Lucania goodei and L. parva are close phylogenetic relatives that differ in abundance along a salinity gradient where L. goodei is found predominantly in freshwater sites, and L. parva is found most often in brackish water sites. It has been suggested that these taxa diverged along a salinity gradient. In order for selection to cause divergence, there must be local adaptation where each species does best in the environment in which it is most often found. In this paper, I test for local adaptation in L. goodei and L. parva during early life-history stages by raising eggs and larvae of each species at 0, 10, 20, and 30 ppt salinity, covering the gradient between fresh water and sea water found along the coast of the Gulf of Mexico. Salinity tolerance differed between the two species. Lucania parva had high survival at all salinities. Lucania goodei had high survival at 0 and 10 ppt, but low survival at 20 and 30 ppt. Hence, there is no evidence for a trade-off in the early life-history stages between these two species. Current research is investigating whether trade-offs occur at later life-history stages.

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