COMPLEX MORPHOLOGICAL RESPONSES OF BENTHIC FORAMINIFERAL SPECIES TO A GRADIENT IN OXYGEN AVAILABILITY IN THE SANTA MONICA BASIN, SOUTHERN CALIFORNIA

Oxygen influences the growth rates and the maximum obtainable sizes of aerobic organisms. However, the within- and among-species responses to oxygen availability remain incompletely studied. Benthic foraminifera, a diverse and geographically widespread group of Rhizarian protists, are an ideal clade in which to examine the influence of oxygen on test size and shape because they survive across a wide range of ambient oxygen concentrations. Here we study four species of benthic foraminifera (Bolivina argentea, B. spissa, Loxostomum pseudobeyrichi, and Uvigerina peregrina) from core-top sediments at 11 sites spanning a steep oxygen gradient in Santa Monica Basin, a borderland basin located off the southern California coast. We calculate the volume and surface area of the test and count the number of chambers of each individual to examine the influence of dissolved oxygen concentration on foraminiferal growth and test morphology. Overall, we observe a positive correlation between maximum test volume and oxygen concentrations when all species are analyzed together, consistent with the physiological theory that oxygen availability should influence maximum obtainable size. However, the among-species pattern in size is not universally repeated within species and trends in test volume-to-surface area ratio (V/SA) are also species-specific. B. argentea and L. pseudobeyrichi decrease in size with decreasing oxygen concentrations as predicted. In contrast, U. peregrina increases test volume and V/SA with decreasing oxygen concentrations, whereas in B. spissa oxygen exhibits no significant correlation with test volume or V/SA. Most of the variation in V/SA observed in U. peregrina occurs due to changes in size rather than changes in shape. U. peregrina also differs from the other species in its mode of variation in overall test size. U. peregrina increases test size by increasing the volume of its chambers, whereas the three other species vary their overall sizes by modulating the number of chambers in the test rather than the sizes of the chambers. These results indicate that among-species responses to oxygen availability cannot be extrapolated from the responses of individual species, nor can the responses of individual species be predicted from macroecological patterns.