Paper Reviewed
Maneja, R.H., Frommel, A.Y., Geffen, A.J., Folkvord, A., Piatkowski, U., Chang, M.Y. and Clemmesen, C. 2013. Effects of ocean acidification on the calcification of otoliths of larval Atlantic cod Gadus morhua. Marine Ecology Progress Series 477: 251-258.

Due to the fact that fish otoliths or ear-bones are composed of aragonite, Munday et al. (2011) have indicated there is a concern that they "could be susceptible to the declining carbonate ion concentrations associated with ocean acidification," which possibility could well be imagined to be quite serious, since they note that "fish ears detect sound, body orientation and acceleration from the position of the otoliths in the inner ear and movement of the otoliths over sensory hair cells," citing in this regard Helfman et al. (1997) and Popper and Lu (2000). And in light of this knowledge, Maneja et al. (2013) have more recently taken it upon themselves to determine the potential effects of an increase in atmospheric CO2 on the calcification of otoliths of Atlantic cod (Gadus morhua), which they accomplished by rearing, from March to May, Atlantic cod larvae in seawater of three different pCO2 concentrations: control (370 µatm), medium (1800 µatm) and high (4200 µatm).

The findings of the seven German scientists are expressed thusly in the concluding paragraph of their paper: "under conditions mimicking global and locally enhanced ocean acidification, otolith growth in Atlantic cod larvae increased," and they add that their growth rates "occurred earlier than under normal conditions." However, they more importantly indicate that "the magnitude of fluctuating asymmetry [the deviation from perfect bilateral symmetry caused by environmental stresses] did not increase, nor did it lead to directional bias in otolith growth or to severe damage in otolith shape." And, therefore, they concluded that "the increase in otolith calcification due to elevated pCO2 might be well within the natural variation in otolith sizes," which they say would enable the fish to maintain what they described as "normal functioning."

References
Helfman, G.S., Collette, B.B. and Facey, D.E. 1997. The Diversity of Fishes. Blackwell Science, Malden.

Munday, P.L., Hernaman, V., Dixson, D.L. and Thorrold, S.R. 2011b. Effect of ocean acidification on otolith development in larvae of a tropical marine fish. Biogeosciences 8: 1631-1641.

Popper, A.N. and Lu, Z. 2000. Structure-function relationships in fish otolith organs. Fisheries Research 46: 16-25.