Morphological variability of two Quadrulella species (Arcellinida: Hyalospheniidae) from the Vlasina Lake region of Serbia

Here we present morphological and morphometrical data from sphagnicolous population of Quadrulella longicollis (Taranek) and both qualitative and quantitative characters of Q. symmetrica (Wallich) from the Vlasina Lake region (Serbia). Both broad and narrow types of Q. longicollis shells were observed, but intermediate forms were not noted. Subsequent data analysis supports the species status of Q. longicollis recently proposed in a molecular phylogenetic study. Based on 603 specimens, eight types of Q . symmetrica shells were observed: barrel-shaped shells, short broad shells without a pronounced neck, elongated pyriform shells, elongated square shells, short square shells, slightly curved shells, short broad shells with a pronounced neck and narrow shells. Intermediate forms between these types were also noted. Coefficients of variation were moderately low for all measured characters in the studied population. The minimal variability (4.5%) was observed for shell length, while maximal variability (9.35%) was recorded for the area of optical section. Analysis of the size frequency distribution suggests that Q. symmetrica is a size-monomorphic species.