Closterium aciculare stands out due to its length of around 500 µm and its very slender shape. The diameter of the cells is usually less than 8 µm. The specimen shown below has a diameter of 7.3 µm. The ends of the cell are slightly curved ventrally and almost pointed. There may be a porus at the end, but this is hard to see even at high magnification (s. fig. 3). The shape of the terminal vacuole is not clearly recognizable in this species. Its possible shape can only be recognized by the few crystals it contains (s. fig. 3). The cell wall is completely smooth, without striations and usually colorless. The similar species Closterium gracile is smaller (90–400 µµ), has fewer pyrenoids (4–8 per chloroplast) and a distinct terminal pore. In addition, girdle bands are often present.