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Sea Urchin Larvae Living in Periodically Low-pH Seawater
Reference
Yu, P.C., Matson, P.G., Martz, T.R. and Hofmann, G.E. 2011. The ocean acidification seascape and its relationship to the performance of calcifying marine invertebrates: Laboratory experiments on the development of urchin larvae framed by environmentally-relevant pCO2/pH. Journal of Experimental Marine Biology and Ecology 400: 288-295.

Background
The authors write that "variation in ocean pH is a dynamic process occurring naturally in the upwelling zone of the California Current Large Marine Ecosystem," where "the near-shore carbonate chemistry is under-characterized and the physiology of local organisms may be under constant challenge from cyclical changes in pH and carbonate ion concentration of unexpectedly high magnitude."

What was done
To explore both aspects of this situation, i.e., to determine (1) the temporal variability of near-shore seawater pH and (2) its effects on calcifying marine invertebrates, Yu et al. did two things. First, they deployed a SeaFET pH sensor (Martz et al., 2010) from 22 July to 17 August 2010 at 8 meters depth at Mohawk Reef, Santa Barbara, California, USA (34°23.66'N, 119°43.80'W) on sandy substrata about 50 meters seaward of that location's kelp forest, which device recorded seawater pH and temperature over a period of 30 seconds every 10 minutes. Second, they raised larvae of the purple sea urchin (Strongylocentrotus purpuratus) in seawater maintained at pCO2 levels ranging from ambient to 1000 and 1450 ppm CO2 (pH 7.7 and 7.5, respectively), while measuring, after three and six days development, "total larval length (from the spicule tip of the postoral arm to the spicule tip of the aboral point) along the spicules, to assess effects of low pH upwelling water on morphology."

What was learned
The four U.S. researchers observed changes of up to 0.32 pH unit over 24-hour periods, and a maximum change between the high and low points of the measurement period of 0.67 pH unit, with a time-averaged value of 7.933 for the entire period, indicating that marine organisms at Mohawk Reef are currently experiencing "low pH and high pCO2 values that are not expected for the open ocean for another ~100 years." As for sea urchin larval development, they report that "even at the highest pCO2 treatments, larval development was normal in terms of timing and morphological appearance," although at both day 3 and 6 larvae in the 1450-ppm CO2 treatment were 7-13% smaller than control larvae.

What it means
Yu et al. state that "the observed developmental progression and survival of cultures was within the norm typically observed for this species at this temperature range." In addition, they indicate that "a lack of developmental deformities at early stages for pCO2 ~1000 ppm has been previously reported for this species (Todgham and Hofmann, 2009), and another local species, Lytechinus pictus, with a similar overlapping portion of its range in southern California (O'Donnell et al., 2010)." And they say "there are even reports that survival is increased in this species and its congener S. droebachiensis under some low pH conditions (Dupont and Thorndyke, 2008)." Hence, it would appear, as Yu et al. conclude, that "the effects of small magnitude in these urchin larvae are indicative of a potential resilience to near-future levels of ocean acidification."

References
Dupont, S. and Thorndyke, M.C. 2008. Ocean acidification and its impacts on the early life-history stages of marine animals. In: CIESM (Ed.). Impacts of Acidification on Biological, Chemical and Physical Systems in the Mediterranean and Black Seas. Number 36 in CIESM Workshop Monographs (F. Briand, Ed.), Monaco.

Martz, T.R., Connely, J.G. and Johnson, K.S. 2010. Testing the Honeywell DurafetŪ for seawater pH applications. Limnology and Oceanography: Methods 8: 172-184.

O'Donnell, M.J., Todgham, A.E., Sewell, M.A., Hammond, L.M., Ruggiero, K., Fangue, N.A., Zippay, M.L. and Hofmann, G.E. 2010. Ocean acidification alters skeletogenesis and gene expression in larval sea urchins. Marine Ecology Progress Series 398: 157-171.

Todgham, A.E. and Hofmann, G.E. 2009. Transcriptomic response of sea urchin larvae, Strongylocentrotus purpuratus, to CO2-driven seawater acidification. Journal of Experimental Biology 212: 2579-2594.

Reviewed 10 August 2011