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Minimal Effects of Ocean Acidification on a Marine Barnacle

Paper Reviewed
Nardone, J.A., Patel, S., Siegel, K.R., Tedesco, D., McNicholl, C.G., O'Malley, J., Herrick, J., Metzler, R.A., Orihuela, B., Rittschof, D. and Dickinson, G.H. 2018. Assessing the impacts of ocean acidification on adhesion and shell formation in the barnacle Amphibalanus amphitrite. Frontiers in Marine Science 5: Article 369, doi: 10.3389/fmars.2018.00369.

According to Nardone et al. (2018), "relatively little is known about how variations in the environment, for example in seawater pH, affect barnacle adhesion and shell formation." And so in considering future changes in oceanic pH that are predicted to result from so-called ocean acidification, the team of eleven researchers set out to conduct such a study. More specifically, they exposed juvenile barnacles (Amphibalanus amphitrite) in controlled-environment aquaria to one of three static seawater pH levels (8.01, 7.78 or 7.5, corresponding to atmospheric pCO2 levels of approximately 400, 800 and 1600 µatm, respectively) for a period of thirteen weeks. Thereafter, they conducted a series of measurements on the barnacles to assess their growth and adhesion properties in relation to the three pCO2 treatments. And what did the experiment reveal?

In the words of the authors, "a reduction in static seawater pH at levels predicted within the next 200 years had little impact on physiological and adhesive metrics in the barnacle A. Amphitrite." In particular, Nardone et al. report that cumulative mortality was not significantly influenced by pH treatment. Neither was tissue dry mass, egg production, adhesive strength, gross morphology of the adhesive glue, barnacle height, shell thickness, shell microhardness, shell crystallinity, atomic disorder, or shell composition affected by seawater pH level. In contrast, elevated pCO2 (reduced pH) did increase barnacle shell mass and the area of the base plate. According to the authors, barnacles grown at pH 8.01 exhibited "approximately 30% lower shell mass and 20% smaller base plate area as compared to barnacles grown at pH 7.50 or 7.78. In addition, the size of the individual calcite crystals were dramatically increased by ocean acidification, with the base plate crystals being nearly twice as large in barnacles in the 7.50 and 7.78 pH treatments compared to the control. About the only potentially negative finding was the observation of slightly elevated damage to barnacle base plate (evidenced by longer crack lengths) following adhesion testing in the pH 7.50 treatment, possibly indicating a reduction in toughness although there was no statistical difference in shell microhardness among the pH treatments.

In light of all of the above, it would appear that the growth and survival of A. Amphitrite is robust under future conditions of ocean acidification.

Posted 18 March 2019