Does ion limitation select for pelvic reduction in threespine stickleback (Gasterosteus aculeatus)? Jennifer L. Rollins 1 , Brian K. Lohman 2 and Michael A. Bell 1 1 Department of Ecology and Evolution, Stony Brook University, Stony Brook, New York, USA and 2 Department of Integrative Biology, University of Texas at Austin, Austin, Texas, USA ABSTRACT Background: Resident freshwater threespine stickleback (Gasterosteus aculeatus) populations exhibit reduced bony armour structures (including the pelvis) that are always maintained in their marine or anadromous ancestors. Environmental ion and calcium limitation may select for armour reduction in freshwater habitats by increasing the energetic costs of importing ions from dilute fresh water. Armour reduction may allow increased somatic growth and reproductive effort. Hypothesis: Gasterosteus aculeatus with reduced pelvic skeletons grow faster than those with robust pelvises in water with low total ion or calcium concentration. Methods: We estimated growth rate (age-adjusted size) of field-caught G. aculeatus from four freshwater populations. Each of these populations is highly variable for the magnitude of pelvic expression, so we could tease apart the effects of pelvic phenotype and population statistically. We also estimated growth (size at 138 days post-fertilization) of specimens with large and small pelvic structures raised in the laboratory under contrasting total ion and calcium concentration regimes. Results: Growth rates of field-caught G. aculeatus did not differ among individuals with small and large pelvic phenotypes within samples from each of four freshwater populations. Similarly, growth of laboratory-reared specimens from two of these populations did not differ among pelvic phenotypes at low total ion or low calcium concentration. Our comparisons provide no evidence that development and maintenance of a larger pelvis reduces growth under conditions of low ion availability either in nature or in the laboratory. Keywords: selection agent, growth rate, calcium limitation, ion concentration, salinity, trade-off. INTRODUCTION A fundamental objective of evolutionary biology is to understand the causes of phenotypic divergence. Associations between phenotypes and habitat variables often provide the initial evidence for divergent natural selection and selection agents (Endler, 1986; Wade and Kalisz, 1990; MacColl, 2011). However, multiple environmental variables may vary together between the Correspondence: J.L. Rollins, Department of Ecology and Evolution, Stony Brook University, Stony Brook, NY 11794-5245, USA. e-mail: jennifer.rollins@stonybrook.edu Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2014, 16: 101–120 © 2014 Jennifer L. Rollins