Early Growth and Development of the Tropical Fish Acanthochromis polyacanthus unaffected by Ocean Acidification

Effects on the calcification and growth due to ocean acidification have been established for several marine invertebrate species. However, little research has been done to examine the effects of ocean acidification on fish. Of the available research, most papers have focused on the acid-base regulatory mechanisms of adult fish, but very few have considered the effects of acidification on juveniles. Munday et al. (2011) studied whether ocean acidification affected the early life cycle of the tropical reef fish, Acanthochromis polyacanthus. The authors believed that an increase in acidity should affect skeletal growth by reducing calcification. They found that increases in dissolved CO2 did not significantly affect the growth and development of these reef fishes. The results suggest that marine fish species may be more tolerant to changes in seawater pH than invertebrates in the same locations. —Emily Putnam
 Munday, P.L., Gagliano, M., Donelson, J.M., Dixson, D.L., Thorrold, S.R., 2011. Ocean acidification does not affect the early life history development of a tropical marine fish. Marine Ecology Progress Series 423, 211–221.

Munday and his colleagues at James Cook University studied the effects of acidification on the growth of A. polyacanthus individuals for the first three weeks of their lives. Freshly hatched A. polyacanthus juveniles were reared in four acidification conditions––the current pH, the projected pH for the year 2100, and two intermediate acidities. At the end of three weeks, the fish were killed and preserved for study. The standard length and weight of each fish was measured to ascertain overall growth. Otoliths, or small ear bones, were removed and examined by photographic analysis to determine the level of asymmetry. Each fish was then stained and measurements were taken for 29 skeletal reference points. Statistical analyses compared the effects due to parentage versus that of acidified conditions.
Munday et al. found that standard length and weight was not significantly different among the treatments. The otoliths were expected to be more strongly impacted by acidified environments because they are composed of aragonite––a compound that is harder to form as pH decreases. However, the authors found that there was no effect of acidification on any aspect of the otoliths. Skeletal measurements agreed with otolith and standard growth measurements––26 of 29 skeletal references were not different amongst the treatments. Statistical analysis showed that these three reference points were not enough to conclude that acidified conditions affected the development of the juvenile. All growth measurements were more strongly connected to genetic variation than to environmental constraints.
Even though early life stages are considered to be more susceptible to environmental conditions, Munday et al. found no negative effects on the early life of A. polyacanthus. They concluded that marine fishes may be more tolerant to ocean acidification than their invertebrate counterparts. One possible explanation for the observed results is that A. polyacanthus juveniles spend all of their early lives on the reef, where large fluctuations in dissolved CO2 levels are common. Thus, this species may be especially tolerant to changes in ocean pH. The authors also posit that 3 weeks may be sufficient for the juveniles to develop to a life stage where they can actively regulate internal acid-base chemistry to overcome the effects of acidification. Taken together, the authors conclude that more expansive studies of species and juvenile-based growth be undertaken to fully consider the effects of ocean acidification of early life development.

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