Occurrence of Amphibians in Saline Habitats: A Review and Evolutionary Perspective

Amphibians are well known as osmotically sensitive organisms due to their highly permeable skin and eggs and, as such, biologists have mostly discounted their presence in saline environments. Yet, from the 1800s to the present day, scientists have repeatedly found amphibians living and breeding in a variety of saline coastal and inland habitats. Despite this plethora of observations, their presence in these habitats is still mostly ignored, and the last (and only) complete literature review documenting amphibians in brackish and saline habitats was completed over 50 yr ago. Here we provide a review of the literature of amphibians in saline waters and present data on 144 species, in 28 families, on every continent except Antarctica. In doing so, we make the case that salt tolerance in amphibians may not be as rare as generally assumed. Through classifying habitats and studies, we conclude that the abilities of dozens of species to locally adapt to coastal and inland saline habitats have been extensively studied, although more work on most observed species is still needed. Our understanding of the evolutionary processes leading to this adaptation is also in its infancy. We summarize the existing knowledge on this subject and present a possible framework toward the development of an evolutionary model of amphibian adaptation to salt, based on genetic variation for salt tolerance in populations and the nature of selection events in osmotically stressful environments. Finally, we discuss some possible limitations on the ability of amphibians to tolerate salt water. Understanding the abilities and constraints of amphibian populations to adapt to salt will become more critical as humans continue to impact the world’s freshwater resources through climate change, landscape modification, and pollution, and these habitats thus become increasingly stressful for amphibians.