Effect of multiple-locus heterozybosity and salinity on clearance rate in a brackish-water clam,Rangia cuneata (Sowerby)

Previous studies of several species of marine bivalves and gastropods have reported a positive correlation between growth or size and level of multiple-locus heterozygosity. There is some evidence that the growth advantage of relatively heterozygous individuals is due to a lower rate of standard or routine metabolism, compared with more homozygous individuals, although heterozygosity-dependent differences in feeding rate may also be involved. The present study examined the relationship between clearance rate in three salinity treatments (5,15, and 25%.) and multiple-locus heterozygosity at nine polymorphic allozyme loci in the clam Rangia cuneata (Sowerby). Clearance rates were determined by disappearance of an algal suspension from a flowing-water system. Allozyme genotypes were determined using starch-gel electrophoresis. The polymorphic loci examined were those coding for a nonspecific esterase (Est), mannosephosphate isomerase (Mpi), leucine aminopeptidase (Lap), 6-phosphogluconate dehydrogenase (6-Pgd), phosphoglucose isomerase (Pgi), isocitrate dehydrogenase (Idh), malate dehydrogenase (Mdh), adenylate kinase (Adk), and phosphoglucomutase (Pgm). Weight-corrected clearance rates increased significantly (P < 0.05) with increasing multiple-locus heterozygosity and decreased significantly (P < 0.05) with increasing salinity. These data support the idea that heterozygosity-growth correlations may be due in part to differences in clearance rate. However, further study is needed to understand the exact physiological processes which relate heterozygosity and growth.