A large scarlet colored sea beast on the planet Argo, the sur-snake has four long tentacles ending in flippers, a pair of yellow eyes, a dorsal fin, and fangs. When a Federation aquashuttle is attacked by it in 2269 (stardate 5499.9), they are able to stun it with a phaser. Studying its unconscious body, they find it has a dual respiratory system, lungs, and gills, and that its skeleton resembles that of a Denebian whale. Later, the U.S.S. Enterprise crew learns that the ambergris-like venom of the sur-snake is the key to reversing the genetic mutation of the surgo-op, thus transforming Kirk and Spock from water-breathers back into air-breathers. Hunting sur-snakes is against the ordainments (rules) of the Aquans.