Geographic variation in thermal tolerance and morphology in a fiddler crab sister-species pair

Abstract

Temperature-adaptive physiological and morphological variation plays a large role in determining species’ geographic ranges and responses to climate change. We examined critical thermal maximum (CTmax) and two thermally relevant morphological traits across multiple populations of two species of fiddler crabs, Leptuca panacea and Leptuca pugilator, spanning a latitudinal thermal gradient from South Padre Island, Texas (26.0850°N) to Long Island, New York (40.9357°N). CTmax was measured on crabs collected in 2015, while morphology was measured on crabs collected between 2012 and 2015. CTmax differed among populations and was greatest in populations experiencing a warmer thermal regime. CTmax did not differ between the two species at the site where they overlapped and experienced identical thermal regimes. These results indicate that large-scale (latitudinal) thermal gradients can shape thermally relevant physiological characteristics. Geographic patterns of the two morphological measurements (carapace width and relative claw length) were not consistent between the two species, and often ran counter to our expectations. Thermoregulatory ability is optimized by large body size and a large claw, and we thus hypothesized that carapace width and claw length would be positively correlated with environmental temperature. Carapace width exhibited a positive relationship with environmental temperature in L. panacea, but conversely exhibited a negative relationship in L. pugilator. Claw length was negatively correlated with temperature in both species. These morphological results highlight the need to consider the multiple, presumed interacting selective pressures shaping morphological variation among populations and species.