The loss of haemoglobin is a unique and highly derived characteristic. However, icefish are more than just a scientific curiosity as they play a crucial role in the Antarctic ecosystem. Without haemoglobin, icefish blood is significantly less viscous than that of many other teleost fish groups. This is important in terms of maintaining metabolic function at an optimal level, as fluids are more viscous at lower temperatures. The oxygen-carrying capacity of icefish blood would be lower, as a result of a lack of haemoglobin, if a number of compensatory measures had not evolved. Icefish devote more energy to cardiac output, have a greater volume of blood and have capillaries with a larger diameter than is commonly found in other teleost groups. Also, the inverse relationship between oxygen and temperature results in a high level of dissolved oxygen in the water column. Thus, there is no selection pressure for haemoglobin expression. Icefish are consequently stenothermic organisms which are highly specialised for life in a narrow range of environmental parameters.
Climate change will place physiological stress on icefish as a result of increasing sea temperatures, and many icefish species are likely to decrease in abundance. More research needs to be carried out to ascertain the effects of temperature changes on icefish physiology and on the effects a decrease in abundance of this important Antarctic fish family will have on the ecosystem as a whole.
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