Chair: Philip Boyd

Laura J. Falkenberg ⁽¹⁾*, Craig A. Styan ⁽¹⁾, Jon N. Havenhand ⁽²⁾

¹ School of Energy and Resources, UCL Australia, University College London, 220 Victoria Square, Adelaide, SA, 5000, Australia
² Department of Marine Sciences, Tjärnö, University of Gothenburg, Strömstad, 45296, Sweden

Background
Changes in environmental conditions are predicted to fundamentally alter marine ecosystems, particularly where they affect the reproduction of species that determine habitat structure. Of key concern are the synergies of global-scale changes, such as ocean acidification, with other stressors, such as ocean freshening (reduced salinity). Forecasting potential responses to these abiotic changes is complicated by variability within species, both at the individual and population levels. It is, therefore, essential to understand how the species may respond to changes in ocean acidification and other key stressors, such as modified salinity, and the variability of such responses.

Methods
We used the invasive oyster, Crassostrea gigas, as a model species to investigate responses to modified conditions. Specifically, we exposed sperm of individuals from two populations (recently invaded, low salinity, Swedish west coast; and established, high salinity, Guernsey, Channel Islands; n = 14 individuals per population) to modified pCO₂ (612, 925, 1250 µatm) and salinity (33, 23, 13 PSU). The response was measured in terms of sperm motility, a key indicator of reproductive potential.

Findings
We found substantial inter-individual variability in sperm motility, as well as general trends. Responses among individuals varied significantly, with some individuals responding strongly positively and others strongly negatively to acidification and freshening. The Guernsey population showed a trend of increasing sperm motility under ocean acidification, whereas the Swedish population showed no response. In contrast, both populations showed a consistent response to freshening, with reduced salinity associated with lower sperm motility.

Conclusions
Variability within and between populations of the same species can result in marked differences in responses to changed abiotic conditions. Such differences are likely to lead to differential recruitment, and local adaptation, potentially causing divergence in dominant habitat, biodiversity and ecosystem services.