Not all cod are created equal. Scratch beneath the surface and an essential difference in their blood appears. Michael Berenbrink from the University of Liverpool, UK, explains that most cod found at the southern extent of their range tend to produce a specific form (genotype) of the oxygen carrying protein haemoglobin in their red blood cells (type I haemoglobin), while individuals from the population further north tend to carry a second form of the protein (type II). And when fish with one or other of the two haemoglobin forms are offered a choice of temperatures, cod carrying the first prefer warmer conditions, while those carrying the second select the chillier water. ‘There is something that translates from the genotype to how the whole organism behaves in a thermal gradient, so there must be something that is different, but what it is we don't know’, says Berenbrink.
Evidence suggested that the different haemoglobins could lie at the heart of the population distributions, if the first form of haemoglobin allowed the fish to transport more oxygen in warm conditions while the second haemoglobin was optimised for oxygen transport in cold conditions. ‘But nobody has ever checked this carefully’, says Berenbrink. With the possibility that climate change may eradicate cod from the Irish Sea by the end of this century, Berenbrink and his colleagues, Samantha Barlow, Julian Metcalfe and David Righton, decided to test how tightly red blood cells from fish in the Irish Sea bind oxygen.
Knowing that she could catch type I and type II individuals in the Liverpool bay, Barlow headed out into the ferocious sea conditions in the dead of winter on charter boats to fish for blood samples. ‘We chose that time of year because there was no difference in the water column temperature and we took all of the fish from a certain spot’, says Berenbrink, explaining that different temperature experiences and stress can dramatically affect how tightly fish red blood cells bind oxygen. And when Barlow returned to the lab with the precious vials of blood, she had to work fast, running the red blood cell proteins on a gel to identify which form of haemoglobin each individual fish carried before selecting the blood group she would test the following day. She then cautiously isolated the red blood cells and stabilised them at three different temperatures (5, 12.5 and 20°C) and three different pHs (7.9, 7.65 and 7.4), before painstakingly adjusting the oxygen concentration in the red blood cells (ranging from 0–100% air) and measuring the subtle colour change as the haemoglobin in the red blood cells absorbed the gas, to determine how tightly the blood cells bound oxygen.
However, when Barlow plotted the characteristic S-shaped oxygen binding curves and adjusted for the internal pH change that occurs naturally as blood cells warm, she and Berenbrink could see that all of the curves essentially overlapped. The different forms of the oxygen carrying haemoglobin did not affect the red blood cells’ ability to bind oxygen at different temperatures, despite the fish's temperature preferences: ‘We are back to the drawing board to figure out what limits cod thermal tolerance’, says Berenbrink.
Realising that the red blood cell oxygen binding was essentially identical, the duo then pooled the data from all 16 samples and calculated how well the fish could absorb oxygen and deliver it to tissues as their temperature and metabolic rates increase under future climate scenarios. However, their calculations suggest that the fish's red blood cells will struggle to meet their increasing oxygen demands at temperatures around 20°C and above. ‘This does not necessarily mean the animal is doomed, just that any improvements in oxygen supply must come from other sources’, Berenbrink concludes.