PATTERNS OF BENTHIC FORAMINIFERAL EMBRYO AND ADULT SIZE VARIATIONS ACROSS MODERN NORTH AMERICAN ENVIRONMENTAL GRADIENTS

The availability of energy in the environment influences the ontogenetic histories of organisms. However, it remains unclear if the physiological controls on organism size remain constant throughout ontogeny and among different environments. Benthic foraminifera grow by adding newly secreted chambers to their tests, recording ontogeny. Because foraminifera are abundant and widespread in the modern ocean, they are an ideal study group for quantifying the physicochemical controls on size at both embryonic and adult life stages across broad environmental gradients. We measured the test dimensions and proloculi of 139 extant rotaliid species inhabiting the North American continental margin from published images of holotype specimens. We merged size data with information on mean annual temperature, dissolved oxygen concentrations, particulate organic carbon (POC) flux, and carbonate saturation for 718 unique localities and 2486 species occurrences. Using a model-fitting approach, we find that temperature shows a significant inverse correlation with the adult/embryo size ratios of North American benthic foraminifera. Depending on the examination of Atlantic versus Pacific or endemic versus cosmopolitan species groups, other environmental variables are significant predictors of foraminiferal adult/embryo size ratios in addition to temperature. The distribution of adult/embryo size ratios of cosmopolitan species living in the eastern Pacific are controlled by variations in dissolved oxygen concentrations and temperature; whereas, cosmopolitan species in the western Atlantic are influenced by a combination of POC flux, temperature, and dissolved oxygen. Temperature alone controls the distribution of adult/embryo size ratios of endemic species from the North American continental margin. The directions of association between environmental variables and adult/embryo size ratios are consistent with predictions based on organism physiology. Moreover, variations in adult/embryo size ratios reflect variations in adult size rather than embryonic size. These findings are consistent with the expectation that environmental constraints related to biovolume or surface area have a greater influence over organism physiology at larger sizes.