Maintained growth in mussel (Mytilus edulis) larvae exposed to acidified under-saturated seawater and physiological tipping point

Chair: Sue-Ann Watson

Alexander Ventura (1), Sabrina Schulz (1), Sam Dupont (1)*

1 Department of Biological and Environmental Sciences, University of Gothenburg, The Sven Lovén Centre for Marine Sciences – Kristineberg 566, SE-451 78 Fiskebäckskil, Sweden

Background
Ocean acidification (OA) severely affects bivalve early life-stages. Commercially and ecologically important blue mussels (Mytilus edulis) can tolerate wide fluctuations in several environmental parameters. However information on their physiological tolerance thresholds in relation to OA is currently lacking. Our aim was to investigate the effects of a wide range of seawater pH on different physiological parameters of M. edulis larvae in order to identify a tipping point beyond which they are no longer capable of carrying out functions necessary to their survival.

Methods
After fertilizing mussel eggs in the laboratory larvae were exposed in triplicate to 5 different pHs (nominal pH 8.1, 7.85, 7.6, 7.35 and 7.1) for 48 days. Mortality, growth, abnormality, calcification and feeding rates were measured throughout the course of the experiment.

Findings
Larvae reared at pHT ≈ 7.1 were not capable of producing a shell and their development was arrested at the trochophore stage. We identified this as a tipping point for blue mussel larvae. However in some of the higher pH treatments (pHT ≈ 7.6, 7.85 and 8.1), including in seawater under-saturated with respect to aragonite, we found some individuals were able to develop a normal shell and their growth rate appeared to be unaffected.

Conclusions
Our results indicate a considerable degree of tolerance to OA in M. edulis larvae up to pHT ≈ 7.1 which we identified as their physiological threshold. Additionally we observed no impact of pH on growth of normally developing larvae, a likely consequence of shifts in energy budget toward calcification which resulted in unaffected net calcification.