BIOCHEMICAL-GENETIC VARIATION AT A LEUCINE AMINOPEPTIDASE (LAP) LOCUSIN BLUE (MYTILUS-GALLOPROVINCIALIS) AND GREENSHELL (PERNA-CANALICULUS) MUSSEL POPULATIONS ALONG A SALINITY GRADIENT

In several marine bivalve species, biochemical genetic variation at a leucine aminopeptidase (LAP) locus is associated with;environmental va riability, primarily salinity fluctuation. Population genetic variatio n at a LAP locus was investigated here in two sympatric mussel species (Mytilus galloprovincialis and Perna canaliculus) from three location s along a salinity gradient in Wellington Harbour, New Zealand. The da ta for M. galloprovincialis and P. canaliculus do not support the hypo thesis that the LAP polymorphism in either species is associated with salinity variation among adult mussels. Due to the absence of small mu ssels among the samples it is not possible to discount the hypothesis that selection acts primarily against juveniles, as it does for M. edu lis in Long Island Sound, USA. Wellington Harbour populations of M. ga lloprovincialis exhibited large and often highly significant heterozyg ote excesses at the LAP locus, whereas populations of P. canaliculus f rom the same locations exhibited large and highly significant heterozy gote deficiencies. The reason for this inter-specific difference in po pulation structure is unknown. If it is the result of selection, this suggests that selection acts differentially upon the two species, beca use demographic attributes and reproductive biology are very similar i n the two species. For both M. galloprovincialis and P. canaliculus, s ignificant levels of population genetic heterogeneity were recorded am ong three locations separated by only 8 to 12 km. Neither species exhi bited shell length-dependent genetic variation at the LAP locus, sugge sting that for these two species the LAP polymorphism is not associate d with variation in shell length.