Filter Feeder [+]Enlarge Credit: Environ. Sci. Technol.

Oysters exposed to oil during and after the 2010 Deepwater Horizon oil spill did not consume significant quantities of that oil, according to a new study (Environ. Sci. Technol., DOI: 10.1021/es302369h).

One of the enduring questions of the Gulf of Mexico disaster is where the oil ended up. Bacteria broke down much of it into small organic molecules that could fuel the Gulf’s food web, according to previous studies. A team of researchers led by marine scientist Ruth Carmichael at the Dauphin Island Sea Lab, a state-run institution in Alabama, wondered whether the region’s famed oysters might also have been consuming oil.

“What we were really interested in,” says Carmichael, “was whether we could see evidence of oil-derived substances making their way into a commercially harvested species and further up into the food web.”

Before the spill, the researchers had already been working with oysters (Crassostrea virginica) at sites along the Alabama and Mississippi coasts that were then hit with high levels of oil. As a result, they had samples from before, during, and after the spill.

Oysters feed by filtering water through their bodies and removing particles. The team measured the ratios of carbon and nitrogen isotopes of the oysters’ tissue, the oil, and the suspended matter in brackish water that makes up the oysters’ normal diet.

The ratios of these isotopes in oil are distinct from those of the oysters’ normal food. The researchers found that the oysters’ flesh and shells, whose composition reflects their diet, showed no significant shift toward oil’s isotopic signature.

The researchers can’t say whether this result means the oysters closed up when exposed to oil, or that they continued feeding normally and somehow avoided ingesting the oil. But, says Carmichael, “we do know the oil didn’t contribute significantly to their diet.”

Carmichael also points out that oysters still could have been exposed to and harmed by oil in other ways. For instance, they could have absorbed polycyclic aromatic hydrocarbons, the most toxic constituents of oil, into their tissues directly. PAHs can cause health problems such as tumors. Oysters may not break the compounds down in ways that would incorporate them into the tissue type and shells that the study analyzed.

Carmichael says the results could be positive because they suggest the Gulf’s oysters did not have “bellies full of oil.” But she says finding evidence of oil consumption might actually have been a good thing, as well. Such a finding could have shown that the oysters were helping clean up the Gulf ecosystem.