Аulacoseira granulata (Bacillariophyta): rimoportulae

Characters of rimoportulae are important in the taxonomy of the genus Aulacoseira, but data on the variability of these structures are scarce. We study the rimoportulae number, position and morphology in the centric diatom Aulacoseira granulata from a population оf the Nizhny Tagil Reservoir (Sverdlovsk Region, Russia). A significant variation in the number of rimoportulae on the half of the valve mantle (1–3) and their location (along the entire surface of the valve mantle) is found. Two different types of the rimoportulae on the inner surface of valves within the same species of Aulacoseira are shown. Most often each rimoportula looks like a long spiral tube, less often such tube is short and straight. Externally, each rimoportula opening resembles a funnel-shaped hole which is often accompanied by an oval hole of an unclear function.