Closterium cells are crescent-shaped or elongate and lack spines. Some are quite straight and needle-like, while others are much broader with curved ends. The ends of the cell are usually tapered and may be pointed or rounded. Each semicell has a single axial, ridged chloroplast with at least one pyrenoid. Occasionally there are two chloroplasts per semicell. The nucleus is located in the center of the cell between the chloroplasts.