Life History, Growth, and Genetic Diversity of the Spotted Gar Lepisosteus Oculatus from Peripheral and Core Populations.

Abstract

I studied the ecology and biogeography of the spotted gar (Lepisosteus oculatus) from core and Great Lakes Region peripheral populations. Peripheral populations occupy the edge of a species’ range and are considered to be important in terms of a species’ ecology, biogeography, evolution, and conservation. Peripheral populations often persist under different environmental conditions from the species’ core populations, and may exhibit adaptations to potentially “harsher” marginal environments. In this study I used common garden experiments, life history analyses, and phylogeography (based on mitochondrial DNA) to address the overall hypothesis that spotted gars from peripheral, Great Lakes Basin populations exhibit distinct life history characteristics and patterns of genetic diversity in comparison to spotted gars from core populations. In common garden laboratory experiments young-of-year spotted gars from peripheral populations exhibited significantly faster growth rates (0.09 cm/day, 0.26 g/day) than core populations (0.04 cm/day, 0.11 g/day, suggesting countergradient variation in growth. Life history analysis based on length-at-age data from 5 field populations (2 peripheral, 3 core) and incorporating thermal opportunity for growth (degree days above 18 °C) indicated significantly higher growth rate in spotted gars from peripheral (1.23 mm/degree day) compared to core populations (0.22 mm/degree day). Catch-curve analyses of the same populations indicated annual mortality rate (A) was lower in peripheral (A = 0.41) compared to core populations (0.56). Analysis of mitochondrial DNA from core and peripheral populations indicated genetic diversity (haplotype diversity, H) was highest in the Mississippi River Basin (H = 0.80), lowest in the Great Lakes Basin (H = 0.00, single haplotype), and most divergent in the western Gulf Coast Basin (H = 0.70, no haplotypes shared with other basins). Overall, the Great Lakes Basin population was shown to be a unique component of the species, and is adapted to life at higher latitudes with shorter growing seasons. As a useful case study, my work can inform gar conservation strategies and lead to a better general understanding of the evolution and maintenance of vertebrate life history patterns and genetic diversity.