The impacts of ocean acidification on sperm swimming behaviour and fertilization success of key Australian marine invertebrates

Ocean acidification is increasingly threatening marine biodiversity. Marine organisms face intense selection pressure for phenotypes that are robust to rapidly acidifying oceans. Greater variability among phenotypes provides greater scope for selection. This variability is a central determinant of the capacity for adaptation, and – if traits that drive robustness are heritable – may ameliorate impacts of ocean acidification. Critical early life history stages possess few protective functions against changing environmental conditions, making broadcast spawning species particularly vulnerable. I explored the extent of inter- and intra-specific variation in responses of gametes to future ocean acidification across five sea urchin species and one polychaete species from Southeast Australia. Flow cytometry and sperm motility analyses were combined to investigate the relation between sperm swimming and mitochondrial function under ocean acidification. All species showed extensive between-male variability in sperm swimming responses to ocean acidification, but sperm of some species (Heliocidaris tuberculata, Tripneustes gratilla) were more robust than others Centrostephanus rodgersii,
Heliocidaris erythrogramma, Holopneustes purpurascens, Galeolaria caespitosa). Enhanced sperm swimming under acidified conditions in some males is likely to increase their reproductive success, supporting the concept of ‘winners’ and ‘losers’ of climate change at an individual level. Acidification-induced reductions in sperm mitochondrial activity in C. rodgersii were not reflected in similar reductions in sperm swimming behaviour. Collectively, these data suggest that capacity for selection of robust sperm phenotypes is present in all species, but that the adaptive benefits of this selection will depend on the degree of heritability of acidification-resistance traits. Shifts in the proportional contribution of robust vs susceptible males to offspring production may severely shift the genetic composition of subsequent generations, with flow-on consequences for species fitness and competitive ability. This may, in turn, alter competitive interactions between species and lead to species-dependent impacts on ecosystem structure and composition.

Schlegel P., 2013. The impacts of ocean acidification on sperm swimming behaviour and fertilization success of key Australian marine invertebrates. PhD thesis, Macquarie University, 227 pp. Thesis.

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