A reappraisal of the genus L eptocylindrus ( B acillariophyta), with the addition of three species and the erection of T enuicylindrus gen. nov

Centric diatoms of the genus L eptocylindrus are common in the marine plankton worldwide. Only two species, L . danicus C leve and L . minimus G ran, so far clearly belong to this genus, whose diversity has not been fully investigated. We investigated frustule and spore morphology as well as three nuclear‐ and three plastid‐encoded markers of 85 L eptocylindrus strains from the G ulf of N aples, and one from the A tlantic US . The strains grouped into five molecularly distinct species with different levels of morphological differentiation. Two species matched the description of L . danicus and produced similar spores but differed in morphometric characters and sub‐central pore position, supporting the description of L . hargravesii N anjappa and Z ingone as a distinct species. L eptocylindrus danicus var. apora F rench III and H argraves, lacking a sub‐central pore and not forming spores, was raised to the species level as L . aporus ( F rench III and H argraves) N anjappa and Z ingone. A fourth species with convex valves was described as L . convexus N anjappa and Z ingone. The fifth species matched the description of L . belgicus M eunier, considered as synonym of L . minimus . However, ultrastructural differences from all other L eptocylindrus supported the erection of the genus T enuicylindrus N anjappa and Z ingone with T . belgicus ( M eunier) N anjappa and Z ingone as type species. None of the sequences matched the L . minimus sequence in GenBank. The species analyzed showed different or partially overlapping seasonal distributions. Despite the addition of the new taxa, the ancient diatom lineage of the L eptocylindraceae shows a relative species poorness and considerable morphological stasis.