Otolith development and elemental incorporation in response to seawater acidification in the flounder Paralichthys olivaceus at early life stages

Ocean acidification can influence the formation, development and functions of calcified structures in marine organisms, such as otoliths, which are mainly composed of calcium carbonate (CaCO3) and function in orientation, balance, sensory perception and locomotion in fish. This study investigated the impacts of seawater acidification (pH 8.10, 7.70 and 7.30, roughly corresponding to the ocean acidification under RCP 8.5 scenario predicted by the IPCC) on somatic growth, otolith (aragonite) morphology and microchemistry in the flounder Paralichthys olivaceus at early life stages (ELSs, exposed to acidified seawater via pCO2 from embryonic to juvenile stages for 52 days). The results demonstrated that seawater acidification promoted otolith growth (mass and size) but did not change their geometric outlines. Seawater acidification did not alter the somatic growth or otolith elemental incorporation (Sr, Ba and Mg) in the flounder. Seawater acidification increased the occurrence of abnormally developed calcitic otoliths (calcite) which considerably differed from the aragonitic otoliths in surface and crystal structures. Additionally, elemental incorporation (Sr:Ca and Ba:Ca) appeared to be higher in aragonitic otoliths than in calcitic otoliths, which was likely related to their unique manners of formation. Our results agreed with the broad literature, in that seawater acidification showed species-specific influences (positive or no effect) on otolith size but did not affect somatic growth, otolith shape or elemental incorporation of fish at ELSs. These findings provide knowledge for evaluating the ecological effects of ocean acidification on the recruitment and population dynamics of fish in the wild.