Growth rate and reproductive differences in a widespread estuarine harpacticoid copepod (Scottolana canadensis)

We demonstrate the presence of significant genetically based differentiation in growth rate (g dry weight d^-1) and reproductive traits (percent reproductive females and mean clutch size g dry weight^-1) among females of an harpacticoid copepod (Crustacea),Scottolana canadensis (Willey), taken from a broad range of latitudes and reared in the laboratory under the same conditions. As temperature increases (15°–25° C), the growth rate of southern-derived copepods continues to increase, while that of northern-derived copepods levels off or decreases. Southern-derivedS. canadensis also have a higher percentage of reproducing females at high temperature (25°C) when rations (cells ml^-1) are reduced, while northern-derived females are at an advantage at low temperature (15°C). Both life-history traits indicate local adaptation to maximize scope for growth and reproduction at prevailing temperatures. The data support our hypothesis that evolution has occurred to maximize feeding minus metabolic energy expended, and that this maximization requires changes in feeding efficiency with differing temperatures.