Key aspects of aquaculture of the Flat oyster (Ostrea chilensis): induced thermotolerance and non-invasive larval assessment technique

Abstract

Key aspects in the aquaculture development of the Chilean flat oyster (Ostrea chilensis), requires the development of a non-invasive assessment technique to facilitate the visualisation of larvae within incubating oysters and an assessment of the effects acute thermal challenges confer on the adult oysters. Non-invasive techniques investigated in this study were anaesthesia: magnesium chloride hexahydrate, laboratory reagent grade and AQUI -S® and imaging techniques: HF-MRI and CT Scan. Adult oysters of 8 per replicate in mixtures of 2 L freshwater and 3 L seawater were anaesthetised under nominal concentrations (magnesium chloride: 30, 40, 50 and 60 g/L; AQUI -S® 20, 40, 80 and 100 μL/L) of each anaesthesia respectively plus 2 controls: Control 1: 2 L freshwater and 3 L seawater without magnesium chloride, and Control 2: 5 L seawater only and without magnesium chloride. AQUI-S® trials mimicked those for magnesium chloride using 4 concentrations respectively (20, 40, 80 and 100 μL/L). Parameters for HF-MRI and CT Scan were adjusted according to the imagery capacity respectively. For magnesium chloride, no significant differences were found in the number of oysters anaesthetised at each concentration respectively. Percent mortality was also insignificant across concentrations with similar percent mortality in one of the control groups. No oysters were anaesthetised using AQUI-S®. Both imagery techniques were also unsuccessful. Overall, magnesium chloride was discovered to be most effective at visualising larvae within incubating oysters using 30 g/L established as the optimal concentration to induce anaesthetised state in adult O. chilensis oysters. The effects of acute thermal challenges were assessed by quantifying heat shock protein (HSP) 70 expressed, after subjecting oysters to a temperature gradient (11-38 0C). Pre-stressed treatment group were heat shocked at 20 0C for 1 h before administration of main acute heat shock in the temperature gradient for 3 h. Pre-stress treatment was performed with the aim to induce thermotolerance in the O. chilensis oysters in order to induce production of HSP70 to protect the oysters from the following main heat treatment shock. High variability in HSP70 expression was found between pre-stress and naïve groups. Mortality was recorded higher in the pre-stressed group (38%) than in the naïve group (19%). Mortality was observed only at the highest nominal temperature band 34-38 0C. Overall, the present study successfully developed a non-invasive assessment technique for the visualisation of larvae within incubating oysters and examined HSP response for the first time in adult O. chilensis oysters. The non-invasive technique developed in this study will thus facilitate obtaining a visual of larval presence within incubating oysters which has never been achieved before. Also, this technique will identify broodstock which will be particularly useful during spawning seasons especially when broodstock numbers are minimal. The novel technique for O. chilensis has the great potential to identify broodstock for conditioning through to spawning of larvae, and facilitate fertilisation trials of superior individuals under selective breeding, which could significantly enhance commercial potential and maintain the market share of O. chilensis against other highly productive oysters. Induced thermotolerance could markedly improve stock performance efficiency at low cost and promote resilience during handling, transportation and relaying operations.