On Monday, my husband and I were over at his parents’ house and he got busy shucking some oysters a friend had dropped off — Eastern oysters, Crassostrea virginica. I was chatting with my father-in-law when my husband started hollering about something from the kitchen, something strange he’d found in the oyster he had just opened.
He comes over and shows us the shell, and there, perched right inside was a very round, very tiny, very translucent crab. Its innards were clearly visible, fluffy-looking red streaks, and we figured it was dead, since it was inside an oyster and the oysters had been in the fridge. We had quite the laugh, thinking about the silly thing crawling into an oyster at a moment when its shell was gaping open. But we were also totally puzzled: my father-in-law was convinced it was a blue crab; my husband thought it to be a fiddler; I had no earthly idea. So we put it into some tap water mixed with alcohol (gin, to be exact, since it was the nearest thing handy on the counter), reckoning that might “preserve” it since I said right away I wanted to take it to a local university to find out what it was.
A few minutes later, I held it up to a window to check it out more clearly. What it was doing, clearly, was moving — very slowly, very sluggishly, obviously dying but still alive nonetheless. So we put it back into fresh water in a small jar (…okay, so it was a pill bottle, since again, it was the nearest small container that was handy), and popped it into the fridge.
The next afternoon, we dropped it off at George Mason University’s biology office. I had to leave a note, since all the faculty were in a meeting, but the grad student at the desk was all, “Wow, cool. Oh, wow. Cool.”
I never expected such a fast reply, but an assistant professor e-mailed me that very evening:
The critter you found was an oyster pea crab (a full grown female). These are quite common, but can surprise the unsuspecting oyster consumer (as you discovered). They live most of their lives with the oyster in its shell. They feed on food particles filtered by the oyster and sometimes on the oyster itself.
Thanks for sharing it. We will make it part of the collection for our invertebrate zoology class.
I was relieved that it was this kind of crab, unsettling as it was to find it inside an oyster, and not another species I’d come across in my Google crab research: the Chinese mitten crab (Eriocheir sinensis), so named for its furry pincers.
Though they’re non-native, it seems that they are not yet what you’d call widespread: as of February 2008, the Smithsonian’s Marine Invasions Research Lab had reported only 13 individuals captured from the Chesapeake Bay, along with a number of others along the coast in New Jersey, New York and in various places in the Great Lakes, where they are believed to have originated through release of ships’ ballast water there. But, according to the USGS, a lone female can lay up to 1 million eggs at one time:
Mitten crabs spend most of their time in fresh water but move to brackish water to reproduce (at least 26 ppt). Their primary food is aquatic vegetation, but will also feed on mollusks, fish, and other crabs. Mature adults are 38-50 mm in carapace width. A female can lay up to 1 million eggs in a one event. One effect of the mitten crab could be the collapse of support structures following successive years of mitten crab burrowing (Ingle, 1986). In the Far East, mitten crabs are an intermediate hosts of lungflukes, Paragonimus ringeri, which occur in humans (Ingle, 1986)
The USGS adds that where it has occurred in California, the mitten crab has become troublesome because of its tendency to congregate around water pumping facilities’ intake pipes.
One way to approach thinking about invasives–along with other elements of environmental change, whether or not you generally agree with it–is this: risk management. (Thanks to my brother for this idea; I read an old post of his several days ago that mentioned risk management.) What are the risks of non-native species introductions–to humans, to native flora and fauna, to infrastructure, to the economy?
I’ll wrap this up by saying that I was on my way to try and find an easy-to-summarize, negative economic example of invasives, but got sidetracked by what looks to be a really interesting news website: Sightline Daily.