Road salt compromises functional morphology of larval gills in populations of an amphibian

Abstract

Throughout much of the world, winter deicing practices have led to secondary salinization of freshwater habitats, where numerous taxa are vulnerable to elevated salinity. Many amphibians are of particular concern because of their permeable skin and reliance on small ponds and pools, where salinity levels can be high. The early life-history stages of amphibians that develop in these habitats are especially sensitive to salt exposure. Larvae developing in salt-polluted environments must osmoregulate through ion exchange in gills. While salt-induced changes to the physiology of ion exchange in amphibian gills is generally understood, functionally relevant changes in gill morphology remain poorly described. Yet the structure of gills should be an important component affecting their ionoregulatory capacity, for instance in terms available surface area. Larval amphibian gills also play critical roles in gas exchange and foraging. Thus, changes in gill morphology due to salt pollution potentially affect not only osmoregulation, but also respiration and feeding. Here, we used a chronic exposure experiment to quantify the effect of salinity on larval gill morphology in populations of the wood frog (Rana sylvatica). We measured a suite of morphological traits on gill tufts, where ionoregulation and gas exchange occur, and on gill filters, which are used in feeding. Larvae raised in high salinity conditions had gill tufts with lower surface area to volume ratio, while epithelial cells on these tufts were less circular but occurred at higher densities. Gill filters showed increased spacing, which can potentially reduce their efficiency in filtering food particles. Together, these changes seem likely to diminish the ionoregulatory and respiratory capacity of gill tufts, and compromise feeding functionality of gill filters. Thus, a singular change in the aquatic environment from a widespread pollutant has the potential to generate a suite of consequences via changes in gill morphology. Critically, this suite of negative effects is likely most detrimental in salinized environments, where ionoregulatory demands are higher, which in turn should increase respiratory demands along with energy acquisition demands through foraging.