Determining how biotic and abiotic variables affect the shell condition and parameters of Heliconoides inflatus pteropods from a sediment trap in the Cariaco Basin
Authors: Oakes Rosie L. , Sessa Jocelyn A. 
Published: April 2020

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.