ADULT MORPHOLOGY IN BENTHIC FORAMINIFERA IS CONTROLLED BY EMBRYO SIZE AND FOOD RESOURCES ALONG A STEEP OXYGENATION GRADIENT

Intraspecific morphological variation in foraminifera often covaries with environmental conditions and understanding the mechanisms driving this variation is critical for paleoenvironmental and palaeoecological interpretations. Here, I examine morphological changes in five benthic foraminiferal species (Uvigerina peregrina, Buliminella tenuata, Loxostomum pseudobeyrichi, Bolivina argentea, and Bolivina seminuda) using computed-tomographic imaging to examine morphological responses to changing benthic oxygenation across a deglacial deoxygenation event recorded in sedimentary records from the Gulf of Alaska. The process of oxygen diffusion implies that these single-celled protists should be smaller and have higher surface-area-to-volume ratios (SA/V) at lower oxygen, however specimens from the lowest oxygen conditions had the lowest SA/V and largest total volume. Examination of test volume through ontogeny indicates that specimens achieve large size by having a large initial chamber and not via faster growth. Thus, adult size is a passive result of embryonic size. Larger embryonic cells may have a survival advantage in low-oxygen settings if they accommodate more vacuoles for nitrate storage to fuel anerobic metabolic processes, however, not all species capable of nitrate metabolism responded similarly along the oxygen gradient. The association between large size and low-oxygen was strongest in U. peregrina and B. tenuata, which are both sensitive to organic matter (food) availability. Previous work suggests this low-oxygen event was maintained by enhanced organic carbon export, thus, the morphological changes observed along the paleo-oxygen gradient may be driven by the production of larger embryos, and their survival advantage, when high food availability is high. These results highlight the multidimensionality of environmental controls on morphology and imply that adult morphology in these benthic foraminifera is determined early in ontogeny.