Fall 2007

A DIME-SIZED TROPICAL crab that has invaded coastal waters in the Southeast United States is having both positive and negative effects on oyster reefs, leaving researchers unable to predict what the creature’s long-term impact will be.

Tiny green porcelain crabs are now found in large numbers within oyster reefs on the coast of the Southeast United States.

Unlike native crabs that eat baby oysters, mussels and fish, the green porcelain crab Petrolisthes armatus is a filter feeder, extracting its food from the water much as oysters do. The fast-reproducing invader therefore isn’t directly attacking oyster populations, though it may be competing with them for food – and may impact the predators that normally attack the oysters.

Georgia Tech researchers have spent more than three years studying the effects of the crab, and recently reported their findings in the journal Biological Invasions. The research, believed to be the first to document effects of the crab on oyster and mussel populations off the Southeast coast, was supported by the U.S. Environmental Protection Agency, the National Oceanic and Atmospheric Administration, and the Harry and Linda Teasley Endowment to Georgia Tech.

“We’re seeing opposing effects from these crabs,” says Mark Hay, a professor in Georgia Tech’s School of Biology. “They are probably having more impact on the ecosystem by being prey than by being predators. Other members of the ecosystem are feeding on them, and that is changing the rate at which fish and other crabs are feeding on the native species.”

The impact of the new crabs is important because oysters are a “foundation species” essential to the health of coastal ecosystems because their reefs provide homes to dozens of other creatures.

“These non-native crabs slow the rate of growth for organisms like oysters that they compete with, but they enhance the ability of those same organisms to survive when young,” Hay says. “They are probably competing with the oysters for food, but the native crabs have switched to eating these green porcelain crabs rather than eating the baby oysters. Even though their growth is suppressed, the baby oysters are not being attacked as much now by the native consumers.”

Though the crabs aren’t killing existing populations of oysters, their long-term impact could still be significant. For instance, Hay notes, their availability as food could potentially increase the population of native crabs, disrupting the delicate balance between those predators and the oysters.

Researcher Amanda Hollebone examines oyster beds for the presence of the non-native crab.

Assessing the long-term impact of the invading crabs has been difficult because the creatures reproduce and grow rapidly, flooding the shallow coastal waters with their young. In research conducted off Skidaway Island and Sapelo Island on the Georgia coast, the researchers found “extraordinarily high” populations of the crab – as many as 11,000 individuals per square meter.

To assess the impacts of the non-native crab population, graduate student Amanda Hollebone placed oysters and mussels into large baskets and located them on mud flats away from existing oyster reefs. Some of the baskets contained only oysters and mussels and were intended to serve as controls; some had a community of oysters, mussels, oyster drills and native mud crabs, while others had the same community spiked with non-native crabs. The distance from the existing oyster reefs was expected to prevent adult green porcelain crabs from reaching the baskets.

However, the researchers found that within a month, the control baskets also had large populations of the green porcelain crabs that had reached the containers as juveniles settling from the water column. Entry of the crabs to the control baskets interfered with the researchers’ ability to compare the traits of communities with and without the non-native crabs.

“You get a true understanding of the sheer densities of these crabs only when you actually pick up or dig through clumps of oysters and oyster shell hash,” says Hollebone, now a temporary assistant professor at Georgia Southern University. “Particularly in the summer months, I was never able to find a patch of oysters in the Savannah area that did not have the green porcelain crab.”

Because the crabs quickly took over the control baskets, the researchers only had valid comparison data for four to six weeks. However, information from their baskets supported the observations made under more controlled – but less natural – conditions at Georgia Tech’s laboratory at the Skidaway Institute of Oceanography near Savannah.

As in the lab experiments, the researchers found that the crabs slowed the growth of small oysters, but not small mussels.

The green porcelain crabs were observed in Florida during 1990s, but have since appeared in large numbers in coastal waters of Georgia and South Carolina. Researchers don’t know if they hitched a ride in the ballast of ships, whether warming water temperatures encouraged a northerly migration – or both.