Chronic heat stress as a predisposing factor in summer mortality of mussels, Perna canaliculus

Abstract

Marine farmers have reported increased incidence of mass mortalities of New Zealand green-lipped mussels, Perna canaliculus, grown on near-shore long-line aquaculture farms. While the causative agents remain unknown, die-off events typically occur during the warmest months of the austral summer and appear correlated to unprecedented summer marine heatwaves that have impacted the eastern Tasman Sea in recent years. To further elucidate the role of chronic heat stress as a predisposing factor to ‘summer mortality’, adult P. canaliculus were incubated at 17, 21 and 24 °C for 13 months. Overall biological performance was impacted in the elevated temperature treatments; mean mass gain (Specific Growth Rate) at 17 °C was 0.21% day−1, but fell to 0.18% day−1 at 21 °C and 0.16% day−1 at 24 °C. A survival tipping point was apparent between 21 and 24 °C, with net mortality of 6 and 10% at 17 and 21 °C, respectively, increasing to 100% at 24 °C. Sacrificial gill and haemolymph samples taken every 4–8 weeks showed little evidence of temperature perturbation upon either haemocyte function or the metabolomic profile during the first 4–6 months of incubation. Beyond 6 months, the proportion of haemocytes showing signs of respiratory burst (superoxide-positive) or apoptosis increased steadily at 24 °C, along with a substantial increase in the levels of amino acids frequently associated with anaerobiosis or antioxidant support. During this period, even the more benign temperature treatments of 17 and 21 °C showed increased haemocyte apoptosis and a metabolite profile consistent with increasing protein catabolism. Prolonged exposure to artificially stabilised ‘summer' temperatures therefore appears detrimental to P. canaliculus, with important implications for the operation of captive broodstock conditioning systems. While stable, elevated temperatures up to 24 °C are unlikely to fully account for current observations of summer mortality, disruptions to energy metabolism and immunocompetency are likely to increase the mussels' vulnerability to pathogens and other stressors. On-going research is required to assess the impacts of realistic, fluctuating temperatures, with consideration of the implications for potential pathogens.