The cold and well-oxygenated waters around Antarctica, which are most importantly stable, are a requirement for the survival of icefish, as variations in these two physical parameters would create a selective pressure that favours the presence of haemoglobin (Sidell and O’Brien, 2006). The lack of haemoglobin is currently thought to be a synaphomorphic trait (derived character trait shared by two or more groups that originated in the last common ancestor) which has been present in the icefish family since they diverged between 5.5 – 2 million years ago (Sidell and O’Brien, 2006). The ability to produce myoglobin has also been lost at a number of stages in the evolutionary history of the family (Figure 6). This loss is thought to be more ancient than that of haemoglobin, as Sidell and O’Brien (2006), were not able to detect the expression of myoglobin in the oxidative skeletal muscles of any icefish sampled. Myoglobin is found in most tissues that serve an aerobic function and is thought to be crucial in the diffusion and intracellular storage of oxygen (Wittenberg and Wittenberg, 2003). The loss of myoglobin is not thought to be selectively disadvantageous as it is not functional in the cold body temperatures of icefish (Sidell and O’Brien, 2006).
The benefits of the loss of haemoglobin are somewhat hard to quantify in terms of the aerobic capacity of icefish, however its loss has lowered the viscosity of blood by 25-40%, which helps to combat the lower viscosity of fluids at low temperatures (Sidell and O’Brien, 2006). The loss of haemoglobin as a respiratory pigment has not reduced the aerobic performance of icefish due to low metabolic demands and the increased solubility of oxygen in seawater and blood at cold temperatures (Clarke, 1983; Clarke and Johnston, 1996).
Icefish have evolved a number of compensatory methods to deal with the lack of haemoglobin. A significantly higher level of energy is devoted to cardiac work, with 22% of resting energy dedicated to this physiological process. This increase in cardiac energy output leads to the pumping of a greater volume of blood around the body per unit time in order to support the body mass of the organism (Sidell and O’Brien, 2006). Icefish also have considerably larger hearts than other red-blooded fish species, which leads to a 4-5 times greater cardiac output (Hemmingsen et al., 1972). Additional adaptations that have allowed icefish to meet oxygen demands for aerobic tissues in the absence of haemoglobin are the four times greater blood volume, and capillaries with a large diameter; which means blood can be transported at low vascular pressures due to a decrease in resistance (Sidell and O’Brien, 2006). These compensatory measures are crucial, as the oxygen carrying capacity of icefish blood is <10% of that found in red-blooded fish species (Holeton, 1970).
Further physiological complications as a result of the cold temperatures and increased viscosity of body fluids are the effects on enzyme kinetics and cytosolic (intracellular fluid/cytoplasmic matrix that separates organelles) in diffusion processes (Sidell, 1991). The mitochondrial capacity of icefish is low compared to species found in warmer waters as a result of the deceleration of gas diffusion and enzyme kinetics at low temperatures (Mintenbeck et al., 2012). The general model of enzymic reactions is that reaction rate increases with temperature up to an optimum rate, before temperature inhibition occurs and reaction rate decreases (Figure 7). Peterson et al. (2007) illustrated this general concept through the use of acid phosphatase as a model. Consequently, mitochondrial energy and oxygen production suffers as a result of reduced diffusion of gas and metabolites to and from the mitochondria (Mintenbeck et al., 2012). These negative effects are overcome in icefish by an increase in the number and capacity of intracellular enzymes, which reduce diffusion distance, and increase aerobic respiratory efficiency (Crockett and Sidell, 1990; Portner et al., 2000). The density of mitochondria and their ultra-structural density is also higher in icefish compared to other fish species. Clarke and Johnston (1996) found that approximately 60% of the muscle fibre volumes of Pleuragramma antarcticum consisted of mitochondria. A number of icefish species have high intracellular lipid concentrations that can be used as an energy store and in the assistance of gas diffusion, increasing the oxygen exchange rate between the blood and mitochondria, and thus energy production (Eastman and DeVries, 1981, Crockett and Sidell, 1990, Kamler et al., 2001).
While the mechanisms listed above were explained by Mintenbeck et al. (2012) principally in terms of offsetting the effects of the reduced efficiency of enzymes at low temperatures, the benefits to icefish in terms of compensation for the lack of haemoglobin are also noteworthy. The increase in oxygen production relative to other fish species helps to maintain energy production despite the reduced efficiency of the oxygen transport system in icefish. Icefish are therefore an excellent example of a group of organisms that are extremely well adapted to the specific environment in which they are found. However, this means that icefish are stenothermic and operate in a narrow ecological and thermal tolerance window, making them especially vulnerable to any changes in the stable conditions in which they have evolved.