Oil droplets have been found beneath the shells of tiny post-larval blue crabs drifting into Mississippi coastal marshes from offshore waters.

The finding represents one of the first examples of how oil from the Deepwater Horizon spill is moving into the Gulf of Mexico's food chain. The larval crabs are eaten by all kinds of fish, from speckled trout to whale sharks, as well as by shore birds.

The tiny droplets are visible under the transparent shells of the 2-millimeter-sized crabs collected in Davis Bayou, said Harriet Perry, director for the University of Southern Mississippi Gulf Coast Research Laboratorys Center for Fisheries Research and Development.

The research lab is working with Tulane University on a study that includes Davis Bayou in Ocean Springs.

"In all the years we've collected, I've never seen anything like that, said Perry. "Just to make sure, we did send some of the megalopa (the post-larval crabs) to a testing firm in Pensacola and the droplets we saw were hydrocarbons."

Perry is an assistant professor in the university's Department of Coastal Sciences. She has compiled an extensive list of publications and research over her 42 years at the GCRL.

The oil droplets also are being found in fiddler crab larvae. Perry said the crabs may lose the droplets when the molt and shed their shells.

"These larvae are up in the marsh and they are picked off by small fish, small speckled trout and those type of things, so that's the way these things get into the food chain," she said.

The female blue crabs carry fertilized eggs around for months before they hatch around the barrier islands. Currents carry the larvae offshore, then move them back to the protective marshes in a month-long cycle, she explained. The crabs remain in the shore areas until adulthood, she said.

"If they lose their habitat that would be crucial to fisheries sustainability," said Perry. "We are going to lose recruitment (young crabs) just because oil covers so much of the ground where the larvae are."

About 41 percent of the offshore larval pool is covered by oil, she said.

Besides crabs, the larvae grounds include spring and summer spawned fish. "Red snapper are out there now."

Oil covering the offshore larval grounds threatens a year's class of crabs, she said.

"Those guys have to have habitat to come back to and if the habitat is impacted, if we have decimated our marshes, then they have no place to grow up," said Perry. "It is just critical we try to keep our marshes from being impacted by that oil."

GCRL will set up six of its own sites to collect young crabs beginning in July in a study that has been ongoing for 11 years, she said.

"We have a good idea of what should be coming," Perry said.