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. Pteropods have been nicknamed the “canary in the coal mine” for oceanacidification because they are predicted to be among the first organisms tobe affected by changing ocean chemistry. This is due to their fragile,aragonitic shells and high abundances in polar and subpolar regions wherethe impacts of ocean acidification are most pronounced. For pteropods to beused most effectively as indicators of ocean acidification, the biotic andabiotic factors influencing their shell formation and dissolution in themodern ocean need to be quantified and understood. Here, we measured theshell condition (i.e., the degree to which a shell has dissolved) and shellcharacteristics, including size, number of whorls, shell thickness, andshell volume (i.e., amount of shell material) of nearly 50 specimens ofthe pteropod species Heliconoides inflatus sampled from a sediment trap in the Cariaco Basin,Venezuela, over an 11-month period. The shell condition of pteropods fromsediment traps has the potential to be altered at three stages: (1) when theorganisms are live in the water column associated with ocean acidification,(2) when organisms are dead in the water column associated with biotic decayof organic matter and/or abiotic dissolution associated with oceanacidification, and (3) when organisms are in the closed sediment trap cupassociated with abiotic alteration by the preservation solution. Shellcondition was assessed using two methods: the Limacina Dissolution Index (LDX) andthe opacity method. The opacity method was found to capture changes in shellcondition only in the early stages of dissolution, whereas the LDX recordeddissolution changes over a much larger range. Because the water in theCariaco Basin is supersaturated with respect to aragonite year-round, weassume no dissolution occurred during life, and there is no evidence thatshell condition deteriorated with the length of time in the sediment trap.Light microscope and scanning electron microscope (SEM) images show the majority of alteration happened todead pteropods while in the water column associated with the decay oforganic matter. The most altered shells occurred in samples collected inSeptember and October when water temperatures were warmest and when the amountof organic matter degradation, both within the shells of dead specimens andin the water column, was likely to have been the greatest.   The hydrographic and chemical properties of the Cariaco Basin vary seasonally due to the movement of the Intertropical Convergence Zone(ITCZ). Shells of H. inflatus varied in size, number of whorls, and thicknessthroughout the year. There was not a strong correlation between the numberof whorls and the shell diameter, suggesting that shell growth is plastic.H. inflatus formed shells that were 40 % thicker and 20 % larger in diameter duringnutrient-rich, upwelling times when food supply was abundant, indicatingthat shell growth in this aragonite-supersaturated basin is controlled byfood availability. This study produces a baseline dataset of the variabilityin shell characteristics of H. inflatus pteropods in the Cariaco Basin and documentsthe controls on alteration of specimens captured via sediment traps. Themethodology outlined for assessing shell parameters establishes a protocolfor generating similar baseline records for pteropod populations globally.

biogeosciences

Determining how biotic and abiotic variables affect the shell condition and parameters of &amp;lt;i&amp;gt;Heliconoides inflatus&amp;lt;/i&amp;gt; pteropods from a sediment trap in the Cariaco Basin