Evol Ecol Res 16: 101-120 (2014) Full PDF if your library subscribes.
Does ion limitation select for pelvic reduction in threespine stickleback (Gasterosteus aculeatus)?
Jennifer L. Rollins1, Brian K. Lohman2 and Michael A. Bell1
1Department of Ecology and Evolution, Stony Brook University, Stony Brook, New York, USA and 2Department of Integrative Biology, University of Texas at Austin, Austin, Texas, USA
Correspondence: J.L. Rollins, Department of Ecology and Evolution, Stony Brook University, Stony Brook, NY 11794-5245, USA.
e-mail: jennifer.rollins@stonybrook.eduABSTRACT
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.
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