Effects of reduced pH on the growth and survival of postlarvae of the donkey’s ear abalone, Haliotis asinina (L.)

Abstract

High atmospheric CO₂ levels primarily from burning fossil fuels have increased CO₂ concentration in the surface water of the ocean, this in turn has resulted in a drop in seawater pH known as “ocean acidification” (OA) and presents a potential threat to calcifying marine organisms. Up to the present, however, the impacts of OA on the early developmental stages of tropical abalone were not known. For the first time in the Philippines, we investigated the direct effects of reduced pH on the growth and survival of the postlarvae of the donkey’s ear abalone, Haliotis asinina L. H. asinina is the only commercial species of haliotid in the country. We reduced the ambient pH (7.99) of experimental seawater by bubbling food-grade CO₂ to obtain the desired pH levels as expected by the year 2100. Survival of H. asinina postlarvae was negatively affected by reduced pH. Mean survival of postlarvae significantly decreased from 86.3 % at ambient conditions to 47.2 and 18.3 % at pH 7.62 and 7.42, respectively, after 20 days (P < 0.001 for both). Survival at pH 7.81 (69.9 %) was comparable with ambient conditions (P > 0.05). Mean shell lengths of postlarvae also differed significantly among treatments and across exposure period (P = 0.000 for both). Postlarvae reared at pH 7.62 and pH 7.42 exhibited smaller shell size (138 and 124 µm, respectively) compared to those at ambient conditions (175 µm). Postlarvae reared at pH 7.81 showed larger shell length (162 µm) compared to the postlarvae reared at pH 7.42 (124 µm). Across time, significant increase in shell size from day 5 to day 20 was observed only in postlarvae reared at ambient conditions (P = 0.000) and in the pH 7.81 treatment (P = 0.006). Daily growth rate decreased significantly from 2.83 µm day⁻¹ (ambient) to 0.95 and 0.30 µm day⁻¹ at pH 7.62 and 7.42, respectively. Our results suggest that growth and survival of postlarval H. asinina was found to be sensitive to reduced pH of seawater. This implies that future decrease in pH unit of the world’s oceans will most likely threaten the natural population of this economically important species.