Assessing annual variability in the shell thickness of the pteropod <i>Heliconoides inflatus</i> in the Cariaco Basin using micro-CT scanning

Abstract. Pteropods have been nicknamed the canary in the coal mine for ocean acidification because they are predicted to be among the first organisms to be affected by future changes in ocean chemistry. This is due to their fragile, aragonitic shells and high abundances in polar and sub-polar regions where the impacts of ocean acidification will manifest first. For pteropods to be used most effectively as indicators of ocean acidification, their natural variability in the modern ocean needs to be quantified and understood. Here, we measured the shell condition (i.e., the degree to which a shell has dissolved) and shell characteristics, including size, number of whorls, shell thickness, and shell volume (i.e., amount of shell material) of nearly fifty specimens of the pteropod species Heliconoides inflatus from a sediment trap in the Cariaco Basin, Venezuela sampled over an 11-month period. The water in the Cariaco Basin is supersaturated with respect to aragonite year-round, and hydrographic and chemical properties vary seasonally due to the movement of the Inter Tropical Convergence Zone (ITCZ). Shell condition was assessed using with two methods: the Limacina Dissolution Index (LDX) and the opacity method. The opacity method captured changes in shell condition only in the early stages of dissolution, whereas the LDX recorded dissolution changes over a much larger range. Shell condition did not deteriorate with the length of time in the sediment trap. Instead, the most altered shells occurred in samples collected in September and October when water temperatures were warmest, and the amount of organic matter degradation in the water column was likely to have been the greatest. Shells of H. inflatus varied in size, number of whorls, and thickness throughout the year. The number of whorls did not correlate with shell diameter, suggesting that shell growth is plastic. H. inflatus formed shells that were 40 % thicker and 20 % larger in diameter when nutrient concentrations were high during times of upwelling, compared to specimens sampled from the oligotrophic rainy season. This study produces a baseline dataset of the variability in shell characteristics of H. inflatus in the Cariaco Basin and establishes a methodology for generating similar baseline records for pteropod populations globally.