Anaesthetic induced relaxation of the winged pearl oyster, Pteria penguin, varies with oyster size and anaesthetic concentration

Stress and mortality of pearl oysters during nucleus implanting for round pearl and mabé pearl production can be reduced using appropriate anaesthetics that allow improved access to nucleus implanting sites. This study evaluated the efficacy of three different concentrations of benzocaine (0.25, 0.50 and 1.20 g L-1) and 1-propylene phenoxetol (2.50, 3.00 and 3.50 mL L-1) when presented to ‘small’ (dorso-ventral height [DVH], 78.7 ± 1.6 mm), ‘medium’ (DVH, 118.2 ± 2.0 mm) and ‘large’ (DVH, 149.3 ± 1.1 mm) cohorts of the winged pearl oyster, Pteria penguin. Results showed the following general trends across treatments with both anaesthetics: (1) greater proportions of large oysters became relaxed compared to small oysters; (2) large oysters required shorter exposure times to become relaxed than small oysters; (3) for each size class of oyster, an increase in anaesthetic concentration resulted in an increased proportion of relaxed oysters; and (4) ‘mantle collapse’ (where the mantle collapses away from the shell) was only recorded in large oysters in treatments with higher concentrations of anaesthetics. The most effective concentration of benzocaine to use with small, medium and large Pt. penguin was the highest level tested in this study (1.20 g L-1). Similarly, the highest concentration of 1-propylene phenoxetol tested (3.5 mL L-1) was also the most effective with all three size classes of Pt. penguin. These treatments caused mantle collapse in large oysters, for which use of lower, less effective anaesthetic concentrations may be considered preferable, to avoid potentially negative impacts of mantle collapse on subsequent mabé pearl production. As well as efficacy, choice of anaesthetic should consider ease of preparation and preparation time. Benzocaine requires dissolving in methyl alcohol and heating to 88–92˚C, while 1-propylene phenoxetol is readily soluble in seawater.