Nerophis ophidion is slender and elongate, with a smooth worm like body. Females may grow to 30 cm, males to 20 cm. The slender snout is straight and equal in length to the rest of the head, with a small mouth. Fins are absent apart from a small dorsal fin. Variable in colour but usually green, darker dorsally, lighter green on the sides with whiteish-green or yellow below. Females may have pale blue markings on head and underside, and some fish have vertical rows of spots on the front half of the body.As with seahorses, eggs are incubated by the male pipefish, which carries the eggs in a shallow abdominal groove. Swimming occurs through undulations of the dorsal fin. Spawning occurs from May to August.