What was done
The authors evaluated the impacts of elevated seawater pCO2 (1264 ppm vs. 375 ppm) on the early development of, and the larval metabolic and feeding rates of, a model marine organism: Strongylocentrotus purpuratus, commonly known as the purple sea urchin. This was done via a protocol where growth and development were assessed daily, for a period of three weeks, in terms of total body length, body rod length, postoral rod length and posterolateral rod length, as well as mortality and feeding and metabolic rates.

What was learned
First of all, the five researchers learned that daily mortality rate (DMR) was significantly higher under control conditions (DMR = 2.7% per day) in comparison to that under high seawater pCO2 (DMR = 2.2% per day). Secondly, in the elevated CO2 treatment, larval development was about 8% slower, such that it took slightly longer for equivalent development stages to be reached in the high CO2 treatment.

What it means
As a result of the slower development of the larvae in the high CO2 treatment, at any given time the individuals in this treatment were found to be smaller and less well developed than those in the control treatment; and if that was the only comparison to have been made in this study, the effects of elevated CO2 might have been thought to have been negative. But if comparisons are made on the basis of development stage, as was also done in this study, it would have been found - and was! - that there were no long-term physical differences between the larvae living in the high and low CO2 treatments.

In light of these observations, it can be appreciated that in studies designed to reveal the effects of atmospheric CO2 enrichment upon various species of marine life, treatment comparisons should be made at equivalent development stages of the organism being studied; since at such points along the life cycle of the purple sea urchin, there were no significant physical differences between individuals raised in the control and CO2-enriched conditions. As Stumpp et al. thus declare at the conclusion of their enlightening study, "we suggest that body length is a useful scale of reference for studies in sea urchin larvae where a morphological delay in development occurs," and that "using time post-fertilization as a reference may lead to misinterpretation of data," i.e., to wrongfully assuming a negative result, when in reality there may be no deleterious effect of ocean acidification. This is essentially the same conclusion reached by Stumpp et al. (2011a) in a companion paper, where they say that "in studies in which a stressor induces an alteration in the speed of development, it is crucial to employ experimental designs with a high time resolution in order to correct for developmental artifacts," which protocol "helps prevent misinterpretation of stressor effects on organism physiology."

Reference
Stumpp, M., Dupont, S., Thorndyke, M.C. and Melzner, F. 2011a. CO2 induced seawater acidification impacts sea urchin larval development II: Gene expression patterns in pluteus lrvae. Comparative Biochemistry and Physiology, Part A 160: 320-330.