Monactinus biwaensis (Negoro) Matsuoka et Kayama was described as a new species in 1954 as Pediastrum biwae Negoro from Lake Biwa in Central Japan. This species is characterized by projections from adjacent marginal cells that either face each other or diverge. However, there was also the view that P. biwae was not an independent species but rather a variety of P. simplex as P. simplex var. biwaense. Subsequently, no other water systems yielded Pediastrum species with this morphology, and P. biwae was long considered endemic to the Lake Biwa–Yodo River system. Measurement of the side ratio of marginal cells in Monactinus simplex collected from Lake Biwa and molecular phylogenetic analysis revealed that M. biwaensis and M. simplex are clearly distinguishable. This result strongly suggests that two morphologically similar species, M. simplex and M. biwaensis occur in Lake Biwa at present. Fossils of M. biwaensis have been found in the Ayama Formation of the Late Pliocene Kobiwako Group and in the Ma0 Member of the Early Pleistocene Osaka Group, leading to the assumption that this species was endemic to Lake Biwa–Yodo River System. However, M. biwaensis was also found in the Middle Pleistocene Karato Formation on Himeshima Island of Oita Prefecture and in the Sahama Mud Member along the coast of Lake Hamana, which are distant from the Lake Biwa–Yodo River System. In addition to this, M. biwaensis has also been observed in surface sediments from Harima Nada and Isa Bay, both located outside the Lake Biwa–Yodo River System. Furthermore, M. biwaensis occurs in other countries outside Japan, primarily in the Southeast Asian region. Taken together, these findings clearly indicate that M. biwaensis is not endemic to the Lake Biwa–Yodo River System.
Matsuoka et al. (Fri,) studied this question.