Introduction

The evolution and expression of protein antifreezes in cold-water marine fishes which permitted them to thrive in otherwise lethally frigid marine habitats represents one of the most clear-cut and remarkable forms of cold adaptation that nature has invented. Antifreeze proteins (AFs) in polar fishes presumably evolved under the selective pressure of cold temperature as the polar oceans cooled to the freezing point of seawater (−1.9 °C) over their respective paleogeographic time scales. The Antarctic coastal waters today are perennially at −1.9 °C and ice-laden (Littlepage, 1965), while the Arctic and some north temperate waters experience similar conditions in boreal winters. Marine teleosts face the danger of freezing in these environments because they are hyposmotic to seawater; the salt content in their blood depresses the colligative or equilibrium freezing point only to about −0.7 °C (Prosser, 1973). They are thus supercooled with respect to ambient freezing seawater, and cannot avoid freezing in the presence of ice. Fishes living in cold waters generally have higher blood salt content; Antarctic fishes, for example, have enough salt to depress the colligative freezing point to −1.1 °C to −1.3 °C (DeVries, 1982; Ahlgren et al., 1988) but this is still insufficient to prevent freezing. Presence of AFs in the blood and body fluids of AF-bearing fishes depress the freezing point further to a few tenths below −1.9 °C, preserving the body fluids in the liquid state (DeVries, 1982). Freezing point depression by AFs is via a non-colligative mechanism; AF molecules absorb to specific faces of ice crystals (Knight, Cheng & DeVries, 1991; Knight, Driggers & DeVries, 1993; Knight & DeVries, 1994) and inhibit ice growth through the Kelvin effect (Raymond & DeVries, 1977).