Introduction
L ive transport has great potential to maintain the quality of cultured seafood (Fotedar & Evans, 2011). The live transport approach, however, involves different handling processes, which in mussels could cause stress and mortality (Nguyen et al., 2020) . By inducing metabolic depression before transport, live mussels may be less sensitive to stress caused by handling processes associated with live transport . In this study, conditions that trigger metabolic depression were explored using a commercially important green- lipped mussel, Perna canaliculus, as a model species, by measurement of cardiac activity.
Materials and methods
Mussels were exposed to different treatments i.e., temperature (4°C, 6°C, 8°C and 14°C (control)), oxygen level (0.5mgO2L-1, 1mgO2L-1, 3mgO2L-1 and 8mgO2L-1 (control)) and anaesthetic concentration (MgCl2; 0gL-1 (control), 30gL-1 , 40gL-1 and 50gL-1) for two hours. During the exposure, eight mussels from each treatment were sampled at two time points (i.e., 30 minutes and 120 minutes after the exposure started) respectively for measurement of heartbeat (bpm) for 20 minutes.
Results
Linear mixed-effects models showed that temperature, oxygen level and MgCl2 concentration, and the exposure duration had an interactive effect (p < 0.05) on mussel HR (Figure 1). HR of mussels at low temperatures was depressed by 50–100%, whereas HRs of mussels exposed to MgCl2 decreased by 36–97%. Low oxygen levels only dropped mussels’ HRs by 1.5–51%.
Discussion
Depression of metabolic rate was most pronounced in the temperature and MgCl2 treatments. Reasons explaining this finding are body temperature of ectothermic organisms depends on the environmental temperature, which governs the biochemical reactions and, subsequently, metabolism and performance within thermal tolerance windows (Cheng et al., 2018). Decreased body temperature due to dropping environmental temperature could reduce ectotherms’ enzyme activity level and thus reduce metabolism, which could be indicated by depressed physiological rates such as respiration and heart rates (Schulte, 2015). Bivalves can also depress their metabolism by shifting from aerobic to anaerobic metabolism to tolerate prolonged hypoxic conditions (Stevens & Gobler , 2018), which was also demonstrated by depressed HRs of P. canaliculus at 0.5mgO2L-1 over exposure time. Immersion in MgCl2 solution could relax the heart and adductor muscles of P. canaliculus, resulting in reduced HRs as Mg2+ blocks Ca2+ from entering the cell, which obstruct the release of acetylcholine to initiate muscle contraction (Azizan et al., 2021 ; Namba et al., 1995).
By comparison , low temperature and immersion in MgCl2 solution both successfully induced metabolic depression of mussel P. canaliculus. Studies on conditions in the combination of low temperature and MgCl2 should be explored to see if they will further suppress mussels’ metabolism.
References
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