The idea of fish freezing solid and then miraculously “coming back to life” is a fascinating concept. While it is not a simple yes or no answer, certain fish exhibit remarkable adaptations to survive extremely cold conditions. This article explores the science behind how some fish endure freezing temperatures, highlighting the biological mechanisms that allow them to persist in environments that would otherwise be lethal.
Not All Freezing Is Equal
The nuance of “freezing” in a biological context is important to understand. True freezing, where ice crystals form inside the cells of an organism, is almost always fatal for fish. This cellular ice formation causes irreparable damage to membranes and organelles. The concept of “coming back to life” is not a resurrection from being frozen solid like an ice cube, but rather a recovery from a state of extreme cold where vital functions are significantly slowed or suspended without critical cellular damage.
Some fish employ strategies to manage ice formation or avoid it entirely. These strategies differentiate between ice forming outside the cells, which can sometimes be tolerated, and ice forming inside cells, which typically leads to death. Controlled management of ice formation can involve supercooling, where water remains liquid below its normal freezing point, or mechanisms that prevent ice crystals from growing large enough to cause harm.
Nature’s Antifreeze: Biological Mechanisms
Certain fish have evolved specific biological adaptations to survive freezing temperatures. A primary adaptation involves the production of antifreeze proteins (AFPs) or antifreeze glycoproteins (AFGPs). These specialized proteins bind to tiny ice crystals that may enter the fish’s blood, preventing them from growing larger and causing cellular damage. This binding action effectively lowers the freezing point of the fish’s bodily fluids below that of the surrounding water, a phenomenon known as thermal hysteresis.
Another strategy is supercooling, where fish can lower their body temperature below 0°C without ice formation. This is achieved by keeping their blood free of ice nucleators, which are substances that can initiate ice crystal growth. However, supercooling carries an inherent risk: if a supercooled fish comes into contact with an external ice crystal, it can trigger rapid and fatal freezing throughout its body. Antifreeze proteins can also play a role in preventing this triggered freezing by inhibiting the growth of any ice that might form.
Fish That Defy the Cold
Fish species with exceptional cold tolerance are found in the polar regions. Arctic and Antarctic fish, such as the Arctic cod and various Notothenioids, including the Antarctic icefish, possess high concentrations of antifreeze proteins in their blood. These proteins allow them to inhabit waters that are consistently below the freezing point of typical fish blood, sometimes as low as -1.9°C.
Antarctic notothenioids, which evolved their antifreeze glycoproteins over millions of years, are particularly adapted to the icy Southern Ocean. Arctic cod developed similar antifreeze glycoproteins through convergent evolution, arriving at the same solution independently. Some Antarctic icefish have even lost red blood cells and hemoglobin, relying on the high dissolved oxygen content of their cold marine environment to supply oxygen.
When Freezing is Fatal
Despite their remarkable biological mechanisms, there are thresholds beyond which even the most cold-adapted fish cannot survive. Factors that can lead to fatal freezing include prolonged exposure to extremely low temperatures that overwhelm their protective systems. Rapid temperature drops can also bypass their adaptations, leading to uncontrolled ice formation.