Limited evidence for local adaptation to salinity and temperature variability in San Juan Island populations of the copepod Tigriopus californicus (Baker)

Local adaptation has been studied in a broad range of taxa for decades. However, we have limited understanding of how often it occurs in variable environments. Whether phenotypic plasticity can evolve in distinct ways among populations experiencing different patterns of abiotic variability is unclear. Abiotic conditions in coastal marine habitats can be highly heterogeneous at small spatial scales, which might promote local adaptation. The harpacticoid copepod Tigriopus californicus (Baker, 1912) has become a model system for testing whether phenotypic differences among populations are a result of local adaptation. To identify potential selective pressures in the field, temperature and salinity were measured in high shore pools for six months at three sites on San Juan Island in Washington, USA. A common garden experiment with factorial combinations of seven temperature and two salinity (32, 55 ppt) treatments was conducted on these distinct populations. Two temperature treatments varied daily, both with an average of 20 °C, but different ranges (low amplitude: 15–25 °C, high amplitude: 10–30 °C). The other five treatments were the average, maximum, and minimum temperatures held constant. Fecundity, survivorship, and development were characterized across two generations. There were strong interactive effects of temperature and hypersalinity on copepod culture dynamics, but these effects differed among populations. Abiotic patterns in the field were correlated with few observed differences in population phenotypes, thus limited evidence for local adaptation was found. For these populations, differences in selective pressures among sites might not be strong enough to overcome the influence of genetic drift.