Light and scanning electron microscopic observations on auxospores of two Epithemia species (Rhopalodiales, Bacillariophyta)

We describe here the first SEM observations on the auxospores of two species of Epithemia, E. reicheltii and E. lacus-douglasi, from Douglas Lake, Michigan, USA. Both species were found in abundance with auxospores in October of 2024. The auxospores of both species were semicircular in shape, initially covered with an organic matrix, which split as incunabular caps. Internal to this matrix were transverse incunabular rows of quadrangular, imbricating scales overlapping towards the valve ends, with the longer abaxial edges fimbriate and the other edges straight. And, internal to the scales, was a single, siliceous covering – the primary longitudinal perizonial band – which had transversely-oriented fissures. Internal to the primary band were three (or four) dorsally positioned secondary longitudinal perizonial bands. The complex coverings of the auxospores of these Epithemia species are unique amongst the diatoms.