Two new species of Oricymba (Bacillariophyta) from China, described with reference to the structure of the apical pore field

Abstract

Epilithic diatoms form an important component of the benthos in freshwater rivers. As part of a study on the epilithic diatoms in rivers from Hunan and Guizhou Provinces, central China, two new Oricymba species, O. sinensis sp. nov. and O. fanjingensis sp. nov., were observed using light and scanning electron microscopy. Oricymba sinensis possesses a conspicuous dorsiventrality, which differs from other Oricymba species. Moreover, O. sinensis has very shallow grooves along either side of each marginal ridge and higher areola density than other Oricymba species. Oricymba fanjingensis has a unique combination of characters that includes apiculate apices of the valve, distinct grooves along either side of the marginal ridge, and a valve surface ridge parallel to its adjacent marginal ridge. This surface ridge is observed for the first time in Oricymba. The valve external and internal ultrastructure is clearly established for these two new species. In particular, their apical pore fields possess internal apertures of each pervalvar row of foramina that are covered by an undulate flap-like silica strip, which does not completely occlude the internal apertures. Some species of the genera Cymbella and Delicatophycus have a similar internal structure in the apical pore field. Thus, the ultrastructure of the apical pore fields should be considered an important distinguishing feature for Cymbelloid diatoms and is worth investigating systematically.