Tropical crab invades Georgia oyster reefs -- but the long-term impact can't be...( A dime-sized tropical crab that has ...)

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 creatures long-term impact will be.

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 isnt directly attacking oyster populations, though it may be competing with them for food and may impact the predators that normally attack the oysters.

Researchers at the Georgia Institute of Technology have spent more than three years studying the effects of the crab, and are reporting 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.

Were seeing opposing effects from these crabs, said Mark Hay, a professor in Georgia Techs 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 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 noted. They are probably competing with the oysters for food, but the native crabs have switched to eating these gre
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