Potamogeton iriomotensis Masamune.

Plants annual or perennial, in fresh water. Rhizome inconspicuously present or absent. Stems filiform, terete, ca. 0.5 mm in diam., simple or sparsely branched; dormant turions axillary, narrowly fusiform, with 3-5 acerose leaves. Leaves dimorphic; stipules convolute, 6-10 mm, membranous, free from leaf base. Submerged leaves sessile, linear to filiform, 2.5-7 cm × ca. 1 mm, 3-veined. Floating leaves usually alternate, opposite just below peduncle, petiolate; petiole 1-1.5 cm; blade opaque, ovate to ovate-oblong, rarely lanceolate, 1.5-2.5 cm × 3-11 mm, leathery, 7-veined, base rounded or cuneate, apex acute or obtuse. Spikes with 3-5 whorls of opposite flowers; peduncles 0.8-1.5 cm. Carpels 4. Fruit obovoid, 1.5-2.5 mm, laterally compressed and shortly pedicellate, abaxial keel cristate; beak 1-1.2 mm, slender. Fl. and fr. May-Sep.

Ponds, paddy fields. Anhui, Fujian, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Liaoning, Sichuan, Taiwan, Zhejiang [Japan (including Ryukyu Islands), Korea, Russia].