Metabolic rate and osmoregulation in desert spring amphipods

Desert springs contain many endemic taxa and are of conservation concern due to anthropogenic activities that are expected to increase environmental salinity. Understanding the nature of osmoregulation is necessary to predict how non-vagile aquatic organisms will respond to changes. In the Chihuahuan Desert, the amphipod genus Gammarus Fabricius, 1775 is composed of two lineages. These lineages have species that currently inhabit springs ranging from 0.4 to 7.8 parts per thousand (ppt). All Gammarus in this region are of conservation concern because each is endemic to a single spring system. We exposed individuals of Gammarus colei Walters, Cannizzaro, and Berg in Walters, Cannizzaro, Trujillo and Berg, 2020 and Gammarus seideli Canniz-zaro, Walters and Berg, 2018 species occupying low-salinity springs, to the range of salinities found in the Chihuahuan Desert. We measured metabolic rates as VO2 to examine the energetic cost of osmoregulation. We also measured the hemolymph osmolality of G. colei and compared that with the isosmotic line to determine the degree of osmoregulation. Neither species in-creased its metabolic rate across increasing salinities. However, G. colei showed an increase in hemolymph osmolality. Despite the divergence (-66 million years ago) between the two lineages, it appears their physiological tolerances have converged, suggesting that both lineages may be able to persist when exposed to moderate salinity changes.