Way Too Easy Bein’ Green?

On the U.S. east coast, a small crustacean is a large problem. The European green crab – you guessed it, an invasive species – is decimating shellfish stocks and disrupting coastal ecosystems from Maine to Virginia. It’s a major menace in Maine; despite their smallness (their carapace measures only about three inches), they have an outsized appetite for soft-shell clams, one of the state’s most important fisheries.

Green crabs, Carcinus maenas, have been in North America for over two centuries. With a mottled, multihued shell, they’re not completely green. They were first spotted in Massachusetts in 1817, probably transported on ships’ hulls or in ballast water. Since then, their range has expanded north and south, and elsewhere around the globe. They’re now on every continent except Antarctica. They’re voracious, omnivorous predators, eating everything from shellfish to marine worms – even their own offspring at times. A single crab may chow down on forty clams a day. In their wake, New England clam populations have plummeted. And they’ve devastated ecologically valuable seagrass beds and salt marshes by clipping the plants’ shoots, presumably to expose prey. 

Green crabs are especially good at making more green crabs. A single ripe female may produce close to 200,000 eggs. They’re highly adaptable, surviving in salinities from full seawater to nearly fresh, and in temperatures from freezing to 86 degrees Fahrenheit. 

Given their ravenous appetites, impressive fecundity, and broad environmental tolerance, green crab populations have soared where they were introduced. They’re in the top 100 of the World’s Worst Invasive Alien Species. They were the first marine organism listed by the Aquatic Nuisance Species Task Force of the U.S. Fish and Wildlife Service. The Task Force even established a special Green Crab Control Committee. Several states, including Maryland, have designated them as a regulated nuisance species. But their numbers keep growing.

So what to do?

We’ve tried every imaginable control method. We’ve regulated them, scooped them off the seabed, trapped them, placed a bounty on them, used them as bait, and ground them up into fertilizer. We’ve hardly made a dent in their populations. Recently, control strategies have moved to marketing green crabs as culinary items – perhaps a case of If you can’t beat ‘em, eat ‘em. Entrées include whole soft-shell crabs and minced green crab patties and empanadas. One entrepreneur has even devised a green crab cocktail, the Oyster and Crab Shooter; for a recipe visit GreenCrabCafe.com. You might also check out The Green Crab Cookbook, available on Amazon.

Not sold? You’d have lots of company. Still, a combination of these control approaches, with a focus on effective management strategies, will pay dividends over time. But by then will we still have a clam fishery? Or oysters?

So far, Chesapeake Bay has avoided serious green crab impacts. The invaders have moved into mid-Atlantic coastal waters, areas around Ocean City, and into the mouth of the Bay. Will they advance farther up the estuary? Scientists aren’t sure. On one hand, the Bay’s entrance is the southernmost extent of the crabs’ present range. The Chesapeake’s higher temperatures and lower salinities than their native European waters may keep them from coming up the Bay. On the other hand, the species’ broad environmental tolerances and evident genetic adaptability, even to a changing climate, may facilitate their spread.

It may be that another iconic crab species is presently checking the advance of the invaders. The Bay’s beloved blue crab, Callinectes sapidus, is one of the few regional predators of European green crabs; some scientists even think the blues are the chief deterrent to the spread of the greens. But adult greens eat baby blues, so the outcome may be a wash.

Regardless, it pays to strongly support green crab management strategies, indeed effective control measures for all invasive species in the Bay. You can do your part by learning more about these unwelcome crustaceans and by not transporting them from one area to another, either carelessly in bilge water on unwashed boat bottoms, or deliberately, as bait items. And you could always try eating them.

Bon appetit!

Henry (“Hank”) Parker is a scientist and writer who previously lived in Annapolis but now lives in Vermont.