LAUSR

A new species of pelagic diatom, Pleurosigma pacificumsp. nov., is described from the tropical Western Pacific Ocean. It has the typical features of Pleurosigma, including a slightly sigmoid raphe, intersected transverse and oblique striae, and loculate areolae with external opening slits and internal poroids. Morphologically, P. pacificum belongs to a species group of Pleurosigma with lanceolate valves, including P. atlanticum Heiden & Kolbe, P. nubecula W. Smith, P. indicum Simonsen, and P. simonsenii Hasle. However, P. pacificum differs by its smaller lanceolate valve and smaller intersection angle as well as elliptical areolae without a silica bar. The SSU rDNA and rbcL sequence data place P. pacificum in a basal position relative to other species of Pleurosigma. Our molecular phylogenetic analyses did not support the monophyly of lanceolate and slightly sigmoid species. Thus, the sigmoidality of valve outline cannot be considered as a criterion to define the species group.