Uncovering sibling species in Radiolaria: evidence for ecological partitioning in a marine planktonic protist.

Phylogeography of unicellular plankton, as representative pelagic organisms, is fundamental to understanding their evolution in the ocean. Historically, these microplankton were believed to have cosmopolitan distributions achieved through passive transport and little potential for speciation because of a lack of geographic barriers in the oceans. Recent phylogeographic studies of these microplankton, however, have often revealed high diversity and fine-scale geographic distributions. These apparent contradictions may result from poor knowledge of the spatial distributions of pelagic microplankton in the water column. More information about both geographic and vertical distributions of pelagic populations could reveal the dispersal pathways, gene flow, and resulting diversifications in the open ocean. Here we demonstrate that two genetic types of the radiolarian morphospecies Spongotrochus glacialis with morphological differences are vertically segregated into the upper and lower surface waters within the pycnocline of the North Pacific Subtropical Water. This vertically separated distribution of two sister species is associated with distinct ecological partitioning. These two species could survive on different food resources from their respective environments: one in oligotrophic surface waters by using nutrients from symbionts, and the other at greater depths by depending on both heterotrophic and symbiotic nutrition. Moreover, molecular divergence-time estimates suggest that the two species diverged during the period of oligotrophic surface-water development in the Pacific Ocean. Our findings suggest that genetic isolation in the vertical dimension occurs through ecological partitioning even in the absence of physical barriers in the pelagic oceans.