Up and down much of the East Coast, from the sandy shores of North Carolina to Narragansett Bay and farther north, to the Maine’s rocky coast, there is an underwater turf battle being waged.
Tiny Asian shore crabs, a red and brown striped species native to Japan, Korea and China, have for more than a decade been challenging local crabs and other aquatic species for food and shelter.
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The effects of the subaqueous scuffle appear relatively benign – at least for now. The alien crustaceans largely have gone unnoticed by fishermen. Marine biologists concede that the Asian crabs have caused no obvious ecological damage.
But foreign aquatic species have invaded waters elsewhere in the country with devastating effects, and New England biologists say the introduction of any non-native species raises red flags.
“The question is how are these crabs fitting into the ecosystem and the food web,” said Nancy O’Connor, an associate professor in the biology department at University of Massachusetts Dartmouth who has studied the crabs since the mid-1990s. “What are they eating? What’s eating them? The impacts have yet to be determined.”
The Asian shore crabs, sometimes called Japanese shore crabs, are thought to have appeared first on the coast of New Jersey in 1988, when they arrived in ballast water or clinging to the side of cargo ships. Since then, the species has spread all the way up to Penobscot Bay in Maine and as far south as North Carolina. The crabs, which are no larger than three inches wide, don’t suit the American palate, though the Japanese serve them as a pickled delicacy.
O’Connor first encountered an Asian shore crab in 1993, when a student found one on Cape Cod. The next year, O’Connor and her students found a number of Asian shore crabs in Buzzards Bay, and within a few years they could be found throughout Narragansett Bay.
Although the Asian crabs feed on juvenile mussels and hard-shell clams, their growing presence apparently has not alarmed local shellfishermen.
“We have heard that they were making their way up the coast, but we haven’t seen any problems from them,” said Mike McGiveney, president of the Rhode Island Shellfishermen’s Association. “But obviously we’re concerned about any invasive species in the bay. Anything that likes to eat shellfish without cocktail sauce is a concern for us.”
James T. Carlton, director of the maritime studies program at Williams College and Mystic Seaport in Mystic, Conn., who is considered a national expert on invasive marine species, said the fishing industry needs to monitor the Asian crab and any other foreign aquatic species.
“The Japanese crab represents the kind of ecological roulette that we play with invasive species,” Carlton said during a phone interview. “The crab itself, as abundant as it is, may not have destroyed any shellfish or fin-fish operations. But what if the next invasive species is the one that wipes out an entire fishery, like the oyster industry?”
Carlton said the proliferation of the Asian shore crab should serve as a “heads up” to the fishing industry to get behind legislative efforts to clamp down on the regulation of ballast waters on ships, which threaten to carry more alien marine species to southern New England. Carlton said invasive species already are arriving on the East Coast at the rate of one interloper every two years.
Just last week, legislation was introduced in the U.S. Senate that would stiffen some shipping regulations to prevent the introduction of invasive species. In a report Carlton wrote last year for the Pew Oceans Commission, he calls for tighter shipping regulations and $50 million to help guard against aquatic invaders.
Kelly Simmons, one of O’Connor’s graduate students at UMass Dartmouth, is studying the Asian shore crab’s interaction with mud crabs, which are the only native crabs along the Rhode Island and southeastern Massachusetts coast.
Her theory: The resilient and more-aggressive Asian shore crab might be encroaching on the mud crab’s habitat, eating its food and muscling into the mud crabs’ favored nooks along rocky shores and cobble beaches.
If so, Simmons said it could potentially throw out of whack a delicate marine ecosystem.
“I’m studying how these two crabs’ interactions affect the bigger picture of the food-web dynamics,” Simmons said. “I’m looking for patterns to show that the (shore crab) being here is somehow affecting – the assumption is negatively affecting – the mud crabs in their natural habitat.”
Simmons has been collecting the two species of crabs since June. In rubber boots and work gloves, clutching a clear plastic tray, she scrambles over rocky tidal pools near Roger Williams University, peering under rocks in search of both Asian and mud crabs.
Back in the lab, preliminary studies have shown that Asian crabs appear to be bullies.
Simmons uses a glass jar filled with seawater to study the interaction between the two species. She fastens a small plastic film canister over a hole drilled in the side of the jar and observes as two crabs duel over the right to occupy the makeshift shelter. In most cases, the Asian crab – even when giving up substantial size – wins the battle.
The Japanese crab also could be moving in on the turf of green crabs, which itself is an invasive species to the East Coast, although it has been here much longer and has settled into its appropriate place in the food chain. Recreational fishermen, for example, use green crabs as bait for sport fish such as tautog.
Stanley Cobb, a professor of biological sciences at the University of Rhode Island who has had several graduate students study the Asian crab, said it is not as simple as replacing one crab species with another.
He points to research by another URI biology professor, Cheryl Wilga, which has shown that sharks, which love green crabs, won’t touch the Asian crabs. Cobb says the finding illustrates the fragility of marine food chain.
Quantifying the extent of the Asian shore crabs’ proliferation has been difficult, researchers say, but anecdotally, the crabs appear abundant.
“My experience is that (the Asian crabs) seem to outnumber mud crabs two-to-one,” Simmons said. “They’re definitely one of the most numerous species out there.” In Long Island Sound, Carlton said the Asian crab, which didn’t arrive there until around 1994, has rapidly become the most abundant crab species.
Cobb, the URI professor, said the bountiful presence of a species that did not exist in Narragansett Bay a decade ago is striking.
“It’s really amazing,” Cobb said. “You can go down to the intertidal, turn over a rock and all you see are the Asian shore crabs.”
