Ionoregulation in the cold: a possible barrier to freshwater colonization?
University of New Brunswick
To test if ionoregulation in winter cold is a barrier to freshwater colonization at high latitudes in fishes, I investigated the ionoregulatory responses of euryhaline marine-adapted Mummichogs (Fundulus heteroclitus) and freshwater-adapted Banded killifish (Fundulus diaphanus) held in freshwater at 14°C and 0, 4, 14, and 28 days of exposure to winter temperature of 4°C. I predicted that the Banded killifish would be able to maintain their ionoregulation better then Mummichogs in the cold freshwater. I measured maximal activities of gill Na+-K+-ATPase and H+-ATPase, two key ionoregulatory enzymes, and plasma chloride concentrations as a measure of osmotic status. There were no significant differences between species and no effect of cold acclimation on plasma chloride or enzyme activities. The lack of ionoregulatory perturbation by winter cold in either species, especially the seawater-preferring mummichog, do not support the hypothesis that ionoregulation in winter is a barrier to freshwater colonization in fishes.