How does rising atmospheric CO2 affect marine organisms?

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How CO2 Impacts Reproduction Among Cinnamon Anemonefish
Miller, G.M., Watson, S.-A., McCormick, M.I. and Munday, P.L. 2013. Increased CO2 stimulates reproduction in a coral reef fish. Global Change Biology 19: 3037-3045.

The authors write that "ocean acidification is predicted to negatively impact the reproduction of many marine species, either by reducing fertilization success or diverting energy from reproductive effort." And they say that "while recent studies have demonstrated how ocean acidification will affect larval and juvenile fishes, little is known about how increasing partial pressure of carbon dioxide (pCO2) and decreasing pH might affect reproduction in adult fishes." So they decided to do something about it. And do it they did.

What was done
Miller et al.'s experiment employed three 8000-L recirculating aquarium systems, each set to a different CO2 and corresponding pH level. These treatments consisted of a current-day Control CO2 (430 ľatm), a mid-century Moderate CO2 (584 ľatm) and an end-of-century High CO2 (1032 ľatm). Eighteen pairs of cinnamon anemone fish (Amphiprion melanopus) - which they collected from Australia's Great Barrier Reef - were placed into each of these three aquariums, after all individuals had been weighed and had their lengths measured. Then, at the start of the experiment, pairs of the fish were placed into individual 45-L tubs with continuous water flow at winter non-breeding temperatures and ambient pCO2 values, which were gradually adjusted over a two-week period to the desired levels. Then, temperature was increased by 0.5°C per week, until the average summer breeding temperature was reached in the first week of November 2010, after which the pairs of fish remained in these conditions for a 9-month period that included the summer breeding season, during which period various assessments of breeding success were made.

What was learned
The four Australian researchers report that "unexpectedly, increased CO2 dramatically stimulated breeding activity." First of all, they indicate that over twice as many pairs of the fish bred in the Moderate and High CO2 treatments (67% and 55%) compared to the Control treatment (27%); and they add that "pairs in the High CO2 group produced double the number of clutches per pair and 67% more eggs per clutch compared to the Moderate and Control groups." As a result, they determined that "reproductive output in the High group was 82% higher than that in the Control group and 50% higher than that in the Moderate group." And they make a point of noting that "despite the increase in reproductive activity, there was no difference in adult body condition among the three treatment groups," and "there was no significant difference in hatchling length between the treatment groups."

What it means
In the words of Miller et al., "this study provides the first evidence of the potential effects of ocean acidification on key reproductive attributes of marine fishes and, contrary to expectations, demonstrates an initially stimulatory effect in response to increased pCO2."

Reviewed 18 December 2013