By Melissa Gaskill of Nature magazine

Heart-breaking pictures of seabirds covered in black crude oil, arresting as they are, can miss the hidden story of an oil spill's impact on wildlife.

Exposure to even tiny concentrations of the chemicals present in oil can also cause harmful biological effects that usually go unnoticed, according to a study published today in Proceedings of the National Academy of Sciences.

"It's striking that even though the analytical chemistry doesn't indicate exposure, the biology does," says Andrew Whitehead, a biologist at Louisiana State University in Baton Rouge, who led the study. "We can measure all the chemistry we want in the environment, but if want to know whether organisms have been exposed, we have to ask them."

The researchers studied the impact of the 2010 Deepwater Horizon oil spill on Gulf killifish (Fundulus grandis) living in the Gulf of Mexico. They collected water and tissue samples three times from marshes where the killifish lived: once in early May, before oil from the blown well had arrived; once in late June, when oil had reached the marshes; and again in late August, after oil was no longer visible. They collected samples from six sites, but only one -- in Barataria Bay, Louisiana -- was heavily oiled following the spill.

Key species

Whitehead has previously shown that exposure to polychlorinated biphenyls (PCBs) can cause harmful gene expression changes in killifish, which are an important food source for many species, including economically important ones such as red snapper. Because PCBs and the polycyclic aromatic hydrocarbons (PAHs) found in crude oil have similar biological effects, the researchers looked at their impact on the same set of genes.

They found analogous changes in gene expression in killifish from the marshes, and in killifish embryos exposed to contaminated water samples in the lab. These changes have previously been shown to cause developmental abnormalities, decreased embryo survival and lower reproductive success. "It doesn't take much PAH to mess with development," Whitehead says.

"The ability of fish larvae to survive has a huge effect on the population down the road," says biologist Lee Fuiman, director of the University of Texas Marine Science Institute in Port Aransas, who was not involved with the study. "A small change in the percent survival equals a large change in the adult population."

Persistent pollution

In addition to the changes in gene expression, the effective surface area of gill tissue was reduced, potentially impairing the ability of the fish to absorb oxygen.

Study co-author Fernando Galvez, also at Louisiana State University, is now investigating how killifish embryos are affected by exposure to sediments from the study's sample sites, which he says had concentrations of hydrocarbons that were "through the roof". High concentrations of hydrocarbons in marsh sediments can persist for years, continuing to expose organisms to their effects -- something that was seen long after the Exxon Valdez spill in Prince William Sound, Alaska, in 1989.

"The best predictors of long-term, population-level consequences after the Exxon Valdez spill were not dead animals in the water, but sub-lethal effects," Whitehead says. "The same indicators that we're studying in these killifish".

This article is reprinted with permission from Nature magazine. It was first published on September 26, 2011.