What are pea crabs and why are they in my oysters?

If you've ever encountered a pea crab in your life, it's very likely that you saw one while eating an oyster! These little critters live in the gills of oysters, and sometimes will come out to play when your half shell is sitting on a plate. Many chefs and diners find them unappetizing or a nuisance, especially if they're scuttling across an oyster, but since little is known about them in the culinary world, they may be misunderstood. In 1907, The New York Times published an article titled "A Rare Delicacy, Little Known" and called pea crabs "one of the sweetest and quaintest viands known to man... so generally neglected that more than 50 per cent of the people who think they know something about good eating have never tasted the dish." So what are pea crabs? And what should you do the next time you see one?

About the Pea Crab

Pea crabs (Pinnotheres ostreum) or oyster crabs (Zaops ostreus) are small soft-bodied crabs that live in bivalves such as oysters and mussels. They are kleptoparasites, which means they steal food from their host to survive. Once they enter an oyster, they live inside the oyster's gills and feed on the food that filters in. Since both crabs behave similarly, we will refer to both generally as "pea crabs."

Pea crabs find their oyster hosts very early on when both are still larvae. The crabs spawn a month after the oysters, which allows them to find oyster spat settled out of the water column. Pea crabs are free-swimming in the early stages to seek out their oyster hosts, but males remain free-swimming for life to find mates from oyster to oyster. Once female larvae find their hosts, they remain there until maturity and lay their eggs inside. That's why you may find more than one pea crab in an oyster.

Although pea crabs are kleptoparasites, their effects on oysters vary during the time of year and do not seem to increase oyster mortality.* Aside from gill damage, an oyster can grow healthily while hosting a pea crab, but it's health will diminish when there is less food in the water because the pea crab will feed first. The oyster will not die, but its meat will become thin. During the summer and fall when food is plentiful, oysters can be very plump even with the presence of pea crabs because the oysters are filtering enough food to feed themselves and their guests. Though their relationship is defined as a parasitic one, the oyster and the pea crab have more of a commensal relationship when compared to other parasites and hosts.

Why are there pea crabs in certain oyster varieties?

Pea crabs can be found on the Atlantic Coast from North America to Brazil and along the Pacific coast -- they live where their hosts live. That being said, pea crabs seem to be more common in certain oyster areas over others. To investigate this variability in parasite prevalence, a team of scientists conducted a large-scale experiment across 700 km of the southeastern Atlantic coastline. They planted juvenile oysters in six locations with similar environmental properties and measured the rate of pea crab infection, which varied from 24 to 73% across sites.

This experiment was significant because it demonstrated a positive correlation between oyster recruitment and pea crab infection, meaning when there were more natural oyster spat in the water, there were also more pea crabs. The highest levels of pea crab infection occurred in the Georgia/South Carolina region, which also had the highest levels of natural oyster spat. This finding says a lot because we typically hear about pea crabs in wild oyster varieties from Long Island Sound and the Mid-Atlantic.

These two regions are homes to many wild oyster fisheries, so many of the oysters will naturally spawn and release oyster spat into the water column during the summer. Baby oysters and pea crabs favor the same water conditions, so if the oyster spat thrive in these waters, the pea crabs will also, especially while baby oysters are abundant!

So what about cold water oysters? Why does there seem to be less pea crabs in them? When we consider oysters from Massachusetts, many of the oysters are farmed, not wild set. Growers purchase oyster seed at a size that is less susceptible to pea crab infection. Then what about the wild fisheries in Onset or Wellfleet? I posed this question to Tanya Rogers, an author of the correlation experiment, and she hypothesized that "[water] temperature probably does play a role."

Crab development is affected by temperature, and if temperatures are too cold, which they never were in the Southeast, but they might be [in Massachusetts], crab populations could be limited by cold temperature more so than oyster recruitment.
— Tanya Rogers, PhD Student at Kimbro Marine Science Center Lab

I Found a pea crab -- What should I do with it?

Despite its spider-like appearance, pea crabs were known to be a delicacy and one of George Washington's favorite foods! Some in the Maryland area still regard them with love and enjoy eating them raw. In the same New York Times article, the author suggests a number of ways pea crabs "may be prepared for the table." Surprisingly, there were no modern recipes online, so we listed a few recipes found in old articles and cookbooks below. So instead of yelping and disposing your pea crabs next time, look at them in a new light and try some of these recipes:

Arrange a small mound of oyster crabs on a crisp leaf or lettuce and cover lightly with a mayonnaise that has been tinted a faint pink by the use of beet juice. Garnish with tiny bits of lemon.

The crabs are also delicious if fried in much the same manner as one would fry whitebait. To do this dip the crabs in flour, afterward in cold milk, and finally, in powdered cracker crumbs. Put them in a colander and shake them gently... then drop them into very hot fat, but do not let them stay over three minutes. Garnish with fried parsley before serving.
— The New York Times, "A Rare Delicacy, Little Known"
HOW TO COOK SHELL FISH, BY OLIVE GREEN, P. 235

HOW TO COOK SHELL FISH, BY OLIVE GREEN, P. 235

How To Cook Shell Fish, by Olive Green, p. 236-237

How To Cook Shell Fish, by Olive Green, p. 236-237


How do you feel about pea crabs? Do you like them or do they annoy you? Have you ever had them raw? Share your experiences and thoughts on pea crabs with us!

 

References

J.E. Byers, T.L. Rogers, J.H. Grabowski, et. al., Host and Parasite recruitment correlated at a regional scale, Oecologia (2014) 174:731-738.

A.M. Christensen, J.J. McDermott., Life-history and biology of the oyster crab, pinnotheres ostreum say, Biol Bull 114:146-179.

Unknown author, Oyster Crabs - The Epicure's Delight, The New York Times, November 9, 1913.

Unknown author, A Rare Delicacy, Little Known, The New York Times, December 15, 1907.

Olive Green, How To Cook Shell Fish, G.P. Putnam's Sons, New York & London: 1907.