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Seven species of coral--Dichocoenia stokesii, Montastrea annularis, Agaricia agaricites, Acropora cervicornis, Porites furcata, P. astreodes, and P. divercata--were experimentally exposed to three concentrations of drilling mud obtained from an offshore oil well in the Gulf of Mexico. The whole mud, collected from the mud pit of a well at a drilling depth of 4,000 m, was diluted with seawater to produce concentrations of 100, 316, and 1,000 µL/L. Corals were exposed to each of the three concentrations and control seawater for 96 hours to observe behavioral response. Response to drilling-mud concentrations was measured as percent of polyps retracted. Some experiments were conducted in laboratory aquaria with Gulf Stream water, but the most significant experiments were conducted at Carysfort Reef, Florida Keys, using similar aquaria located in 3 m of water. Polyp behavior was determined with serial closeup photography which allowed counting of retracted, partially retracted, and nonretracted polyps in each colony.
All species except Montastrea annularis and Agaricia agaricites survived exposure to 1,000-µL/L mud during the period of testing. In two tests with Acropora cervicornis, one group survived exposure to the mud and the other died. All other corals except dichocoenia stokesii and Porites divercata showed significant (p<0.05) polyp retraction during exposure to 100-µL/L mud concentration, whereas 316-µL/L mud was the minimum concentration which induced significant polyp retraction in Porites divercata. Polyps of Dichocoenia stokesii did not react to any of the three concentrations.
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