Sperm performance and sexual selection in a rapidly changing ocean for broadcast spawning marine invertebrates.

Sperm are often considered to be simple packets of ‘swimming DNA’, yet spermatozoa are the most diverse cell type in the animal kingdom as a result of the strong selective pressure on their success in achieving successful fertilisation. The majority of benthic marine invertebrates reproduce by freely releasing their sperm directly into their surrounding seawater. But this environment into which sperm are released is now changing at an unprecedented rate as a result of warming, acidification and deoxygenation, in combination with the continued input of historic and modern contaminants, raising the potential for the selective pressures acting on sperm performance and fertilisation to change. Here I present our work demonstrating impacts of these changes in ocean physico-chemistry on sperm function at all biological levels from physiology and swimming performance to fertilisation ecology and sperm competitiveness.

In the polychaete Arenicola marina we observe ocean acidification (OA) exposures reduce average sperm swimming speeds across a population, with impacts increasing with time. In sea urchins, we demonstrate a strong inverse relationship between individual fertilization success rate under current conditions and the change in fertilization success under OA. Individuals with a high fertilization success under current conditions had reduced fertilization under OA, while individuals with a low fertilization success under current conditions improved. OA also alters the behaviour and speciation of many ionisable contaminants, such as metals and pesticides, which we show can alter their spermiotoxic effects resulting in additional indirect effects of OA for coastal invertebrates living in contaminated areas. Finally, I discuss our work demonstrating the implications of these alterations in function for sperm competition and sperm egg recognition, important aspects of sexual selection and which ultimately determines the winners and losers in a future ocean.