Analysis and critical evaluation of structural features in four Cymbellaceae taxa from New Caledonia

Delicatophycus costei (Lange-Bertalot et Moser) Wynne, Cymbella bourrellyi Maillard, Cymbella latarea Maillard and Cymbella pernodensis Maillard are endemic to New Caledonia and show uncommon morphological features compared to the "sensu stricto" definition of their respective genus. Delicatophycus costei was described based on light microscopy and scanning electron microscopy (SEM) observations, confirming the generic LM diagnosis. However, the external view in SEM is atypical as both valve face and mantle are covered by longitudinal ribs, overlaying part of the zig-zag striae, generally accepted as the determinant generic feature. The three species of Cymbella have both apical pore fields and "stigmata". However, they show some specific morphological features compared to the more classically shaped Cymbella species. Cymbella bourrellyi has a narrow hyaline area on the valve face and the areolae are bordered internally by small outgrowths. Cymbella pernodensis is distinguished by internal biseriate striae. The internal morphology of Cymbella latarea is characteristic in having typical biseriate striae and bisected apical pore fields. In these three species, internally, the stigmata are continuous with the striae and appear as simple and only weakly separated structures at the end of the striae. The three species are also characterized by differences in the number of areola rows per stria externally and internally.