The physiological significance and evolutionary biochemistry of a spatially and phylogenetically widespread aminopeptidase polymorphism will be investigated in two bivalve species, Mytilus edulis and Modiolus demissus. Digestive physiology will be studied by a combination of autoradiography and histocytochemistry during starvation/feeding experiments. The probable role of these enzymes in the mobilization of cellular protein will be investigated by correlating known temporal variations in enzyme activity with the postreproductive condition of maximum mobilization of cellular protein for energy. The molecular basis for the maintenance of this ubiquitous genetic polymorphism will be investigated in both species. Purified enzyme will be used for antibody production for microcomplement fixation (MCF) experiments. Known activity differences between populations and individual genotypes will be investigated to establish the basis of the different enzyme activities. Radioisotope pulse labelling experiments will determine the relative turnover rates of the LAP enzyme in populations and in individuals exhibiting different activity levels. A complete enzymatic characterization of the electrophoretic LAP alleles in both species will be done under varying assay conditions, to discover the differential functional properties of the individual alleles within each species as well as the similarities of these properties between the two species. Known genotype-dependent sensitivity to ions will be explored in depth with both metal and physiologically important divalent cations. Structural relationships will be estimated among individual alleles within and between each species by determination of the immunological similarity of denatured enzyme. The amino acid compositions of the individual allozymes will be determined if immunological similarities are small.