Edinburgh Napier University

  The transition to air-breathing by formerly aquatic species has occurred repeat-edly and independently in fish, crabs and other animal phyla, but the proximatedrivers of this key innovation remain a long-standing puzzle in evolutio-nary biology. Most studies attribute the onset of air-breathing to the repeatedoccurrence of aquatic hypoxia; however, this hypothesis leaves the current geo-graphical distribution of the 300 genera of air-breathing crabs unexplained.Here, we show that their occurrence is mainly related to high environmentaltemperatures in the tropics. We also demonstrate in an amphibious crab thatthe reduced cost of oxygen supply in air extends aerobic performance tohigher temperatures and thus widensthe animal’sthermal niche. These findingssuggest that high water temperature as a driverconsistentlyexplainsthe numer-ous times air-breathing has evolved. The data also indicate a central role foroxygen- and capacity-limited thermal tolerance not only in shaping sensitivityto current climate change but also in underpinning the climate-dependentevolution of animals, in this case the evolution of air-breathing.