Geographic Variation in the Life History and Morphology of the Pacific Geoduck Panopea generosa

Evaluating the spatial scale of biological variation in marine species aids understanding of the ways in which environmental selection pressures can shape life histories, population demography, and morphological traits. The Pacific geoduck Panopea generosa is an ideal candidate to investigate patterns of life history variation and morphological plasticity, as the species occurs over a large geographic range, inhabits different marine environments, and experiences intense fishing pressure in some locations. Six populations were sampled from Washington to Baja California (Mexico), from a mix of subtidal and intertidal habitats, to evaluate evidence for geographic variation in demography, life history, and shell morphology. Results provided evidence for a latitudinal cline with larger clams occurring in locations characterized by colder water temperature. Von Bertalanffy growth model analyses revealed significant spatial variation in asymptotic lengths across sampling locations. Correlations between environmental variables (sea surface temperatures and chlorophyll a) and growth parameters indicated that clams reached larger sizes at locations that were cooler and had higher primary productivity. Age structure and longevity analyses demonstrated that life expectancy was significantly lower at intertidal sites than at subtidal sites, potentially in response to increased fishing pressure at sites more easily accessed by humans at low tide. Morphological analyses (i.e., geometric morphometrics) revealed significant spatial differences in shell shape that did not follow a latitudinal pattern, but may be better explained by site-specific differences in sediment characteristics or other selective pressures in the environment. In a management context, these results may inform efforts to develop spatially explicit regulations for the Pacific geoduck fishery. Finally, this study provides a general framework for evaluating demographic variability in shellfish populations across large spatial scales.