The surgo-op was a chemically-induced forced mutation that made air-breathing lifeforms capable of living under water. It was designed by the air-breathing ancestors of the Aquans to allow their species to continue to exist after intense seismic activity started turning their planet, Argo, into a water world. The mutation was induced and controlled by the venom of the sur-snake.
In 2269, James T. Kirk and Spock underwent the surgo-op. It was performed by young Aquans in order to save them after they had been attacked by the sur-snake. Doctor Leonard McCoy noted that Kirk and Spock's internal structure had completely changed; they had also grown webbed hands, their metabolism had changed, and their eyes were now covered with a transparent film. McCoy kept them in an aquarium in sickbay while he attempted to reverse the effects, but he was unable to find a solution. Kirk and Spock returned to the Aquan city and learned of the location of information regarding the development of the surgo-op. After getting the information, McCoy learned that an anti-toxin, produced by the venom of the sur-snake, would reverse the surgo-op. (TAS: "The Ambergris Element")