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Volvox  sect.  Volvox  is characterized by having unique morphological characteristics, such as thick cytoplasmic bridges between adult somatic cells in the spheroids and spiny zygote walls. Species of this section are found from various freshwater habitats. Recently, three species of  Volvox  sect.  Volvox  originating from rice paddies and a marsh were studied taxonomically based on molecular and morphological data of cultured materials. However, taxonomic studies have not been performed on cultured materials of this section originating from large lake water bodies. We studied a new morphological type of  Volvox  sect.  Volvox  (“ Volvox  sp. Sagami”), using cultured materials originating from two large lakes and a pond in Japan.  Volvox  sp. Sagami produced monoecious sexual spheroids and may represent a new morphological species; it could be clearly distinguished from all previously described monoecious species of  Volvox  sect.  Volvox  by its small number of eggs or zygotes (5–25) in sexual spheroids, with short acute spines (up to 3 μm long) on the zygote walls and elongated anterior somatic cells in asexual spheroids. Based on sequences of internal transcribed spacer (ITS) regions of nuclear ribosomal DNA (rDNA; ITS-1, 5.8S rDNA and ITS-2) and plastid genes, however, the  Volvox  sp. Sagami lineage and its sister lineage (the monoecious species  V .  ferrisii ) showed very small genetic differences, which correspond to the variation within a single biological species in other volvocalean algae. Since  V .  ferrisii  was different from  Volvox  sp. Sagami, by having approximately 100–200 zygotes in the sexual spheroids and long spines (6–8.5 μm long) on the zygote walls, as well as growing in Japanese rice paddies, these two morphologically distinct lineages might have diverged rapidly in the two different freshwater habitats. In addition, the swimming velocity during phototaxis of  Volvox  sp. Sagami spheroids originating from large lakes was significantly higher than that of  V .  ferrisii  originating from rice paddies, suggesting adaptation of  Volvox  sp. Sagami to large water bodies.

A New Morphological Type of Volvox from Japanese Large Lakes and Recent Divergence of this Type and V. ferrisii in Two Different Freshwater Habitats