Analysis of type material of Pleurosigma formosum and P. decorum (Pleurosigmataceae, Bacillariophyta)

Abstract

The type materials of Pleurosigma formosum W. Smith and P. decorum W. Smith wereexamined. Both species were separated hitherto by size range, valve shape and stria density basedon light microscopy (LM) analysis. However, the type materials show some overlap in the size and striadensity ranges, with subtle differences in the oblique stria intersection angle, which is somewhat smallerfor P. decorum. Scanning electron microscopy (SEM) analysis shows that both species share severalultrastructural features such as a thick saddle-shaped central raphe nodule, very long overlapping centralraphe fissures, short hook-shaped terminal raphe fissures, general morphology of the internal hymenoccludedpores crossed by a recessed bar and presence of some scattered pairs of more deeply recessedpores lacking a bar. However, the type specimens differ in the internal morphology of the hymen-occludedareolar pores, circular, crossed by a recessed bar and rimmed in P. formosum and subcircular to elliptical,crossed by a less recessed bar, not rimmed in P. decorum. The SEM findings thus support Hendey’s viewthat P. decorum and P. formosum are separate but closely allied species.