Meeting Abstract

55.5  Tuesday, Jan. 6  Contrasting latitudinal patterns of countergradient growth variation in silverside fishes (Pisces: Atherinidae) from the Pacific vs Atlantic coasts BAUMANN, Hannes*; CONOVER, David O; Stony Brook University, Stony Brook hannes.baumann@stonybrook.edu

Growth capacity comprises an important trait in fish upon which natural selection can operate. Along latitudinal gradients, growth rate often displays countergradient variation (CnGV) as, for example, in the Atlantic silverside Menidia menidia. In this species, growth capacity increases greatly with latitude, a likely response to shorter growing seasons and the size selectivity of winter mortality. In this study we ask whether CnGV would also apply to Pacific silversides, and if so, how do the reaction norms of temperature-dependent growth compare between the two coasts? We conducted common garden experiments on topsmelt, Atherinops affinis, that were obtained by strip-spawning adults from populations in southern California (Tijuana Estuary, 33N), mid-California (Elkhorn Slough, 37N), and mid-Oregon (Coos Bay, 43N). Individuals were reared in 3 replicates at 15C, 21C, and 27C under ad libitum feeding conditions until approximately 35 mm total length. We found a clear pattern of increasing growth rates with latitude for any given temperature, consistent with what is known for Atlantic silversides. However, the magnitude of the effect was much smaller in Pacific compared to Atlantic silversides. For example, mean growth rates at 27C varied between 0.64 – 0.85 mm/d for southern- and northernmost topsmelt populations, respectively, while M.menidia growth rates at comparable temperatures have been reported to range between 0.8 1.4 mm/d. At low temperatures, the patterns appear to be reversed, with higher and latitudinally more variable growth rates in Pacific than Atlantic silverside populations. These results suggest that the thermal plasticity of growth evolves in direct correspondence with the temperature range normally experienced in nature.