Differential temperature preferences of vertebral phenotypes in Gasterosteus

Abstract

Across a broad diversity of freshwater and marine fish that have wide latitudinal distributions, intraspecific variability in vertebral number shows a robust trend for higher vertebral counts in colder latitudes (Jordan’s Rule). Variability in this meristic trait is determined during early larval development by a combination of heritable and temperature-mediated influences. We experimentally evaluate for the first time whether different vertebral phenotypes from a population actively segregate across a temperature gradient, and if so, whether the segregation is consistent with broad geographical trends across taxa of greater vertebral counts that are associated with cooler waters. Using threespine stickleback (Gasterosteus aculeatus L., 1758) from two populations, we conducted 22 trials with ∼50 fish per trial and a temperature gradient ranging from 2 to 15 °C among trials. Results show that six trials yielded statistically significant or near-significant results, of which five trials were in the predicted direction of more vertebrae in cooler waters. The effects were more expressed in fish with small bodies (35–55 mm) than in fish with larger bodies (60–85 mm) and in longer duration trials. We believe that these data are novel and are consistent with recent studies on swimming efficacy among vertebral phenotypes; the data also hint at much greater ecological functionality than is currently assumed by intrapopulation variation in this meristic trait.