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Ocean Acidification and the Sagittal Otoliths of Marine Fish
Volume 12, Number 46: 18 November 2009

Otoliths, to quote the authors of a recent Brevia item in Science (Checkley et al., 2009), "are bony structures used by fish to sense orientation and acceleration and consist of aragonite-protein bilayers," or as the dictionary we possess more simply states, they are "small vibrating calcareous particles in ... the ears of some animals, especially of fishes."

Noting that atmospheric CO2 enrichment has been calculated, on a purely chemical basis, to decrease the saturation state of carbonate minerals such as aragonite in the world's oceans, the six scientists "hypothesized that otoliths in eggs and larvae reared in seawater with elevated CO2 would grow more slowly than they do in seawater with normal CO2," and to test this hypothesis, they "grew eggs and pre-feeding larvae of white sea bass (Atractoscion nobilis) under a range of CO2 concentrations [380, 993 and 2558 ppm] and measured the size of their sagittal otoliths."

These experiments indicated, "contrary to expectations," in the words of Checkley et al., that "the otoliths of fish grown in seawater with high CO2, and hence lower pH and aragonite saturation, were significantly larger than those of fish grown under simulations of present-day conditions." More specifically, they found that "for 7- to 8-day-old fish grown under 993 and 2558 ppm CO2, the areas of the otoliths were 7 to 9% and 15 to17% larger, respectively, than those of control fish grown under 380 ppm CO2."

But how could this be? ... especially since these findings seemingly contradict the well-known laws of chemistry? The answer is to be found in the fact that living creatures can accomplish many things that dead chemistry can't.

The marine researchers went on to state, in this regard, that young fish are "able to control the concentration of ions (H+ and Ca2+) ... in the endolymph surrounding the otolith," where "with constant pH, elevated CO2 increases CO32- concentration and thus the aragonite saturation state, accelerating formation of otolith aragonite [our italics]."

In like manner, but in the case of corals, we contended almost a decade ago (Idso et al., 2000) that coral calcification is much more than a purely physical-chemical process, stating that it is a biologically-driven physical-chemical process, which is something that is much more complicated. Enlarging on this statement, we stated that real-world data indicate that the photosynthetic activity of a coral's symbiotic zooxanthellae "is the chief source of energy for the energetically-expensive process of calcification," while further noting that long-term reef calcification rates have generally been observed to rise in direct proportion to increases in rates of reef primary production, which are typically enhanced by increases in the air's CO2 concentration, as is indicated by many of the studies described in the recent review of the subject by Idso (2009). Therefore, we suggested that coral calcification -- just like otolith calcification -- should actually increase as the atmosphere's CO2 content rises, which has also been demonstrated to be the case in a number of the studies reviewed by Idso (2009), as well as in additional studies for which we have written and posted reviews in our Subject Index under the general heading of Calcification.

Viewed in this light, the findings of Checkley et al. are seen to be in no way strange or unusual, and should not have been deemed "contrary to expectations." To our way of thinking, at least, they are exactly what we would have expected to see.

Sherwood, Keith and Craig Idso

Checkley Jr., D.M., Dickson, A.G., Takahashi, M., Radich, J.A., Eisenkolb, N. and Asch, R. 2009. Elevated CO2 enhances otolith growth in young fish. Science 324: 1683.

Idso, C.D. 2009. CO2, Global Warming and Coral Reefs: Prospects for the Future. Vales Lake Publishing, Pueblo West, Colorado, USA.

Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO2, global warming and coral reefs: Prospects for the future. Technology 7S: 71-94.