Ontogenic vertical migration and lunar reproduction cycle in planktonic foraminifera: myth or reality?

Fossil shells of planktonic foraminifera are a key substrate for chemical proxies used to reconstruct the state of the ocean during the precipitation of the shell. Planktonic foraminifera species appear to live and calcify across a range of depths, but the stability of their depth habitats and the existence of depth migration during their life remain poorly constrained. This is unfortunate considering how significant the existence of such migration would be for foraminifera-based paleoproxies.

Here we provide direct observations on the evolution of the depth habitat and reproductive cycle of multiple species of planktonic foraminifera in the tropical North Atlantic Ocean. The data are based on daily sampling with stratified plankton tows during the RV METEOR cruise M140 between the 12^thand 25^thof August 2017.

Our results reveal a consistent vertical distribution of planktonic foraminifera with highest densities in the upper 80 m. The largest total standing stock, > 450 ind.m^-3, was observed during the black moon while the lowest ones were sampled around the 3^rdquarter. No clear differences between the ratio of living and dead specimens along the lunar cycle could be observed, suggesting no particular phase of mortality across the population. In addition, individual size measurements for five species Globigerinita glutinata, Globigerinoides ruberpink, Globorotalia menardii,Orbulina universaand Globigerinella siphonifera(>4150 individuals) suggest stable preferential depth habitats with no clear pattern of vertical migration in phase with the ontogeny. The combination of size parameters, and vertical distribution / abundances supports no signal of reproduction in phase with the lunar cycle. In summary, we find no evidence for a persistent ontogenetic depth migration or for a reproduction synchronised by the lunar cycle. Instead, individuals of different sizes (and ages) occur at any time in similar proportions across species-specific stable depth habitats.