Date of Completion


Embargo Period



branchial ionocyte, comparative physiology, directional selection, euryhalinity, free amino acid, ion balance, NKA, NKCC, osmoregulatory divergence, SEM

Major Advisor

Eric T. Schultz

Associate Advisor

John A. Baker

Associate Advisor

Elizabeth L. Jockusch

Associate Advisor

Stephen D. McCormick

Associate Advisor

J. Larry Renfro

Field of Study

Ecology and Evolutionary Biology


Doctor of Philosophy

Open Access

Open Access


Maintaining ion balance in environments of changing salinity is one of the greatest physiological challenges facing aquatic organisms and by comparing populations inhabiting different salinity regimes, we can learn how physiological plasticity evolves in response to local osmotic stress. I characterized the evolution of osmoregulatory responses in representative marine, anadromous, and freshwater (FW) populations of Threespine Stickleback (Gasterosteus aculeatus) by comparing survival and physiological measures in F1-generation fish following salinity challenge. Juveniles from a population landlocked for ~10,000 years displayed ontogenetically-delayed seawater (SW) tolerance, a lower maximum salinity threshold, and did not upregulate the Na+/K+-ATPase (NKA) ion transporter as much as marine counterparts (Chapter 1). Stickleback also responded to salinity stress by remodeling their gill epithelium: I observed a higher density of ionoregulatory cells when juveniles were subjected to both low and high salinities, and the latter treatment induced strong upregulation of ion secretory cells (Chapter 2). Finally, I examined the speed at which osmoregulatory plasticity evolves by comparing halotolerance between an anadromous population and descendants that had been FW-restricted for only two generations (Chapter 3). The lake-introduced group had improved survival in FW, but also retained SW tolerance and had similar increases in gill NKA activity, gill Na+/K+/2Cl- cotransporter abundance, and organic osmolytes in SW. Overall, the differentiated responses to salinity I observed among stickleback populations indicate that osmoregulation has evolved in a manner consistent with local adaptation and following FW invasions, positive selection on FW tolerance acts more rapidly than relaxed selection on SW tolerance.