Surgo-op
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, , into a water world. The mutation was induced and controlled by the venom of the sur-snake, which proved also to be needed to reverse it.

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; their metabolism had also changed, they had experienced pronounced scaling and toughening of their respective epidermises and dermises, they had grown webbing of their fingers and toes, and their eyes were now covered with a transparent film ("nictitating membranes"). 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. ()

de: Surgo-Operation
Medical procedures