Detailed observations of the life history and phylogenetic placements of the freshwater green alga <i>Oocystaenium elegans</i> (<scp>Oocystaceae, Trebouxiophyceae</scp>), with an emended description

SUMMARY Oocystaenium elegans Gonzalves &amp; Mehra (Oocystaceae, Trebouxiophyceae) was collected from a paddy field in Japan. Its life history was reexamined based on detailed observations of cultured material, and an emended description is given. This species is a large spindle‐shaped green alga that grows in puddles and paddy fields and has been reported in India, Nepal and Cuba. Reproduction occurs asexually via autospore formation and sexually via oogamy. Although this oogamy is considered to involve fertilization by globular egg cells and antherozoids, the results of the present study revealed that the egg cells (female cells) remained spindle‐shaped and became globular zygotes after fertilization. The place on the egg cells where the antherozoid attached swelled to become a receptive papilla, after which the antherozoid invaded. Phylogenetic analysis based on nuclear 18S small subunit ribosomal RNA gene sequences showed that O. elegans is included in the family Oocystaceae.