28 SEPTEMBER 2019 • WORLD AQUACULTURE • WWW.WAS.ORG the study assessed the effects of acidification on survival, growth and metabolic rates of oyster larvae. In addition, the study began to identify the processes that may enable survival by first assessing whether food availability enhances resilience of these early life stages. These questions are particularly important because a decrease in survival and size of larvae can significantly impact the commercial industry as a result of decreased yield. Studying metabolic rates is vital to understanding the health of an organism as it can indicate whether an animal is under stress or has high energetic demands that must be met with high food availability. Experimental Design Eastern oysters were spawned and embryos were exposed to seawater with a pH similar to the average global, ambient pH (ambient condition, pH 8.1, pCO2 400 ppm) or seawater with a pH predicted for the end of the century (elevated condition, pH ~7.5, pCO2 1000 ppm). Developing larvae (Fig. 1) were held at 10 larvae/mL at 23 C in a standing system with target pH maintained by diffusing ambient air or CO2 mixed with air. After 24 hours, the number of live larvae was assessed by microscopic observation. Growth was assessed using the ImageJ image analysis program on formalin-preserved samples. Respiration rates were measured by adding two-weekold larvae to a sealed, air-free chamber and measuring oxygen consumption over time. Changes in Physiology Viability of oysters exposed to future pH conditions was significantly reduced compared to oysters maintained at ambient pH conditions (Fig. 2a). Oysters exposed to elevated CO2 were significantly smaller in length than those in the ambient condition after just 24 hours of exposure (Fig. 2b). Oysters under elevated CO2 consumed significantly more oxygen and, therefore, had greater respiration rates than those under ambient conditions (Fig. 2c). Increased respiration rates are indicative of high stress under acidified conditions and more energy spent on survival. Food Limitation Larvae were adversely impacted by ocean acidification and that, perhaps to compensate, larvae increased respiration rates. As metabolic rates are defined as the daily amount of energy an individual requires over a period of time, increasing metabolic rates therefore increases energy demands. If oyster larvae require more energy under conditions of ocean acidification, there may be some mechanism involving energy availability that helps larvae tolerate the stress. Based on this assumption, we tested the hypothesis that resilience to ocean acidification is related to some energy allocation mechanism and the availability of food resources in the form of algae. If oysters are not limited with regard to available energy in algae, then they should be able to cope with the stress of acidification FIGURE 1. Larval (24-h old) eastern oysters. FIGURE 3. Percent mortality for larval eastern oysters (mean ± 95 percent confidence interval) under ambient and elevated pCO2 and two different food concentrations. Different letters indicate a significant difference at p<0.05. FIGURE 2. Percent survival (a), size (b) and respiration rates (c; µmol O2 consumed per individual per hour) of eastern oyster larvae grown under ambient and elevated pCO2 . Mean ± SE. * indicates a significant difference (p<0.05) between treatments.
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