Meeting Abstract

53.6  Monday, Jan. 5  On the adaptive significance of Jordan’s Rule: comparing the temperature-dependence of critical swimming speed among latitudinal populations of the Atlantic silverside, Menidia menidia HICE, LYNDIE A.*; CONOVER, DAVID O.; Stony Brook University; Stony Brook University lhice@ic.sunysb.edu

The Atlantic silverside, Menidia menidia, displays a very strong and spatially fine-scale increase in vertebral number with latitude along the east coast of North America, consistent with Jordan’s Rule. Most of this variation is genetic and such tight clinal patterns implicate natural selection as the cause but its adaptive significance is unclear. High latitude populations are thought to have evolved a greater number of vertebrae to allow for increased body flexibility in colder, more viscous water, however empirical evidence is limited. To test this theory, we hypothesized that at high temperatures, southern Atlantic silverside populations would show significantly higher critical swimming speeds than northern populations, but the reverse would be true at lower temperatures. Critical swimming speed experiments were conducted on southern (South Carolina) and northern (Nova Scotia) populations reared in a common environment. Each population was tested at four larval or juvenile sizes ranging from 10 to 30mm and three experimental temperatures. Swimming speed increased with size in both populations. The southern population exhibited maximum swimming speed at the highest experimental temperature, while the northern population performed best at intermediate temperature. There was no strong evidence of reversal of swimming ability at low temperature as expected. Few studies have investigated the link between vertebral number and swimming ability and these results provide evidence of potential agents of vertebral number selection in the wild.