The Deepwater Horizon (DWH) oil spill, which dumped an estimated 685,000 tons of crude oil into the Gulf of Mexico (GoM) between April and August 2010, is the largest accidental spill in maritime history. While immediate environmental impacts of the spill, such as direct and fatal fouling of wildlife and the physical contamination of coastal areas were easily observed, long-term effects of the spill are still being determined. Efforts to understand the recovery of impacted areas and species of the GoM, and the GoM ecosystem itself, requires knowledge of the ranges and types of initial impacts and the potential longevity of those impacts. This project proposes to study the impact of the spill on the commercially important American oyster Crassostrea virginica. Impact will be assessed as: (1) the pathologic morphological transformation of sensitive tissues because of exposure to oil, and (2) alterations to developmental and growth rates as recorded by specimen shells. In addition to baseline and early impact specimens already collected, specimens will be obtained from coastal areas of Louisiana and Florida, with periodic sampling from July 2013 to March 2014. Preliminary analysis of C. virginica collected in Barataria Bay, Louisiana and Apalachicola Bay, Florida between August 2010 and December 2010, show that 72% of specimens underwent abnormal epithelial transformation (metaplasia), whereas specimens obtained from the latter between December 2011 and the present exhibit normal epithelia, possibly signaling environmental recovery. While these preliminary data support a hypothesis of DWH impact on C. virginica, it is important to consider other potential sources of contamination in the GoM, including natural hydrocarbon seepage, non-DWH activities of the petroleum industry, and terrestrial-based activities delivered by freshwater input. Baselines will therefore be established for each analysis using geographic and historical controls.