World Aquaculture - December 2024

26 DECEMBER 2024 • WORLD AQUACULTURE • WWW.WAS.ORG (Namba et al. 1995). Additionally, mussels treated with MgCl2 were less likely to spawn, a significant factor in the mussel market since gonad fullness affects both quality and price (Heasman et al. 1995). In contrast, low oxygen conditions only reduced metabolism by about 30%, proving less effective than the low temperature and MgCl2 treatments. Moreover, oxygen-deficient environments could have negative consequences due to the reliance on anaerobic metabolism, which may degrade meat quality and cause additional metabolic stress (Wells and Baldwin 1995). Does low metabolism mean low stress level for mussels? A lower metabolism doesn’t always mean lower stress. A decrease in aerobic (when oxygen is present) metabolism, paired with an increase in anaerobic (when oxygen is absent) metabolism, can also lead to a suppressed metabolic rate. This shift toward anaerobic metabolism, however, can negatively affect both the survival and quality of the mussels. To detect if the stress associated with anaerobic metabolism is present or not, we can look into the metabolite composition. An organism’s metabolism involves various pathways that produce metabolites. Stress can disrupt these pathways in different ways in mussels (Dunphy et al. 2018). For instance, when energyrelated substances like succinic and citric acids increase in greenlipped mussels (Perna canaliculus), it indicates that the mussels are using more energy. This often happens because of stress resulting from being out of water during handling (Nguyen et al. 2020). Free amino acid levels in bivalves can change to help them cope with the stress caused by changes in salinity in their surrounding environments (Song et al. 2023). Since different stressors influence specific metabolites, these compounds can be used as biomarkers to detect the presence of stress in mussels. Next steps We will compare the stress levels in mussels that received different pre-treatments after simulated live transport to identify those conditions resulting in the lowest levels of stress biomarkers. Additionally, we’ll assess the costs associated with live transport of mussels using those pre-treatments. It may be more expensive to ship mussels in chilled containers maintained at low temperature, so the application of MgCl2 may offer new avenues to reduce both mussel loss and transportation costs. Developing a pre-treatment to slow mussel metabolism and reduce mortality is vital for a sustainable live mussel trade, lowering food waste and enhancing the supply chain. Our study has shown that using low temperatures and MgCl2 can reduce metabolic rate, as indicated by slower heart rates. It is, however, still unclear whether these methods effectively relieve stress during live transport, and further research is needed. The next step could involve analysing metabolites after pre-treatment to determine if stress has been minimised, using specific metabolites as stress indicators. We also aim to apply this method to other shellfish for a more sustainable live supply chain. FIGURE 2. Mussels may die during long-distance live transport due to handling stress, unless they receive a pre-treatment that reduces their metabolism (estimated by heart rate). Three inducers, each of them with four levels, were evaluated. These will be further narrowed down in order to determine the most efficacious pre-treatment to be applied for mussel live transport. Graph by Vivian Ward. FIGURE 3. (a) Non-invasive infrared sensor attached onto the shell of the green-lipped mussels, Perna canaliculus, and (b) example heartbeat traces measured at 14°C, showing variation among three different individuals. FIGURE 4. Mussels’ heart rate (metabolism) at different (a) temperatures, (b) oxygen levels and (c) MgCl2 concentrations for two hours (Treatment control (Con) for temperature = 14 °C, for oxygen = 8 mg/L and for MgCl2 = 0 g/L). Graphs were re-plotted after Cheng et al. 2024.

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