Three-dimensional analysis of inter-and intraspecific variation in ontogenetic growth trajectories of planktonic foraminifera

Abstract

Abstract Planktonic foraminifera grow by adding chambers onto their tests, modifying their surface area, volume, porosity, and ornamentation with each addition. This process results in a wide variety of adult morphologies and preserves a record of the early morphology. With high-resolution three-dimensional imaging techniques, precise measurements can be made on previously inaccessible parameters like surface area and volume of the test interior. Here, we used three-dimensional images (microCT scans) of extant planktonic foraminifera from plankton tows and sediment traps to measure and compare the variation in chamber and whole test volume within and between seven morphospecies. By incorporating data from Caromel et al., (2015), we were able to explore the extent of interspecific variation in growth (i.e., total volume) with each chamber addition from a total of 12 extant species. All species measured approximately adhered to log-linear growth trajectories through the majority of their ontogeny. Species exhibited characteristic deviations from the log-linear growth trajectory and were grouped into three categories on the basis thereon. The volume of the first chamber—the proloculus—varied by more than two orders of magnitude. However, subtle differences in relative growth rates could make up for the initial variation proloculus size, such that the correlation between proloculus size and biovolume decreased through ontogeny. Intraspecific variation in proloculus size, cavity volume at the final five chamber stages, and terminal calcite volume was pronounced in the seven morphospecies investigated and attributed environmental and ontogenetic differences among individuals. Finally, we generated linear models for estimating cell volume and calcium carbonate test volume from linear measurements (major and minor axis) that are more accurate than the spherical equivalents used previously.