The hard clam, Mercenaria mercenaria, shows a general response to environmental variation. The molar ratio of free taurine to glycine in gill and mantle tissues climbs above 3, while alpha-amino acids and carbohydrates decrease. Subtle adjustments in the total pattern of free amino acids and fatty acids also occur, but these can be readily seen by changes in biochemical diversity and equitability. In an estuary long suffering from hydrocarbons and other agents in petroleum products and sewage, high mortality results from a culmination of natural responses superimposed on abnormal complications. The process apparently starts after a black, polymeric irritant collects in epithelial tissue and eventually occludes the renal sac. This leads, indirectly, to infestations of a parasitic polychaete that is rarely found in hard clams. A syndrome with many facets soon becomes clear, but the situation can be identified and its results predicted by simply observing the responses of taurine and glycine in stressed and normal populations.