Abstract

Diatoms are microscopic algae with silica walls; they evolved relatively recently compared to other algal groups but have the greatest diversity of any and are abundant enough to contribute a fifth of the oxygen in the atmosphere and to have accumulated fossil mineral deposits (diatomaceous earth). Among the most recent and most diverse diatom genera is Mastogloia, with over 370 valid taxa but so far no pattern of phylogenetic relationships among the species. The genus is defined by the presence of chambers (partecta) on the girdle bands, which are as varied as the valve shapes and pore patterns. Mastogloia decussata was described in 1892, but its structure is still poorly understood, including whether it has axial costae and the nature of a V-shaped structure reported in 1970. A similar species, M. singaporensis, differs in having narrower, quadrate partecta that occupy only the middle half of the band; it has axial costae. We have observed several populations with quadrate partecta extending nearly to the apices in samples from Micronesian and Caribbean islands; such specimens have also been reported in the literature misidentified as M. decussata. The objective was to establish the ultrastructure of these species. Four major findings are as follows: (1) We establish the correct description of M. decussata with a neotype and show that it has axial costae; (2) we show that it has compound partecta with intrapartectal cylinders; (3) we describe one new species, M. paddocketkempii Lobban & J.A.B.Valencia, which also has compound partecta; (4) we show that all have dual pseudoloculate valve walls with interior and exterior pseudoloculi in slightly offset hexagonal arrays. The new species occurred in both ocean basins and may include simulacra, but we could not separate them morphologically. Detailed SEM of M. singaporensis shows its structural similarity to the other two species.

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