| The genetic population structure of geoduck clams (Panopea abrupta ) in inland waters of Washington may influence fishery management and aquacultural practices involving this species. To investigate genetic differentiation in geoduck clams, novel di- and tetranucleotide microsatellite loci were developed in geoduck clams by magnetic bead hybridization selection as markers for population genetic analyses. To examine spatial structure, samples were collected from 16 Washington State sites located in the five Puget Sound subbasins, southern Georgia Strait, and the Strait of Juan de Fuca. A collection from Clarence Strait in SE Alaska was included as an outgroup. An average of 95 individuals per collection were genotyped at 11 allozyme and 7 microsatellite loci. In contrast to the microsatellite loci, the allozyme loci were in Hardy-Weinburg Equilibrium (HWE). Deviations from HWE expectations in microsatellite loci were interpreted as being primarily due to primer site sequence variation rather than population level processes such as inbreeding. FST estimates averaged 0.001 for microsatellites and 0.002 for allozymes. Significant differences in genotype frequencies among some collections were detected using both marker classes. For both marker classes, there was no significant correlation between genetic and geographic distance measures. To investigate these results, interpreted as possibly due to stochastic variation in reproductive success and recruitment, two additional samples (n = 1010) were taken at multiple stations. Individuals were aged by counting growth annuli on thin-sectioned chondropores and genotyped as above. Within-site comparisons among both year class and station genotype frequencies were made. In addition, relative to random collections, individual year class data were examined for: (1) reduction in genetic diversity, (2) increased relatedness, (3) reduction in heterozygote deficiencies, and (4) smaller estimates of effective population size (Ne), all sources of evidence of stochastic variation in reproductive success. No evidence of significant bias in reproductive success was detected, however there were significant genetic differences among stations in one of the samples, and year class strength differences between the two samples. Taken together, these findings suggest large variation in larval settlement and underscore the need for further investigation into population dynamics and recruitment processes in geoduck clams. |
