Sea temperatures around the Antarctic Continental Shelf can drop as low as -1.86C, therefore a physiological mechanism is required to prevent ice formation in body tissues. Antifreeze glycoproteins (AFGPs) are an evolutionary adaptation that prevent ice formation in polar fishes by attaching to and blocking the growth of ice crystals (Mintenbeck et al., 2012). The evolution of Antifreeze glycoproteins is thought to have occurred only once in the history of the Notothenioids, and can be traced back before the Suborder diverged (Figure 4). Antifreezes are found in the tissues and organs of Notothenioids and Northern cod (Gadua morhua), and work by attaching the–OH and –NH2 groups in the molecules to oxygen atoms in water. The front of the growing ice crystal is therefore broken up into a series of smaller fronts that cannot grow until temperatures decrease (Willmer, 2004). The synthesis of AFGPs is thought to be regulated by ambient water temperatures, leading to the assumption that a greater number of, or larger AFGPs, are synthesised at lower water temperatures (Wohrman, 1996). The evolution of Antifreeze Glycoproteins was not however enough to cope with the low temperatures experienced in the Southern Ocean, therefore a number of other mechanisms have been required to maintain efficient metabolic function.
Figure 4. A cladogram of notothenioid relationships. The colours used indicate geographical distributions and the wedges are proportional to species diversity in each of the eight families. This particular cladogram was simplified from, and based on, nucleotide sequencing data in Near et al., (2004). The red slot illustrates that all of the Antarctic notothenioids possess antifreeze glycoproteins, and therefore, the trait can be described as synaphomorphic (Taken from: http://www.oucom.ohiou.edu/dbms-eastman/research.htm; accessed 15/11/2014).