AbstractUnderstanding how species adapt to spatial climate gradients can provide clues to potential evolutionary responses to climate change. Species distributed across broad environmental gradients, such as those that occur along latitudes or altitudes, often exhibit adaptive genetic variation. However, little attention has been given to how the type of environmental gradient shapes adaptive responses. To provide insight into this, local adaptation is compared in related fish species across two very different environmental gradients: the Atlantic and Pacific coasts of North America. Local adaptation is first examined in the California grunion (Leuresthes tenuis) and then results are compared to previous work on the Atlantic silverside. Common garden experiments and wild fish studies were used to test for local adaptation among several traits (growth capacity, sex determination, and vertebral number) of the California grunion across three latitudinal populations: Monterey, CA (36.6_N), Malibu, CA (34.0_N), and Ensenada, MX (31.9_N). Consistent genetic differences in growth capacity between latitudinal populations were not observed. Wild southern grunion were slightly larger and grew faster than more northern grunion, likely due to environmental effects. Temperature (p<0.001) and photoperiod (p=0.011) were found to significantly affect sex ratios of laboratory reared fish, indicating that grunion have environmental sex determination (ESD); however the level of ESD did not differ among populations. Mean vertebral numbers in wild grunion were nearly identical for all populations. The lack of latitudinal variation in these traits of the grunion is in direct contrast to the Atlantic silverside, which exhibits a high degree of genetic differentiation in all of the above traits. Results also differ from recent work on the topsmelt, another Pacific coast silverside species. Failure to observe latitudinal variation in the grunion unlike its other taxonomic relatives may be due to its oceanic rather than estuarine habitat, which provides a greater opportunity for broad-scale gene flow and results in a more homogenous environment. Implications for climate change are discussed.