Morphological recognition of cryptic species in the planktonic foraminifer Orbulina universa
One of the key hypotheses of paleoceanography is that planktonic foraminiferal morphospecies record reasonably stable and homogeneous oceanographic and climatic characteristics over their geographic and stratigraphic ranges. The discovery of numerous genetically-defined cryptic species challenges the morphospecies concept in planktonic foraminifera and paleoceanographic interpretations based on them. Here, we present a combined genetic and biometric analysis of Orbulina universa specimens in the Atlantic, Indian and Pacific Oceans. Our study is based on shells retained after DNA extractions. On those genotyped shells, we perform biometric analyses (shell size and thickness, inner porosity and pore surface distribution). Our genetic data confirm the presence of three cryptic species of O. universa in the world ocean, whose distributions are primarily correlated to the productivity of the surface waters. The Mediterranean species of O. universa is most abundant in the vertically mixed and nutrient-rich areas of the low to mid-latitudes, whereas the Caribbean and Sargasso species occur in stratified and oligotrophic subtropical waters. Our biometric data show no correlation between shell size and inner porosity within each cryptic species of O. universa . Combining Principal Component Analyses with MANOVAs performed on shell pore surface distribution, we demonstrate that the three different cryptic species are characterized by significant morphological differentiation. The Caribbean species typically exhibits large pores and higher porosity values, while the Mediterranean and Sargasso species are characterized by smaller pore areas and shell porosity. A model based upon pore surface distribution correctly assigns 60% to 90% of the specimens to their corresponding genotype. Although the inner shell surface of the Sargasso species resembles that of the Mediterranean species, our model demonstrates that the pore surface distributions of these two cryptic species can be distinguished. Finally, the Sargasso species exhibits significantly thinner shells than the two other cryptic species.