If you ever come across an open oyster with a tiny crab-looking critter dwelling comfortably in it, you should eat it. Though your first thought may be to chuck the oyster and crab and complain to your server that your oyster treat had been invaded and occupied by an unwanted houseguest, avoid the knee-jerk reaction and just consider yourself lucky, one of the chosen few. Seriously, you have just received one of those rare gifts of nature that not every oyster provides.
The crab that you find wriggling in your oyster is not your typical blue crab. It's actually a different species entirely. It's called an oyster crab, or pea crab, and if you speak Latin it goes by the name Zaops Ostreum. The oyster crab is a tiny animal that is typically found inhabiting oysters harvested from the Chesapeake Bay area. They range as far south as Brazil, but you won't find any taking residence in oysters from our northern neighbors. Massachusetts eat your heart out. It's most often the female crabs that live inside live oysters, feeding on what the oysters eat, while the males are free-swimming wanderers that fend for themselves. The crabs stunt the oyster's growth, but beyond that live with the oyster in harmony, making a life and reproducing in the oyster until finding themselves under the blade of someone's oyster knife.
To experienced oyster shuckers and gourmands, these small crabs are more than a novelty, they're an absolute delicacy. The New York Times was reporting on the merits of their flavor as far back as 1913, though today I do not think that many people living in the Chesapeake Bay area even know of their existence. That's a hard fact to explain since it's the Bay areas oysters in which the crabs call home. Their flavor has been described as delicate and shrimp-like. From my own experience I think they taste sweeter than most shrimp, with grassy, mineral notes and a little bit of sea salt. They offer a nice crunch and a crazy umami flavor, one you might find in a fine broth that took days to make.
Most of the time oyster crabs are removed from the raw oysters by the shucker or chef before they are served to the guest. I guess it's the industry's way of "protecting" their guests from "undesired" tenants. The majority of diners wouldn't be too excited to find a tiny, living creature in their freshly shucked oyster. Two live animals in one shell is two too many. It's hard to get the word out that these crabs are really delicious. It's even harder to get people to eat food that is still moving. Yes, if you had to, you could save the crabs and cook them, but really that's an unnecessary use of energy. Cooking does nothing to improve on what nature has already provided in this case. All I know is that I'll take my local oysters with crab, as many as I can get, and consider myself lucky. This doesn't mean you will be seeing more restaurants leaving the crabs in the oysters. But now that you know, and if you are lucky enough, it might be a good idea that the next time you settle in at the oyster bar to give the shucker a heads up, that you'll take any oyster found with a crab in it as a packaged deal.