Two new Encyonema species from Europe compared to the type material of E. minutum and E. silesiacum (Gomphonemataceae, Bacillariophyceae)

Cymbella minuta Hilse and C. silesiaca Bleisch, transferred in 1990 to the genus Encyonema, are two species frequently observed in European rivers. The original material of both species was included in Rabenhorst's historical exsiccata series (numbers 1261 & 1802) and illustrated by Kurt Krammer in 1997. Despite this publication, the morphological variability of these two species is poorly known, and interpretations of this variability are often too broad or not precise enough. Recently, several populations of Encyonema displaying intermediate morphological features between E. minutum and E. silesiacum have been observed in Europe, making a reliable identification of these taxa impossible. Based on the currently available literature, two taxa that could not be assigned to an existing species are described as new: E. morvanense and E. vandammeanum. The present paper illustrates and discusses the morphology of the type material of E. minutum and E. silesiacum and compares it with both new species. Encyonema morvanense is distinguished by its typically protracted, rostrate apices, a low length/width ratio, the presence of an isolated pore, and areolae that are discernible under LM. In contrast, E. vandammeanum also exhibits protracted apices, but a higher length/width ratio, areolae that are faintly discernible, and the absence of an isolated pore. Comparisons with other Encyonema species, described in 1997 by Krammer (such as E. brevicapitatum, E. minutiforme and E. simile), are added in order to better understand the differences and similarities between these species.