Geographically Widespread, Non-hybridizing, Sympatric Strains of the Hermaphroditic, Brooding ClamLasaeain the Northeastern Pacific Ocean

We have studied phenotypic variation in six enzymes of Lasaea, a taxonomically complex genus of small brooding clams, from nine northeastern Pacific sites. Each of the individuals examined produced one of five combinations of electromorph patterns. Lasaea phenotypes could be differentiated into two main types, one containing two and the other three phenotype combinations. Samples from each population contained from one to three phenotype combinations and there was no evidence for crossbreeding among phenotypes. These results are strongly at variance with random mating expectations and indicate that the phenotype combinations represent reproductively isolated strains. This is substantiated by a more detailed study of the McNeill Bay, British Columbia, population where both main strains coexist. Electrophoretic characterization of Lasaea from individual 100 cm2 samples of barnacle cover revealed that strains are not spatially segregated. Progeny of (1) pair mating experiments, (2) brooding field individuals, and (3) specimens that reproduced in isolation, all perpetuated the maternal electromorphs. Data from previous studies of reproduction in northeastern Pacific Lasaea suggest that the formation of non-hybridizing strains has resulted either from the predominance of self-fertilization or as a result of pseudogamy combined with meiotic parthenogenesis. We currently favor the former hypothesis. The two main strains are largely conserved between geographically distant sites, despite the lack of a planktonic larval stage. Separation of the main strains in Victoria, British Columbia, populations on the basis of shell color phenotype is to some degree possible, but is often equivocal. Electrophoretic analysis (especially of reddish specimens) is necessary for their reliable identification.