Determining how biotic and abiotic variables affect the shell condition and parameters of <i>Heliconoides inflatus</i> pteropods from a sediment trap in the Cariaco Basin

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 changing ocean chemistry. This is due to their fragile, aragonitic shells and high abundances in polar and subpolar regions where the impacts of ocean acidification are most pronounced. For pteropods to be used most effectively as indicators of ocean acidification, the biotic and abiotic factors influencing their shell formation and dissolution in the modern ocean need 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 50 specimens of the pteropod species Heliconoides inflatus sampled from a sediment trap in the Cariaco Basin, Venezuela, over an 11-month period. The shell condition of pteropods from sediment traps has the potential to be altered at three stages: (1) when the organisms are live in the water column associated with ocean acidification, (2) when organisms are dead in the water column associated with biotic decay of organic matter and/or abiotic dissolution associated with ocean acidification, and (3) when organisms are in the closed sediment trap cup associated with abiotic alteration by the preservation solution. Shell condition was assessed using two methods: the Limacina Dissolution Index (LDX) and the opacity method. The opacity method was found to capture changes in shell condition only in the early stages of dissolution, whereas the LDX recorded dissolution changes over a much larger range. Because the water in the Cariaco Basin is supersaturated with respect to aragonite year-round, we assume no dissolution occurred during life, and there is no evidence that shell 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 to dead pteropods while in the water column associated with the decay of organic matter. The most altered shells occurred in samples collected in September and October when water temperatures were warmest and when the amount of organic matter degradation, both within the shells of dead specimens and in 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 thickness throughout the year. There was not a strong correlation between the number of whorls and the shell diameter, suggesting that shell growth is plastic. H. inflatus formed shells that were 40 % thicker and 20 % larger in diameter during nutrient-rich, upwelling times when food supply was abundant, indicating that shell growth in this aragonite-supersaturated basin is controlled by food availability. This study produces a baseline dataset of the variability in shell characteristics of H. inflatus pteropods in the Cariaco Basin and documents the controls on alteration of specimens captured via sediment traps. The methodology outlined for assessing shell parameters establishes a protocol for generating similar baseline records for pteropod populations globally.