Elevated carbon dioxide and temperature affects otolith development, but not chemistry, in a diadromous fish

•Elevated CO2 affects otolith (ear bone) development in a diadromous fish.•Elevated CO2 and temperature increase otolith perimeter until maximum thermal threshold, after which perimeter reduced.•No effect of CO2 on otolith elemental chemistry, suggesting elements tested are unsuitable tracers of environmental pCO2.

Ocean acidification threatens marine ecosystems by altering ocean chemistry and calcification processes in marine organisms. This study investigated the effects of predicted future CO2 levels, under varying temperature levels, on otolith development (size and shape) and chemistry, with the latter aimed at developing a chemical tracer of environmental pCO2. Juvenile barramundi (Lates calcarifer), a diadromous fish species, were reared in ambient (pCO2: 640 μatm; pH: 7.9) and elevated (pCO2: 1490 μatm; pH: 7.5) pCO2 treatments representing current and projected coastal systems crossed with three temperature levels (26 °C, 30 °C and 34 °C) for 42 days. Otolith shape and size parameters (length, width, perimeter and area) were measured and element concentrations (Na, Mg, Sr, Ba, Li, Mn and B) were quantified using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA ICP-MS). There was an interactive effect of elevated pCO2 and temperature on otolith shape and perimeter, whereas otolith chemistry did not vary among treatments. This study demonstrates that combined elevated pCO2 and temperature can affect the development of important internal structures in diadromous fish, but also suggests that otolith elemental chemistry was not a suitable tracer for pCO2 histories in fish. Future climate change conditions affect an important auditory and balance organ; consequently, rising CO2 levels may interfere with sensory function.

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