Geographic variation in thermal tolerance and morphology in a fiddler crab sister-species pair
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.
The authors thank J. Levinton for providing lab space, assisting with fiddler crab collections in New York, and for providing valuable feedback on this manuscript; P. Munguia for providing lab space in Texas; and S. Hicks, S. Bergeron, and S. Rehage for technical assistance. This study benefited from early discussions with R.S. Greenberg on temperature-driven variation in fiddler crab claw size and sparrow bill size. Partial funding for this research was provided by the Nicholls State University Research Council.
Compliance with ethical standards
Human/animal rights statement
All animals used in the study were collected, treated, and handled following established protocols. As only invertebrates were used in experiments, IACUC approvals were not required.
Conflict of interest
The authors declare that they have no conflict of interest.
- Allen BJ (2007) Costs of sexual selection in the sand fiddler crab, Uca pugilator. Dissertation, Stony Brook University, New YorkGoogle Scholar
- Burnham KP, Anderson DR (2002) Model selection and multimodel inference: a practical information-theoretic approach. Springer, New YorkGoogle Scholar
- Castaneda LE, Lardies MA, Bozinovic F (2004) Adaptive latitudinal shifts in the thermal physiology of a terrestrial isopod. Evol Ecol Res 6:579–593Google Scholar
- Crane J (1975) Fiddler crabs of the world (Ocypodidae: genus Uca). Princeton University Press, PrincetonGoogle Scholar
- Faria SC, Faleiros RO, Brayner FA, Alves LC, Bianchini A, Romero C, Buranelli RC, Mantelatto FL, McNamara JC (2017) Macroevolution of thermal tolerance in intertidal crabs from Neotropical provinces: a phylogenetic comparative evaluation of critical limits. Ecol Evol 7:3167–3176CrossRefGoogle Scholar
- PRISM Climate Group (2012) Oregon State University. http://prism.oregonstate.edu. Accessed 16 Aug 2017
- Roberts B, Espinosa J, Heilman K, Brodie R (2015) Southern males are bigger but northern males are more honest: latitudinal trends in male claw traits of the fiddler crab Uca pugnax. Integr Comp Biol 55:E321Google Scholar
- Shih HT, Ng PKL, Davie PJF, Schubart CD, Turkay M, Naderloo R, Jones D, Liu MY (2016) Systematics of the family Ocypodidae Rafinesque, 1815 (Crustacea: Brachyura), based on phylogenetic relationships, with a reorganization of subfamily rankings and a review of the taxonomic status of Uca Leach, 1814, sensu lato and its subgenera. Raffles Bull Zool 64:139–175Google Scholar
- Vernberg FJ, Vernberg WB (1966b) Studies on the physiological variation between tropical and temperate zone fiddler crabs of the genus Uca. VI. The rate of metabolic adaptation to temperature in tissues. J Elisha Mitchell Sci Soc 82:30–34Google Scholar
- Vernberg WB, Vernberg FJ (1968b) Studies on the physiological variation between tropical and temperate zone fiddler crabs of the genus Uca. VIII. The rate of metabolic adaptation to temperature in tissues of Uca rapax from the northern and southern hemispheres. J Exp Mar Biol Ecol 2:113–123CrossRefGoogle Scholar