Ocean acidification Impacts Otolith Morphology and Mineralogy in Larval Haemulon chrysargyreum

Chair: Martin Grosell

Robert J Holmberg(1), Andrew L Rhyne(2,3), Michael F Tlusty(1,3), Bradford Bourque(2), Eric Wilcox-Freeburg(1), Robyn Hannigan(1)

1 University of Massachusetts Boston, Boston, MA, 02125, USA
2 Roger Williams University, Bristol, RI, 02809, USA
3 New England Aquarium, Boston, MA, 02215, USA

Background
Ocean acidification is expected to influence calcification of otoliths in teleost fishes, which may have profound consequences for hearing and gravisense. Much research has investigated effects on otolith morphology in fish larvae, but little has considered effects on mineralogy. We selected the smallmouth grunt (Haemulon chrysargyreum) for its long larval cycle, and analysed otolith morphology and mineralogy after rearing in pCO2 treatments that simulate future ocean scenarios.

Methods
Multiple thousand H. chrysargyreum larvae were reared in 4 pCO2 treatments (pH 8.1, 7.8, 7.6, 7.3) until settlement. Treatment setpoints were achieved by dosing CO2 into continuously-monitored experimental aquaria with our dynamic pH-stat CO2 dosing system. Aquaria were replicated 5x for 20 independently-controlled experimental units. All 6 otoliths were extracted. Stereomicrographs were analysed for area, perimeter, and circularity. Scanning electron micrographs were scored for crystal habit, mineralogy (proportion of vaterite vs. aragonite), % visible minerals, and core development.

Findings
Fish standard length peaked in moderate pCO2 before levelling and finally plummeting in highest pCO2. Right lapilli circularity decreased and right asterisci circularity increased with increasing pCO2. No differences in sagittae circularity or otolith area and perimeter were observed between treatments. Changes to otolith mineralogy, including enhanced core development and shifting crystal habit, were observed between treatments. Sagittae in which crystal habit shifted from orthorhombic aragonite to 100% amorphous CaCO3 were observed in acidified treatments.

Conclusions
Our study boasted 3x longer exposure relative to earlier studies involving clownfishes, facilitating observation of longer-term ocean acidification effects on teleost reef fishes. It is the first of its kind to observe any effect of ocean acidification on asterisci in fishes. Furthermore, it represents an unprecedented analysis of otolith mineralogy in the context of ocean acidification. We anticipate that the observed effects on otolith mineralogy, manifested as topographical abnormalities, will impair otoconial function in teleosts.