Evolutionary theory predicts that selection will favour phenotypic plasticity in sperm traits that maximise fertilisation success in dynamic fertilisation environments. In species with external fertilisation, osmolality of the fertilisation medium is known to play a critical role in activating sperm motility, but evidence for osmotic-induced sperm plasticity is limited to euryhaline fish and marine invertebrates. Whether this capacity extends to freshwater taxa remains unknown. Here, we provide the first test for plasticity in sperm-motility activation in response to osmotic environment in an anuran amphibian. Unexpectedly, there was no detectable shift in the optimal osmolality for sperm-motility activation after approximately 13 weeks of acclimation (a period reflecting the duration of the winter breeding season). However, in both the low and high acclimation treatments, the optimal osmolality for sperm-motility activation mirrored the osmolality at the natural breeding site, indicating a phenotypic match to the local environment. Previously it has been shown that C. signifera display among-population covariation between environmental osmolality and sperm performance. Coupled with this finding, the results of the present study suggest that inter-population differences reflect genetic divergence and local adaptation.  We discuss the need for experimental tests of osmotic-induced sperm plasticity in more freshwater taxa to better understand the environmental and evolutionary contexts favouring adaptive plasticity in sperm-motility activation.

Male common eastern froglets (Crinia signifera) were acclimated to either low (0 mOsmol kg-1) or high (50 mOsmol kg-1) environmental osmolality and using a split-sample experimental design sperm were activated across a range of osmolality treatments (0, 25, 50, 75, 100 and 200 ± 2 mOsmol kg-1).