Hydrodynamic constraints on the evolution and ecology of planktic foraminifera

The origin and function of a biomineralised skeleton in many of the non-motile groups of plankton remains an open question. Morphological diversity within these groups has often been explained by its relevance to hydrodynamic behaviour, principally buoyancy and settling. Consequently, ecological and evolutionary patterns of morphology have been associated with changes in surface water properties, but these hypotheses have rarely been critically assessed. Computational Fluid Dynamics simulations present a way to quantify the relative effect of size (maximum diameter), shape of the test and density (ratio between calcite and cavity volumes) of the specimen on settling velocity, as all variables can be manipulated independently. Here we interrogate the morphological diversity in planktic foraminifera as model organisms to explore the range of evolutionary options open to plankton to modulate settling velocity under varying environmental conditions. The evolutionary changes in morphology required to accommodate physical changes in the upper water column due to environmental changes, such as increased temperature, are small compared to the ecophenotypic variability of the population. In the modern ocean, the pattern of species distribution with depth is not likely to be determined by hydrodynamics as it is inconsistent with predictions based on settling velocity. These results suggest that intrinsic constraints on size, shape and calcification, such as heritage, exposure of the symbionts to light or oxygen diffusion into the cell, are likely to be more important than hydrodynamic function in determining the depth distribution and test morphology of planktic foraminifera.