Halosphaera K. J. F. Schmitz 1878

Unicellular algae with two stages, Phycoma stage spherical, up to 800 µm in diameter, surrounded by a two-layered wall, numerous chloroplasts usually with pyrenoids, lots of lipid, initially uninucleate, divides to form flagellate stage; flagellate stage with 4 flagella emanating from an anterior pit; single parietal chloroplast, 2-4 pyrenoids, eyespot. Cell surfaces with organic scales in several layers, on the body up to 4 types, including an underlayer of very small square scales and 2 layers of larger scales. Flagella with an underlayer of small square scales overlain by limuloid scales; hair-shaped scales in two opposed rows, switch between the Phycoma stage and the motile cells may be linked to the lunar cycle; motile stage divides by fission. Halosphaera differs from the closely related Pachysphaera (by the cytoplasmic contents of the Phycoma stage being distributed peripherally in Halosphaera but filling the Phycoma in Pachysphaera; motile cells of Pachysphaera are very similar to those of Pterosperma and similar to the fossil Tasmanites.

Halosphaera is a genus of green algae in the order Pyramimonadales.