Phenoloxidase activity in the hemolymph of bivalve mollusks

The oxidative enzyme phenoloxidase has been proposed as a component of inte rnal defense in a variety of organisms. We measured the phenoloxidase activ ity of hemolymph plasma and hemocytes from a selection of marine and freshw ater bivalve mollusks. Phenoloxidase activity is present in the hemolymph a nd hemocytes of Placopecten magellanicus, Argopecten irradians, Geukensia d emissa, Mercenaria mercenaria, Lampsilis teres, and Lampsilis claibornensis . The enzyme activity in the freshwater species was 5 to 10 times higher th an that in the marine animals. Phenoloxidase activity was not consistently stimulated by preincubation of hemolymph plasma or extracts of hemocytes wi th trypsin, bacterial cell wall lipopolysaccharides and fungal cell wall co mponents (zymosan). The role of this enzyme, if any, in internal defense re mains to be elucidated.