Fish, being cold-blooded organisms, have body temperatures that align with their surrounding environment. While it might seem that fish would freeze solid in icy conditions, most fish species do not freeze in their natural habitats. Instead, fish have evolved a variety of remarkable adaptations that allow them to survive and even thrive in very cold water, preventing ice formation within their bodies.
Survival Strategies in Cold Water
Fish employ several biological and physiological mechanisms to prevent their body tissues from freezing. One such mechanism involves antifreeze proteins (AFPs), specialized proteins produced by many polar and subpolar fish. These AFPs bind to any ice crystals that begin to form within the fish’s body fluids, inhibiting their growth and preventing them from causing cellular damage. This action effectively lowers the freezing point of the fish’s internal fluids, allowing them to remain liquid at temperatures that would otherwise cause freezing.
Another adaptation is supercooling, where some fish can maintain their body fluids in a liquid state even when their temperature drops below the typical freezing point of water. This is possible if ice nuclei, which are necessary for ice crystal formation, are absent. However, this strategy carries a risk, as contact with an external ice crystal can instantly trigger freezing throughout the supercooled fish.
Fish also utilize osmoregulation, maintaining a stable balance of salts and water within their bodies, to depress the freezing point of their blood. By adjusting the concentration of solutes in their blood, fish can lower their internal freezing point. For instance, the blood of teleost fish typically freezes at about -0.6°C to -1°C, while polar seawater can be as cold as -1.9°C. Maintaining specific solute concentrations helps bridge this temperature difference.
Behavioral adaptations also play a role. Fish may seek out warmer water pockets, such as deeper areas of lakes or oceans where water temperatures are more stable and typically remain above freezing. Many fish also enter a state of torpor, a reduced metabolic state, to conserve energy in cold conditions.
The Lethality of Internal Ice Formation
While fish possess remarkable adaptations to prevent freezing, the actual formation of ice within their tissues is almost always lethal for most species. When water freezes, it expands and forms ice crystals. In living tissues, these ice crystals can cause irreparable damage at the cellular level. They physically rupture delicate cell membranes and organelles, which are essential for maintaining cellular integrity and function.
The formation of extracellular ice, or ice crystals outside the cells, also draws water out of the cells through osmosis. This process leads to cellular dehydration, concentrating the solutes inside the cells and disrupting biochemical processes critical for survival.
Furthermore, ice formation can impede blood flow and oxygen delivery throughout the fish’s body. The heart, gills, and brain are particularly susceptible to damage from ice crystals, which can disrupt their structure and function, effectively shutting down vital systems.
Species-Specific Adaptations and Tolerance
The diversity of adaptations among fish species reflects their varying tolerances to cold. Fish inhabiting extreme cold environments, such as the Arctic and Antarctic, exhibit highly specialized mechanisms. For example, Antarctic notothenioid fish and certain northern cods produce a type of antifreeze glycoprotein (AFGP). These AFGPs are nearly identical despite the fish being phylogenetically distant, indicating convergent evolution where similar solutions arose independently to address the same environmental challenge.
Notothenioids, which dominate the fish fauna of the Antarctic Ocean, have evolved AFGPs from a pancreatic trypsinogen gene. These proteins protect them from freezing in seawater that is consistently around -1.9°C, a temperature below the freezing point of their blood. Some of these fish, like the Antarctic icefish, have even lost hemoglobin in their blood, relying on the high oxygen solubility in cold water for oxygen transport.
While polar fish have robust antifreeze capabilities, not all species possess the same level of cold tolerance. Some fish in temperate regions may only survive by migrating to warmer waters or by entering a state of torpor in deeper, insulated lake bottoms where the water remains liquid.