The sur-snake was a poisonous aquatic creature native to the planet Argo. The source of the sur-snake's poison was from a venom shot from its teeth. It was noted for having dual respiratory systems, both lungs and gills, and a skeletal structure similar to that of the Denebian whale. It had four long tentacle-like appendages, which were both prehensile in nature, and strong enough to pick up a Starfleet aqua-shuttle and throw it.
Despite its poisonous nature, the sur-snake's venom was used in 2269 to reverse the surgo-op procedure, a mutation Spock and James T. Kirk underwent that transformed them into water-breathers. (TAS: "The Ambergris Element")
The sur-snake was described in the first draft of the script as being "a huge bright amber Sea Monster" that "resembles an elongated whale. Its body is almost snake-like and it has four side tentacles."
In the novelization of the story included in Star Trek Log 5, this creature is called a snake-squid. One of the Aquans referred to it as a cpheryhm-aj.
The entry on Argo in Shane Johnson's The Worlds of the Federation gives the alternate name of the duitra to the sur-snake and notes that the capture of specimens is prohibited by the natives of Argo. Alan Dean Foster's novelization blames this last on the Aquans's hate and fear of aerobic beings, noting that its poison was needed to reverse the surgo-op mutation procedure.