Volumetric analysis of benthic foraminifera: Intraspecific test size and growth patterns related to embryonic size and food resources

Abstract

Morphological variation in benthic foraminifera is often used to reconstruct paleoenvironmental conditions and infer their physiological responses to environmental change. Computed-tomographic imaging allows precise measurements of test volume, test surface area, and the internal volume of individual chambers, thus changes in morphology throughout ontogeny can used to test these morphology-environment relationships. Here, I examine morphological changes in five benthic foraminiferal species (Uvigerina peregrina, Buliminella tenuata, Loxostomum pseudobeyrichi, Bolivina argentea, and Bolivina seminuda) during a deglacial deoxygenation event recorded in marine sediments from the Gulf of Alaska to examine morphological responses to oxygenation. Although the mechanics of oxygen diffusion imply that benthic foraminifera should have higher surface area to volume ratios (SA/V) at low oxygen, specimens from the lowest oxygen conditions had lower SA/V due to their larger size. Examination of test volume through ontogeny indicates that specimens achieve large size by having a large initial chamber and not via higher growth rates. Thus, terminal size is a passive result of embryonic size. U. peregrina and B. tenuata have the strongest association between large size and low-oxygen and are sensitive to organic matter availability. The studied low-oxygen event was likely maintained by enhanced organic carbon export, which provided the resources to produce large offspring. Thus, the morphological changes observed along the paleo-oxygen gradient are likely related to food availability and the survival advantage of larger embryos in low-oxygen settings for some species. These results imply that size and SA/V in benthic foraminifera do not reliably reflect physiological responses to oxygenation.