A swordfish being caught off southern California, USA, by the Pfleger Institute of Environmental Research (PIER) team, USA. Photo credit: PIER.
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.
But first the team needed to obtain fresh samples of the elusive fishes’ blood. ‘Chugey and Diego really did all the hard work, locating and catching these amazing fish’, says Morrison, recalling how the precious blood samples were couriered from California to Vancouver within a day. Then, Morrison had to work fast; ‘My job… was to complete the blood experiments ASAP’, he says. After washing the oxygen out of the swordfish and opah blood with nitrogen, he gradually fed oxygen back into each sample – recording how much of the gas was picked up by the blood cells – first at 10°C for both fish, before repeating the exercise at 15 and 20°C for the opah blood and ramping the temperature straight up to 25°C for the swordfish. Plotting the fraction of oxygen absorbed by the blood as the amount of oxygen in the vicinity increased, Morrison realised that when the quantity of oxygen in the swordfish's blood was relatively low, temperature did not affect how much oxygen it could carry – ideal for blood returning to the gills ready to be recharged. In contrast, as the opah's blood became warmer, it held on to oxygen more tightly at higher temperatures, which is convenient, because heat-retaining structures in the opah's gill warm the blood before it is sent around the body, otherwise the blood would be unable to hold on to all of the oxygen picked up in the gill.
Next, Morrison tested how the oxygen-binding characteristics of the blood could change as it passed through heat exchanger structures – where warm blood leaving a tissue passes close to cold blood vessels from the gills, warming the cooler blood to retain heat that would otherwise be lost. By rapidly heating or cooling the blood, he found that the swordfish's blood lost more oxygen as the temperature rose, which could be problematic for oxygen delivery to warmer organs. In contrast, the ability of the opah's blood to carry oxygen increased as the temperature rose.
But what was driving these differences? Extracting haemoglobin from the fishes’ red blood cells, Morrison discovered that ATP, which is also carried in blood, reduces the protein's temperature sensitivity, minimising the impact of dramatic body temperature changes on the ability of haemoglobin to transport oxygen. In addition, Morrison checked how well the swordfish's blood bound to oxygen at 10°C and it was significantly better than that of other fishes, potentially allowing the animals to take up oxygen from the oxygen-depleted deep waters that they frequent to feed.
