Water-breather
Kirk and Spock unable to breathe normal air
Water-breather was a term used by Leonard McCoy referring to anyone who used gills to breathe underwater, such as James T. Kirk and Spock after surgo-op, or the Aquans who performed said feat.
McCoy stated that this mutation was caused by an unidentified substance or hormone found in the bloodstream that affected their entire metabolism. (TAS: "The Ambergris Element")