Multi-stressor dynamics: Effects of marine heatwave stress and harmful algal blooms on juvenile mussel (Perna canaliculus) survival and physiology.

In New Zealand, the frequency and intensity of marine heatwaves (MHWs) and blooms of the harmful algal species, Alexandrium pacificum, are increasing in areas where there are natural reefs and commercial farms of the mussel, Perna canaliculus. In this study, we assessed the whole organism, tissue and molecular-level response of juvenile (spat) P. canaliculus exposed to these abiotic and biotic stressors, alone and together. Spat of P. canaliculus were held at a control temperature of 17 °C or a MHW temperature of 22 °C for the duration of the experiment and exposed to two environmentally relevant concentrations of the dinoflagellate A. pacificum for 4 days, followed by a recovery period of 4 days. The MHW temperature alone had no detrimental effect on mussel survival; however, exposure to A. pacificum at both temperatures led to reduced spat survival, especially at higher microalgal concentrations (< 90 % survival vs 100 % at the control temperature, no A. pacificum). The combination of both MHW temperature and A. pacificum exposure resulted in higher mortality than either stressor alone (as low as 59 % survival compared to 99 % at the MHW temperature alone). Spat exposed to A. pacificum alone showed up to a 4-fold reduction in byssal plaque production, crucial for spat attachment and therefore survival. Growth rate was >7.5 times slower, and there were increased histological alterations as compared to mussels in the control treatment. Similarly, spat exposed to A. pacificum showed significant changes in gene expression, particularly in pathways related to lipid metabolism and detoxification. Spat exposed to MHW temperatures alone had fewer differentially expressed genes, most being heat shock proteins. These findings emphasise the importance of understanding multi-stressor impacts in marine environments, particularly in the context of climate change and harmful algal blooms (HABs).