Herbs, submerged marine; rhizome robust, with persistent fibrous remains of leaf sheaths. Leaves 2-6, narrowly linear or ribbon-shaped, sheathing at base, with many parallel veins, unequal at apex. Plants dioecious; flowers unisexual. Male inflorescence shortly pedunculate, enclosed by 2 spathal bracts; flowers numerous, minute, shortly pedicellate, breaking off just before anthesis, floating on surface of water at maturity; tepals reflexed; stamens 3; anthers subsessile, latrorsely dehiscent; pollen grains very large. Female inflorescence 1-flowered, with a long peduncle, enclosed by 2 overlapping, scarcely fused, strongly keeled spathal bracts; sepals narrowly elliptic; petals linear; carpels 6, fused; styles 6, bifid. Fruit irregularly dehiscent. Seeds few.

One species: widely distributed along the coasts of the Indian and W Pacific Oceans.