MEPS 461:175-186 (2012)  -  DOI: https://doi.org/10.3354/meps09802

ABSTRACT: Spatial adaptations that maximize a species’ fitness in its local environment, particularly in the form of countergradient variation (CnGV = genotypic influences counteract environmental effects on trait variation) and cogradient variation (CoGV = genotypic influences accentuate environmental plasticity), have been documented in many estuarine and anadromous fishes. However, their prevalence in more oceanic fishes, where extensive gene flow and reduced environmental selection may preclude local adaptation, remains unclear. If oceanic species lack genetic structure, they may be limited in their ability to adapt to global climate change. We examined the potential for adaptation in oceanic fishes by testing whether an oceanic silverside, the California grunion Leuresthes tenuis, displays CnGV in growth capacity and CoGV in vertebral number across a latitudinal gradient as found in estuarine silversides. Common garden experiments revealed that reaction norms for growth in response to temperature differed little among 3 L. tenuis populations from different latitudes. Growth rates of wild adult L. tenuis measured from scale analysis suggested that growth may be slightly faster at more southern latitudes, likely due to environmental variation. Mean vertebral number in wild L. tenuis, determined from radio-graphs, were nearly identical for all populations. Overall, our results did not provide evidence for spatial adaptations in these traits of L. tenuis, in contrast to the strong patterns of latitudinal trait adaptation in other, more estuarine silversides. This study provides support for the finding that higher gene flow levels and weaker environmental selection pressures of oceanic species limit local adaptation.

KEY WORDS: Latitudinal gradient · Gene flow · Adaptation · Growth capacity · Vertebral number · Climate change · Environmental selection