The role of physiological variation in delimiting geographical ranges of parapatric cryptic species

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A species’ geographic range is defined by a variety of factors, including dispersal, biotic interactions, historical biogeography, and physiology. Chief among these are the stresses imposed by temperature, but experimental work often tests the physiological limits of temperature alone. Here, I will argue that organisms have adapted to a multidimensional set of environmental stresses, and that these can impact physiological tolerance in a synergistic fashion. To test this, I propose a study of the physiological and genomic basis of range differences between two cryptic species with partially overlapping ranges and high dispersal abilities.Melampus bidentatus nov. sp. N and M. bidentatus nov. sp. S are cryptic species of high intertidal marsh snails. Collectively, the two species span nearly 25 latitudinal degrees, over which they are exposed to a wide range of environmental conditions; both species survive cold winters via freeze tolerance. Interestingly, they have partially overlapping ranges with discontinuous regions of allopatry and different northern range limits. Where their ranges overlap, they are found in complete sympatry at locales with intermediate ocean salinity, but at high- and low- salinity locales are segregated between the species. Previous tests of cold tolerance have found no difference in the degree to which both species are freeze tolerant, suggesting that cold alone does not explain their range differences. I propose using a combination of whole-genome comparisons, physiological tests, and field surveys to test the idea that interactions between low temperature and sea salinity will explain range differences between these two very similar species. These results would have important implications in understanding the role of the environment in limiting species boundaries, the genomic basis of adaptive differences among close relatives, and in predicting potential range shifts under climate change.