Molecular evidence for high dispersal distribution of planktonic foraminifera between Pacific and Atlantic Oceans

Planktonic foraminifera, marine zooplankton with calcareous tests, are widely dispersed in all oceans. Because the diversity of these foraminifera is controlled by the environmental components (sea temperature, salinity, etc.), distinct species assemblages occur in several provinces from tropical to subpolar area dependent on circulation patterns of the ocean surface water. Most of the Recent species originated in the Miocene to Pliocene. . However, the worldOs ocean has been separated by the physiographic changes during the Neogene, yet the species recognized morphologically occur worldwide. In particular, the warm surface waters of the Atlantic and Pacific have been isolated from one another for at least 3.5 my and maybe longer. In this study, we present a molecular phylogeny inferred from 18S partial small subunit ribosomal DNA sequences using the 10 morphospecies obtained from the western Pacific Ocean. The phylogenetic analyses indicate a polyphyletic origin and two major clades; one is composed of the Candenidae and Globorotaliidae with certain benthic foraminifera and another by the Globigerinidae. The sequences were compared with sequences from the same species from the Atlantic Ocean, eastern Pacific Ocean and Mediterranean Sea shown; the genetic distance was estimated about 67 OTU. The partial 18S SSUrDNA of the 10 morphological taxa have almost same sequences, (less than 1% difference). No regionally separated populations are apparent within the same taxa, in particular, the Globorotaliidae and Globigerinoides sacculifer. However, 5 taxa have two or three cryptic species, respectively. These cryptic species cannot be identified by the morphological differences except for Orbulina universa. Thus, planktonic foraminifera have high dispersal distribution and have maintained their sequences in their huge population. On the other hand, the cryptic species of 5 species diverged after the Pliocene. As they belong to a group of shallow dwelling plankton, they likely adapted to the high variability of the oceanic condition after the Neogene in the course of their genetic changes.