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Roger, L. M., Richardson, A. J., McKinnon, A. D., Knott, B., Matear, R., and Scadding, C. 2012. Comparison of the shell structure of two tropical Thecosomata (Creseis acicula and Diacavolinia longirostris) from 1963 to 2009: potential implications of declining aragonite saturation. – ICES Journal of Marine Science, 69: 465–474.Thecosomata (shelled pteropod molluscs) are calcifiers that play an important role in the ocean carbonate cycle. Ocean acidification as a result of the uptake of CO2 affects pteropods by increasing dissolution rates of their aragonite skeletons. Two species of pteropod found in Australian tropical waters were studied, Creseis acicula and Diacavolinia longirostris. To assess the changes in their aragonite shells, shell morphology, growth patterns, structure, size, and porosity are described for both species, from material collected at seven sites between the 1960s and the 2000s. Shell characteristics were used to explore variations over time potentially related to ocean acidification. The aragonite saturation level (Ωarag) of surface waters was hindcast and a decline equivalent to −10% (average of the seven sites) was found. Simultaneously, variations in shell thickness were recorded (C. acicula by −4.43 µm, D. longirostris by −5.37 µm) over the study period along with a significant increase in shell porosity (C. acicula: +1.43%, D. longirostris: +8.69%). The work, although not conclusive, does suggest that pteropods off Northern Australia may have been influenced by the decline in Ωarag over the past few decades. Such adverse effects could ultimately affect thecosome survival and that of their predators. [ABSTRACT FROM PUBLISHER]