Cocconeis fetscheriana sp. nov. (Bacillariophyta) from streams in California (USA) with reference to the cingulum variability within the placentuloid species complex

A novel diatom species, Cocconeis fetscheriana, is described from streams in California (USA). Detailed light and scanning electron microscopy of its frustules reveal a unique combination of characters. These are a highly variable and often irregular valve outline, rapheless sternum valves with characteristic dumbbell-shaped areolae, forming 7–14 longitudinal rows per hemi-valve, a lanceolate, slightly diagonal sternum separated from the valve apices by up to four areolae, robust closed smooth valvocopula attached to sternum valve mantle by an undulate rim, and raphe valve with distinct submarginal areolae, which always circumradiate the raphe apices. Teratological forms are commonly observed in this novel species and are discussed. Environmental data and morphological comparisons with similar Cocconeis taxa with open valvocopulae are provided. The importance of detailed characterization of the cingulum structure with a particular focus on the rapheless sternum valve valvocopula as diagnostic features in Cocconeis is discussed. Examples of cingulum ultrastructure variability are included from additional undescribed Cocconeis species from another stream in California.