Interspecific comparisons of anuran embryonic epidermal landscapes and energetic trade-offs in response to changes in salinity.

Background: Freshwater salinization is an emerging stressor in amphibian populations, and embryonic stages are most vulnerable. To better understand the variation in embryonic osmoregulation, we challenged embryos of two phylogenetically diverse anuran species, Xenopus laevis and Lithobates (Rana) sylvaticus, along a gradient of non-lethal salinities. We hypothesized embryos at higher salinities will display epidermal plasticity as a coping response and increase energy expenditure related to osmoregulation demands, thereby reducing energy for growth and development.

Results: Scanning electron microscopy revealed an extra mucus-secreting cell type and higher ionocyte proportions in the X. laevis epidermis, suggesting more osmoregulatory machinery than L. sylvaticus. Under elevated salinity, X. laevis displayed greater increases in goblet cell proportions, mucus secretion, and reductions in ionocyte apical area compared with L. sylvaticus. Although both species increased oxygen consumption rates and reduced body length with elevated salinity, these effects were proportionally greater in L. sylvaticus at the highest salinity, and only this species slowed developmental rates.

Conclusion: These findings support the hypothesis that frog embryos respond to salinity by altering the cellular landscape of their epidermis. We show that epidermal cell types, as well as the magnitude of epidermal plasticity and energetic trade-offs in response to salinity, vary among amphibian species.