Phenotypic biomarkers in selectively-bred families of the Greenshell™ mussel (Perna canaliculus): Anaerobic enzyme and shell gape behaviour as biomarkers of prolonged emersion tolerance

Abstract Breeding Greenshell mussel ( Perna canaliculus ) families resilient to live shipment stress offers an opportunity to increase export revenue for the New Zealand mussel industry. We tested two key phenotypic responses (enzyme activity and shell gape) of selectively bred mussel families grouped ( n  = 3 × families/grouping) into those that showed ‘good’ (LD 50  = 16.9–17.5 days), ‘average’ (LD 50  = 13.3–13.9 days) or ‘poor’ (LD 50  = 9.2–9.6 days) survivorship during a 30 day emersion trial. Mapping daily fold change of anaerobic enzyme activity revealed two phases in enzyme activity levels regardless of family. Specifically, an initial down-regulation of the enzymes alanopine dehydrogenase, and strombine dehydrogenase was observed in days 1–4 of the trial. This was then followed by marked up-regulation of enzyme activity, although the day at which up-regulation occurred varied among families; with ‘good’ surviving families up-regulating anaerobic metabolism earlier than either ‘average’ or ‘poor’ surviving families (i.e. 1–4 vs 6–9 and 6–8 days respectively). Such a result suggests that switching to anaerobic metabolism earlier confers greater tolerance to emersion stress. Peak level of enzyme activity, however, appears less important to withstanding prolonged emersion. Shell integral (valve angle and duration of gape) was used to compare gape behaviour among mussel families with ‘good’ and ‘poor’ emersion tolerance. Once emersed, mussels exhibited two behavioural phases; i.e. valves were initially closed for longer periods in the early stages of emersion (days 1–2). This was then followed by longer, more pronounced gaping. Moreover, ‘good’ surviving families switched behaviours earlier than ‘poor’ surviving families which presumably facilitated greater levels of gas exchange and helped to prolong survival. Thus, mussel families that were tolerant to prolonged emersion initiated favourable metabolic and behavioural adjustments earlier within the trial. Whilst the exact mechanisms underpinning emersion tolerance are complex, these two phenotypic responses offer potential biomarkers of emersion tolerance in selective breeding programmes of bivalve species.