Effects of one-year exposure to ocean acidification on two species of abalone.

Ocean acidification (OA) resulting from the absorption of excess atmospheric CO2 by the ocean threatens the survival of marine calcareous organisms, including mollusks. This study investigated the effects of OA on adults of two abalone species (Haliotis diversicolor, a subtropical species, and Haliotis discus hannai, a temperate species). Abalone were exposed to three pCO2 conditions for 1 year (ambient, ~ 880, and ~ 1600 μatm), and parameters, including mortality, physiology, immune system, biochemistry, and carry-over effects, were measured. Survival decreased significantly at ~ 800 μatm pCO2 for H. diversicolor, while H. discus hannai survival was negatively affected only at a higher OA level (~ 1600 μatm pCO2). H. diversicolor exhibited depressed metabolic and excretion rates and a higher O:N ratio under OA, indicating a shift to lipids as a metabolism substrate, while these physiological parameters in H. discus hannai were robust to OA. Both abalone failed to compensate for the pH decrease of their internal fluids because of the lowered hemolymph pH under OA. However, the reduced hemolymph pH did not affect total hemocyte counts or tested biomarkers. Additionally, H. discus hannai increased its hemolymph protein content under OA, which could indicate enhanced immunity. Larvae produced by adults exposed to the three pCO2 levels were cultured in the same pCO2 conditions and larval deformation and shell length were measured to observe carry-over effects. Enhanced OA tolerance was observed for H. discus hannai exposed under both of the OA treatments, while that was only observed following parental pCO2 ~ 880 μatm exposure for H. diversicolor. Following pCO2 ~ 1600 μatm parental exposure, H. diversicolor offspring exhibited higher deformation and lower shell growth in all pCO2 treatments. In general, H. diversicolor were more susceptible to OA compared with H. discus hannai, suggesting that H. diversicolor could be unable to adapt to acidified oceans in the future.