Most fish are infamously cold blooded, but a super-league of fish has bucked the trend. Sharks, tuna, swordfish and the smalleye Pacific opah recycle heat within their bodies to maintain some tissues - including their brain, eyes and specific muscles - at significantly warmer temperatures to soup-up performance. However, there is a drawback. The protein haemoglobin, which transports oxygen from the gills and releases it into tissues, tends to function optimally over a narrow range of temperatures. Yet, warmer-blooded fish, such as tuna, have managed to tweak the oxygen-carrying protein to be less vulnerable to temperature, allowing their haemoglobin to successfully deliver oxygen regardless of body temperature. Knowing that hot-headed swordfish routinely plumb the chilly depths while warm-bodied smalleye Pacific opah rarely ascend from their cool deep-water homes, Diego Bernal from the University of Massachusetts, USA, and Chugey Sepulveda (PIER), with Nicholas Wegner from the National Oceanic and Atmospheric Administration, USA, and Phillip Morrison and Colin Brauner from The University of British Columbia (UBC), Canada, decided to find out how the oxygen transporting ability of the haemoglobin carried by the two fish varies at different temperatures.
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