Evolution from Benthos to Plankton: Rare or Common?

Various groups of planktonic organisms, including foraminifera, have evolved from benthic ancestors. The question remains whether such evolution and expansion into a new ecological domain occurred only once (or very few times) per clade or whether such evolutionary events were more common. Foraminifera invaded the planktic realm in the Jurassic, but was this the only time? Small subunit ribosomal RNA gene sequences unequivocally reveal gene flow between morphologically similar, Recent planktic and benthic forms: the microperforate biserial planktic foraminifer Streptochilus globigerus and the benthic foraminifer Bolivina variabilis are the same biological species. Genetically identical organisms thus are living as cosmopolitan benthos on the shelf, and as plankton in the surface mixed layer 600 nautical miles offshore in the central Arabian Sea. Trace element (Mg/Ca) analysis of the tests indicates that the planktic form grows actively within the surface waters as a component of the planktic assemblage, and is not just transported from coastal waters. These data indicate that some foraminiferal species, like some diatom species, may survive both as plankton and benthos (tychopelagic lifestyle). Clearly, such tychopelagic organisms have the potential to evolve into true planktonics, especially at times when the pelagic niche is relatively empty, e.g. after mass extinctions such as the one at the end of the Cretaceous. Tychopelagic foraminifera are endowed with ecological advantages, enabling them to rapidly re-colonize the pelagic planktonic domain from the benthos and the reverse. Relatively common benthos to plankton evolution would explain the unusual, temporally and biogeographically discontinuous distribution of microperforate planktic biserials throughout the Late Mesozoic and Cenozoic, and would be a strong argument that a planktic lifestyle is not necessarily a valid synapomorphic character in phylogenetic analysis.