperm from her single mating. Nothing is known about the fate of the males; perhaps they return to mussels to live out their lives or maybe they wander forlornly in the plankton until overtaken by the Grim Reaper; nobody knows. In Puget Sound mating swarms occur in late May.
It seems that a great variety of marine invertebrates serve as hosts for many species of pea crabs. Thus gaper pea crabs live in the mantle cavities of horse clams while burrow pea crabs, as their name implies, live in the burrows of ghost shrimps, mud shrimps, and fat innkeeper worms - yes, that's their real name! There's another species that lives on or in the bodies of gumboot chitons and California sea cucumbers. Still others live in the gills of sea squirts, on the under surface of sand dollars, in scallops, and the list goes on. Each species of pea crab responds positively only to chemicals produced by its particular host species.
The advantage of these symbiotic arrangements for pea crabs is that they get to live in protected environments while scavenging food from their hosts. In many cases they are commensals, that is, they apparently do no harm to the animals that harbor them. But sometimes they can be detrimental and so are classed as parasites. The mussel crab is a case in point for it robs a significant amount of food from its host and also feeds on the protective mucus layers that cover its tender tissues thus injuring the mussel's gills.
The seas are full of tiny, inconspicuous creatures like pea crabs but most of us are unaware of them. We become so enthralled with the more spectacular animals of the ocean; whales, birds, sea lions, and sharks, that we forget that there can be drama in a tidal pool, an eel grass flat, or even a glass of sea water. We really don't need fancy boats or expensive equipment in order to make interesting observations about marine life. As Rachel Carson has taught us in her wonderful books, all we need is an observant eye, patience, attention to detail, and an active mind. Also, it sometimes helps to think small.
Additional Reading:
G.C. Jensen: Pacific Coast Crabs and Shrimps. Sea Challengers.
Ricketts, Calvin, and Phillips: Between Pacific Tides. Stanford University Press.
E. Kozloff: Seashore Life on the Northern Pacific Coast. Univ.of Washington Press.
Rachel Carson: Under the Sea Wind and The Sea Around Us.
Barnes: Invertebrate Zoology. Saunders.