It was originally described by Wilhelm Schmidle as a heterokont, and has also been placed in the family Ulotrichaceae.

[3] Planctonema consists of short, uniseriate filaments surrounded in a common mucilaginous sheath.

At the ends of the cell poles are vacuoles containing conspicuous oil globules.

[1] Similar genera include Psephonema (formerly considered to be synonymous with Planctonema), and Planctonemopsis, described in 2017.

Psephonema has shorter cells than Planctonema (with a length–width ratio of 1.5–2, while Planctonemopsis has chloroplasts with pyrenoids).