A new species of the larger porcelaneous foraminifer Borelis provides novel insights into Neogene to Recent western Pacific palaeobiogeographical dispersal patterns

Only three species of alveolinoidean larger foraminifera occur in present-day tropical shallow-water marine settings. Alveolinella quoyi thrives in the Central Indo-Pacific and Eastern Indo-Pacific, Borelis pulchra in the Central and Eastern Indo-Pacific and in the central Atlantic, whereas Borelis schlumbergeri inhabits the IndoPacific Ocean. The northernmost record is that of Alveolinella quoyi from the shallow-water settings in Okinawa (central Ryukyu Islands, Japan). A new porcelaneous larger foraminiferal species, Borelis matsudai sp. nov. (Alveolinoidea, Borelinae), is established herein, based on specimens discovered in present-day shallow-water sediments from Sekisei Lagoon, southern Ryukyu Islands (Japan). This is the northernmost record of the genus in the western Indo-Pacific Warm Pool. The architectural characters of this species suggest its phylogenetic relationship with the Borelis pulchra group. A comprehensive literature survey of fossil and modern records of Borelis pulchra over the past 30 million years shows that the Middle Miocene constriction of the Indonesian Seaway and the Late Miocene-Early Pliocene restriction of the Indonesian Throughflow impacted the species-level dispersal of this species in the Indo-Australian Archipelago. Driven by the Kuroshio dispersal route Borelis matsudai sp. nov. likely appeared in the southern and central Ryukyu Islands at least from the Chibanian (Middle Pleistocene). This species represents a marginal part of a population in significant contact with its ancestor (B. pulchra), which is widespread southward since the Oligocene (Philippines). With ongoing global warming possible occurrences of Borelis matsudai sp. nov. in the northern Ryukyu Islands, where it has not yet been found, are expected. The Sekisei Lagoon represents, therefore, a biogeographical stepping stone relay station in northward migration of the shallow-water benthic organisms along the Kuroshio dispersal route.