Chair: Andrew McMinn

Victoria Cole ^(1)*, Laura Parker ⁽²⁾, John Wright ⁽¹⁾, Elliot Scanes ⁽¹⁾, and Pauline Ross ⁽¹⁾

¹ School of Science and Health, Western Sydney University, NSW, 2751, Australia
² School of Biological Sciences, The University of Sydney, NSW, 2006, Australia

Background
Molluscs are known to be highly vulnerable to climate change, yet species-specific responses differ across the globe. Climate change provides the opportunity for some molluscs to be more successful, and for introduced species to become invasive. It is unknown how climate change will affect interactions between introduced and endemic molluscs, and the diverse communities they support. It was hypothesised, climate change would affect competitive interactions between endemic and introduced mussels, and subsequent recruitment and behaviour of colonising infauna.

Methods
Novel outdoor mesocosm experiments tested effects of temperature and/or pCO₂ on endemic Trichomya hirsuta and introduced Mytilus galloprovincialis mussels, and colonisation of infauna. Experiments consisted of orthogonal combinations of temperature (ambient 22C or elevated 25C), pCO₂ (ambient 400μatm or elevated 900μatm), mussel species (endemic or introduced), and mussel configuration (endemic, introduced, or both), n=3 replicates. This experimental design allowed quantitative tests of competition between mussel species, and active choice behaviour of colonising infauna. Competition between mussel species was measured as the effects on clearance rates, Standard Metabolic Rates (SMR), growth, and maternal investment (egg size).

Findings
Elevated pCO₂ increased the competition between endemic and introduced mussels that was not apparent under ambient conditions. Competition decreased clearance and SMR of T. hirsuta, there was no influence on growth or the size of eggs produced by either species, but the eggs of M. galloprovincialis were larger under elevated pCO₂.

Temperature and pCO₂ influenced the infauna that recruited in both species of mussels. There were significant changes to behaviour of infauna such that many species preferred to colonise one species of mussel over another.

Conclusions
Competition between endemic and introduced molluscs, choices of infauna, and subsequent biodiversity, will be altered in a high CO₂ world. These types of mesocosm experiments are required if we are to predict the future for temperate reef biodiversity.