Fish living in Gulf of Mexico marshes exposed to last year's oil spill have undergone cellular changes that could lead to developmental and reproductive problems, a group of researchers reported Monday in the journal Proceedings of the National Academy of Sciences.

The study, which was funded by the National Science Foundation and the BP-funded Gulf of Mexico Research Initiative, suggests scientists are just beginning to capture the spill's ecological impact. The team of researchers from Louisiana State, Texas State and Clemson universities focused on the killifish, a minnowlike fish that is abundant and a good indicator of the health of wetlands.

Killifish residing in areas affected by the spill showed cell abnormalities, including impaired gills, two months after the oil had disappeared, researchers found. Killifish embryos exposed in the lab to water from the same site, which had only trace amounts of chemicals in it, developed cellular abnormalities as well.

"Their biology is telling us that they've been a), exposed to these chemicals and b), affected by them in negative ways," said Andrew Whitehead, an associate professor of biology at LSU and the paper's lead author. "Very low-level exposures can cause these toxic effects."

The group sampled killifish at six field sites spanning from Barataria Bay in Louisiana to Mobile Bay off Alabama. Researchers started sampling before oil made landfall in May 2010 and continued sampling as late as September 2010. They used satellite imagery as well as photographs to pinpoint where oil had hit marshes.

Whitehead said the findings were cause for concern because the fish, also known as bull minnow or cacahoe, were showing the same initial signs of toxicity that appeared in herring and harlequin ducks after Alaska's 1989 Exxon Valdez spill. These populations crashed, and some have yet to recover.

Tom Mueller, a BP spokesman, said in an e-mail that BP is reviewing the study and cannot comment on its conclusions or underlying data. "However, we support the efforts underway to better understand the potential, long-term impacts of the oil spill," he said.

Whitehead said the results show that just because fish from the gulf have passed federal inspections, it does not mean these species are unaffected by the spill.

"You can have a fish that's safe to eat but is still not healthy," he said, adding that as sediment containing hydrocarbons is dredged up by storms, it could expose species over time. "The sediments are going to act as this long-term reservoir of oil, of potential exposure."