Morphology and molecular phylogeny of Pleurosigmapacificum sp. nov. (Pleurosigmataceae), a new tropical pelagic species from the Western Pacific Ocean.

A new species of pelagic diatom, Pleurosigmapacificum sp. 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 rbc L 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.