Degree Name

International Bachelor of Science (Honours)


School of Earth, Atmospheric and Life Sciences


Phillip Byrne


Globally, anthropogenic forces are causing the osmolality (concentration of dissolved solutes) of aquatic environments to increase, with the salinisation of freshwater bodies of particular concern. With rapid increases in salinity in freshwater environments becoming an increasing threat to biota, research is needed to determine the adaptive ability of individual species. Whilst the responses of whole organisms to changing environmental osmolality have been extensively studied, the responses of sperm are mostly unknown. In externally fertilising species, the osmolality of the fertilisation medium is known to play a key role in the activation of sperm motility, which is essential for successful fertilisation. Between species, and even populations, sperm motility activation has been found to peak at specific osmolalities, and sperm performance rapidly declines at extreme osmolalities. As the osmolality of environments fluctuate or shift, osmolality-induced phenotypic plasticity in sperm traits may allow for optimal sperm motility activation, however, there is currently little research into this possibility. This is especially true when considering anuran amphibians (frogs and toads), with no current understanding of the potential for phenotypic plasticity in anuran sperm in response to changing osmotic environments. This thesis aimed to investigate osmolality-induced plasticity in sperm motility activation in the common eastern froglet, Crinia signifera. A previous study in this species has shown that the optimal osmolality for sperm motility activation is reflective of the breeding site osmolality from which the frog originates. The thesis aim was addressed by acclimating 16 adult male frogs, which were collected from a single population, to low (0 mOsm kg-1) or high (50 mOsm kg-1) osmolality for a period of 13 weeks and then assessing sperm performance across a range of osmolalities. Sperm was obtained via testicular maceration and a split-sample experimental design was used, where sperm were activated in six different activation medium osmolalities: 0, 25, 50, 75, 100 and 200 mOsm kg-1. Following activation, sperm motility (% sperm motile; % sperm progressively motile) and sperm velocity (average path velocity, VAP; curvilinear velocity, VCL) were assessed using computer assisted sperm analysis. It was predicted that there would be a significant relationship between the acclimation treatment and the activation medium osmolality, with optimal sperm performance reflective of the acclimation treatment osmolality. Results of this study revealed that there was no effect of the acclimation treatment on sperm performance, however there was a significant effect of the activation medium osmolality, with sperm motility and sperm velocity negatively associated with osmolality. Additionally, sperm performance was optimal when activated in an activation medium osmolality closest in value to the breeding site osmolality. When considered in the context of the previous study in Crinia signifera which demonstrated inter-population variation in optimal sperm osmolality, these findings suggest that differences between populations are likely the outcome of genetic divergence and local adaptation, as opposed to phenotypic plasticity. Importantly, this study is the first to indicate the potential for local adaptation in anuran sperm traits in response to the osmotic environment. The insights this study has provided have far-reaching implications for the conservation of threatened anuran species, because certain species may have limited capacity to respond to changes in salinity, with the potential for large negative effects on fertilisation success and population viability.

FoR codes (2008)

060806 Animal Physiological Ecology



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.