Re-assignment of , a newly-recorded species in China, to Stephanosphaerinia clade (Chlamydomonadales, Chlorophyceae): insights from morphological and phylogenetic analyses.

A newly-recorded green algal species in China, identified as Raineri, exhibits distinctive morphological characteristics. The thallus of this specimen is spherical or slightly irregular, calcified and firmly adheres to stones in running water. The prostrate part consists of loose, thin-walled tissue, with extended filaments being cylindrical and sparsely branched. The terminal sporangia are only slightly larger than the vegetative cells, with smaller cell dimensions and a higher length-to-width ratio. Biflagellate zoospores were formed within terminal sporangia. Each cell contains one or two pyrenoids. Ultrastructural observations revealed that the pyrenoids are traversed by thylakoid membranes and no plasmodesmata were observed between adjacent cells, confirming that should be excluded from the order Chaetophorales. Phylogenetic analyses of 18S rDNA, ITS and A sequences indicate that belongs to the Stephanosphaerinia clade of Chlamydomonadales. Within this clade, and forms a distinct, independent branch and cannot be assigned to any currently-known genus within Chlamydomonadales, but the taxonomic status of other species within the genus , including the type species, remains unresolved. Therefore, the name has been retained. Further in-depth research, particularly focusing on the type species , is needed to refine the classification system and resolve the taxonomic uncertainties within this genus.