Invasive Crabs Changing Ecology of Narragansett Bay

By ecoRI News staff

Asian shore crabs are quite small. (Niels-Viggo Hobbs)

Asian shore crabs are quite small. (Niels-Viggo Hobbs)

KINGSTON, R.I. — Niels-Viggo Hobbs has spent a great deal of time in recent years exploring tide pools and the rocky shoreline of Rhode Island, and he said that the ecology of the shore has changed dramatically in the past two decades because of one relatively recent invader: the Asian shore crab.

“Twenty years ago if you went to the rocky shore and turned over rocks, you would have found mostly one or two species of native crabs,” said Hobbs, a doctoral candidate at the University of Rhode Island. “But within a year of the Asian shore crab showing up in Rhode Island in 1998, if you turned over those same rocks, you’d have something like 20 or 30 Asian shore crabs.”

This dramatic change in the composition of crabs in the region may have far-reaching implications for the ecology of Narragansett Bay. Many of the native crab species in Rhode Island are difficult to find. Some have been pushed into deeper water, where they are preyed upon by fish and other species they seldom encountered in the past. Others, such as the spider crab, seem to be unaffected by the invaders.

Asian shore crabs aren’t large — their shells are typically an inch or less across — but they reproduce quickly and are more tolerant of cold and being out of the water than the region’s native crabs. Hobbs said a 1-meter area that may have harbored a dozen native crabs two decades ago now harbors hundreds of Asian shore crabs.

“They eat whatever they can get their claws on, and they reproduce like crazy, so they have a lot of mouths to feed,” he said. “But they also become food for other marine life as well.”

Lobsters are one such species for which the Asian shore crab is a double-edged sword. While the shore crabs may consume large quantities of larval lobsters, the crabs are also probably eaten in large numbers by adult lobsters. Commercially important fish, such as black sea bass, also benefit from the availability of large quantities of Asian shore crabs.

Hobbs said the green crab, which is the second-most abundant crab species on Narragansett Bay shores, is also an invasive species. But it arrived in the region in the ballast of ships about 200 years ago — long before scientists paid much attention to crabs — so it’s difficult to know how its arrival may have affected the marine environment.

“The one thing we know happened after the arrival of green crabs is that snail shells got thicker and tougher,” Hobbs said. “They were apparently really good at cracking open snail shells, so the snails evolved thicker shells.”

The focus of Hobbs’ research is on the cost of aggression among crabs of the same species — for instance, how does fighting among themselves affect how quickly they grow and mature, how likely they are to be injured or die, and other factors. So he put pairs of crabs in glass jars for what he called “crab fight club” to observe how aggressively they behaved.

He found a great deal of variation. Spider crabs, which have the smallest claws, exhibit almost no aggression at all, whereas the rare lady crab is just the opposite.

“Those results likely correlate with how abundant they are in the wild,” Hobbs said. “Hundreds of spider crabs can be piled up in one small area, and if they were as aggressive as lady crabs then they would just kill each other.”

What is most interesting to Hobbs is that Asian shore crabs aren’t all that aggressive. Since they don’t fight among themselves, they can rapidly build up large populations, making it difficult for native crabs to share the same habitat.

“One of the major mechanisms helping the Asian shore crab succeed is that they’re not all that aggressive to their own species,” Hobbs said. “They’re incredibly tolerant of other Asian shore crabs, allowing them to be in such abundance that they can act like a wall keeping other species from moving into an area.”

Has the invasion of Asian shore crabs and green crabs made the Narragansett Bay ecosystem unhealthy?

“I can’t say it’s a less healthy ecosystem; it’s just a different ecosystem than it would have been without them,” Hobbs said. “There could be negative ecosystem effects — in addition to outcompeting native crabs, Asian shore crabs eat a lot of mussels, so they could be having an effect on important filter feeders in the bay. But there is no evidence that they’ve had such an impact that the ecosystem changes are significantly negative.”