The Cranky Crab That Restores Cordgrass

A European Green Crab looking particularly crusty (Photo by dreed41 / CC BY-NC-SA 2.0)

The European green crab’s demeanor has made it a long-despised and feared invasive. Originally carried from Europe’s northern coastline across the pond by ballast water, this cranky crustacean has aggressive tactics that destroy the species it hunts and drive away native competitors. But now researchers reveal in a study that this aggression is serving a positive purpose, as it helps to heal the cordgrass stretches of Cape Cod, New England.

Cordgrass, a plant that grows in waves on salt marshes and tidal flats, has come to symbolize the disastrous impacts wrought by overfishing in Cape Cod. When a saltmarsh ecosystem has been damaged, it shows up in balding stretches, as cordgrass thins and dies off. The cause of this disappearance is another crab, the Sesarma reticulatum or purple marsh crab, which feasts on fresh new cordgrass shoots.

But like everything in the food chain, the purple marsh crab’s rising impact is driven by something else. As intensive fishing and crabbing has taken hold of the landscape over the past several decades, the purple marsh crab’s natural predators were driven down, leaving the species to spread and dine copiously on cordgrass. The crabs also burrow beneath the marsh, leaving soil weakened and more likely to erode.

It might seem unlikely that the green crab, renowned especially for its destructive tendencies, would improve this scenario. “It eats about everything,” said marine ecologist and study author Mark Bertness of Brown University to Science. “In terms of biodiversity, it’s hell on wheels.”

So how has it rescued the marsh? Well, ‘rescuing’ might be an overstatement—Bertness points out that the marshes are already altered so badly that they need more than just the green crab’s help. But nevertheless, the researchers found that patches of marsh populated with more green crabs were faring better than those without. They noticed that not only were the invasive creatures occupying burrows like those of the purple marsh crab, but that they were actually taking over the burrows of purple marsh crabs they had either killed or driven out.

This explains the green crab’s two-pronged strategy: it pushes out competitors by pure force—with the help of powerful pincers and its larger size—and also by simply scaring the daylights out of purple marsh crabs. The researchers established this via two experiments. First, they found that purple marsh crabs left in cages with green crabs had only a 14 % survival rate. The rest were crushed and dismembered. In cages where just one green crab paraded in amongst a number of purple marsh crabs, the latter rather comically saw fit to stay underground for the duration of the month-long experiment.  

By moving in and evicting purple marsh crabs from cordgrass saltmarsh, the green crab has helped to manage what was once an unchecked population. “It’s important to acknowledge that introduced species will in some cases provide an unintended benefit, and this is a cool one,” said Edwin Grosholz, an ecologist at the University of California, Davis, not a study author, to Science. However, he acknowledged that it couldn’t be a one-size-fits-all approach. “[The green crab] may have a positive effect in New England,” he said. “Its track record elsewhere is quite different.”

In Cape Cod, the crab’s contribution may help cordgrass gather just enough momentum to regrow in patches, and perhaps to slowly spread its soft waves across the marsh again.