Abstract
Ocean acidification (OA) is predicted to be a major driver of ocean biodiversity change. At projected rates of change, sensitive marine taxa may not have time to adapt. Their persistence may depend on pre-existing inter-individual variability. We investigated individual male reproductive performance under present-day and OA conditions using two representative broadcast spawners, the sea urchins Lytechinus pictus and Heliocidaris erythrogramma. Under the non-competitive individual ejaculate scenario, we examined sperm functional parameters (e.g. swimming speed, motility) and their relationship with fertilization success under current and near-future OA conditions. Significant inter-individual differences in almost every parameter measured were identified. Importantly, we observed strong inverse relationships between individual fertilization success rate under current conditions and 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. Change in fertilization success ranged from −67% to +114% across individuals. Our results demonstrate that while average population fertilization rates remain similar under OA and present-day conditions, the contribution by different males to the population significantly shifts, with implications for how selection will operate in a future ocean.
Highlights
Ocean acidification (OA) is expected to become one of the greatest drivers of global ocean biodiversity change [1]
motile sperm (MOT) was significantly affected by OA in both species; an increase in sperm MOT in response to OA conditions was observed in L. pictus whereas in H. erythrogramma, MOT decreased at pH 7.90 but increased slightly again at pH 7.70 (95.39% under current conditions, 94.28% at pH 7.90, 96.95% at pH 7.70)
OA is predicted to be a significant driver of global ocean biodiversity change this century, with species-specific ‘winners’ and ‘losers’ already being estimated based on populationlevel sensitivities [1,42]
Summary
Ocean acidification (OA) is expected to become one of the greatest drivers of global ocean biodiversity change [1]. Understanding how OA affects sperm function and fertilization success on an individual basis is required to build a better understanding of the potential that existing phenotypic variation in sperm performance traits will influence how marine species will respond globally. The individual male-pooled female design provided a ‘common’ fertilization environment for the males thereby reducing variability caused by influential gamete incompatibility and increasing the likelihood of accurately identifying inter-individual trait variability in male responses to OA [39,40] We note that this design does not consider the competitive fertilization scenario that can influence individual reproductive performance [24,25]. For H. erythrogramma, 16 males were included in fertilization success analyses but only 14 males were included in sperm parameter analyses due to problems with CASA video analysis under current conditions. Figures were produced using GraphPad PRISM v. 7.03 for Windows (GraphPad Software, La Jolla, CA, USA, www.graphpad.com)
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