Freshwater tolerance of fourspine stickleback (Apeltes quadracus) marine and lake populations

Abstract

Fishes have evolved freshwater tolerance multiple times, using different physiological mechanisms, and we have much to learn about how fish can adapt to freshwater. Sticklebacks in the family Gasterosteidae are becoming an important model system in which to study the evolution of freshwater tolerance. In this study I measured freshwater tolerance of a freshwater (lake) and a marine population of Fourspine Stickleback (Apeltes quadracus) to test overall tolerance in this species and determine if there is evidence of freshwater adaptation in the lake population. To assess tolerance, both populations acclimated to near isosmotic water (10 ppt) and were exposed to an acute freshwater (0 ppt) challenge and monitored over 48 days of freshwater acclimation. Survival, body water content, tissue ion content and standard metabolic rate were measured from fish from both populations during the study. We found that both the marine and lake populations were able to cope with freshwater transfer, as both had high survival rates and could maintain body-water homeostasis. My data suggests that ancestral Fourspine Stickleback that were landlocked at the end of the last ice age may have had higher freshwater tolerance than some other Gasterosteidae species, allowing them to easily colonize post-glacial lakes.