Description from Flora of China
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].