Water-breather

Kirk and Spock changed

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")

See also