Researchers said the dime-sized tropical crab -- Petrolisthes armatus -- is leaving scientists unable to predict what the creature’s long-term impact will be.

Unlike native crabs that eat baby oysters, mussels and fish, the green porcelain crab is a filter feeder, extracting food from the water much as oysters do. Therefore, it isn’t directly attacking oyster populations, although it might be competing with them for food. And that might impact the predators that normally attack the oysters.

"We’re seeing opposing effects from these crabs," said Mark Hay, a professor in the Georgia Institute of Technology's School of Biology. "They are probably having more impact on the ecosystem by being prey than by being predators."

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, the researchers said.

The findings are reported in the journal Biological Invasions.