How Antarctic fish survive in freezing waters without turning into blocks of ice

Nature · Published 3 October 2026 Swipe more like this →

In short: In the Southern Ocean, where temperatures drop below freezing, fish have evolved extraordinary biological hacks to stay alive. From blood that functions like antifreeze to species that lack red blood cells entirely, these creatures thrive in an environment that would be lethal to almost any other vertebrate on Earth.

The Southern Ocean is a harsh, frigid habitat, yet it teems with specialized fish that have mastered the cold. The most striking adaptation is the presence of glycoproteins in their blood, which act as a natural antifreeze. These molecules prevent the fish from freezing internally by binding to ice crystals that enter the body through food or the surrounding water. This mechanism is so effective that it allows these species to survive in temperatures as low as -1.86 °C.

Perhaps the most remarkable example of this evolutionary ingenuity is the icefish. Unlike most vertebrates, the icefish lacks red blood cells and hemoglobin, the protein responsible for carrying oxygen. Instead, it transports oxygen through its blood plasma. This adaptation is part of a broader suite of traits that allow these fish to occupy diverse niches, from the seafloor to the open water column, despite lacking the swim bladders common in other bony fish.

To achieve neutral buoyancy without a swim bladder, many Antarctic fish have evolved skeletons with reduced mineral content and increased lipid levels. This allows them to hover in the water column with minimal energy expenditure. While these adaptations are highly successful, they come with trade-offs. For instance, some species have lost the ability to express heat shock proteins typically used by organisms to cope with temperature spikes, leaving them uniquely vulnerable to rapid environmental changes.

The study of these fish provides a window into adaptive radiation, the process by which a single ancestor rapidly diversifies into many species. Scientists use genetics and paleontology to trace how these lineages evolved as Antarctic waters cooled over millions of years. Today, these fish remain a vital, if fragile, component of the Southern Ocean ecosystem, serving as a testament to the extremes of biological resilience.

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