Climate change is causing habitat salinity to transform at unprecedented rates across the globe. While much of the research on climate change has focused on rapid shifts in temperature, far less attention has focused on the effects of changes in environmental salinity. Consequently, predictive studies on the physiological, evolutionary, and migratory responses of organisms and populations to the threats of salinity change are relatively lacking. This omission represents a major oversight, given that salinity is among the most important factors that define biogeographic boundaries in aquatic habitats. In this perspective, we briefly touch on responses of organisms and populations to rapid changes in salinity occurring on contemporary time scales. We then discuss factors that might confer resilience to certain taxa, enabling them to survive rapid salinity shifts. Next, we consider approaches for predicting how geographic distributions will shift in response to salinity change. Finally, we identify additional data that are needed to make better predictions in the future. Future studies on climate change should account for the multiple environmental factors that are rapidly changing, especially habitat salinity.

Euryhalinity is present in diverse aquatic taxa and requires flexible osmoregulation to field the challenges posed by differing salinities. Na, K-ATPase (NKA) is a ubiquitous ion pump in the gills of fishes and, for some species, paralogs of the catalytic α-subunit (NKA α1a and α1b) exhibit reciprocal expression between fresh- and seawater, termed paralog-switching. We investigated the expression and evolution of NKA paralogs in Alewife (Alosa pseudoharengus), a euryhaline and migratory fish. Comparisons between landlocked and diadromous life history forms and migrant and pre-migrant ontogenetic stages were used to study shifts in NKA paralog expression related to freshwater or seawater specialization. We exposed juvenile diadromous and landlocked alewives to freshwater (0 ppt) and seawater (30 ppt) for 2, 5, and 15 days. Additionally, we sampled migrant and pre-migrant alewives from the natal freshwater environment or after 24 hours in seawater. Diadromous Alewife exhibited salinity-dependent paralog switching, and the freshwater-specialized landlocked life history form showed greater upregulation of NKA α1b in seawater. Migrant Alewife showed a loss of freshwater readiness traded for seawater specialization through greater reliance (via upregulation) on NKA α1a in freshwater. Molecular phylogenies show Alewife NKA paralogs originated independently of paralogs in salmonids and other members of Euteleosteomorpha. This study demonstrated that NKA paralog switching is tied to halohabitat profile and that duplications of the ancestral NKA gene provided the substrate for multiple, independent molecular solutions for supporting a diadromous life history.