Seawater around the poles is colder than the freezing point of a typical fish’s blood, yet whole communities of fish live there year-round. The ones that thrive are not just tough. Their bodies carry built-in antifreeze and cold-tuned chemistry that warm-water fish lack. Two unrelated groups at opposite ends of the planet even ended up inventing nearly the same antifreeze on their own.1PubMed. Convergent evolution of antifreeze glycoproteins in Antarctic notothenioid fish and Arctic cod
The Fish That Live in the Coldest Water
Around Antarctica, the dominant fish are notothenioids, a family that includes Antarctic cods and icefish.2PubMed Central. Genomics of cold adaptations in the Antarctic notothenioid fish radiation They all descend from one ancestor that lived about 22 million years ago.3PubMed. Adaptations and Diversity of Antarctic Fishes: A Genomic Perspective That single ancestor branched into roughly 120 to 140 species today,4PubMed. Adaptations and Diversity of Antarctic Fishes: A Genomic Perspective and together they make up about 90% of the fish by weight on Antarctica’s continental shelf.5PubMed. Adaptations and Diversity of Antarctic Fishes: A Genomic Perspective Millions of years isolated in steady cold have left them able to tolerate only a very narrow temperature range.6PubMed Central. Molecular ecophysiology of Antarctic notothenioid fishes
The water they live in is extreme. Salt lets seawater stay liquid a little below 0°C, down to around -1.9°C near Antarctica, colder than the point where an ordinary fish’s less salty blood would freeze.
The Arctic has its own cold specialists. Polar cod are found throughout the central Arctic Ocean, living just under the sea ice.7Polar Biology. Under-ice distribution of polar cod Boreogadus saida in the central Arctic Ocean and their association with sea-ice habitat properties Young polar cod gather most thickly where ice is thicker and more widespread.8Polar Biology. Under-ice distribution of polar cod Boreogadus saida in the central Arctic Ocean and their association with sea-ice habitat properties They are also abundant where the tiny ice-dwelling crustaceans they feed on are plentiful.9Polar Biology. Under-ice distribution of polar cod Boreogadus saida in the central Arctic Ocean and their association with sea-ice habitat properties In turn, polar cod are a key food for seals, whales and seabirds. The Greenland shark, which can grow over 5 metres long, is an iconic Arctic species.10PubMed. Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus)
The deep sea is cold too. Snailfish are the most common backboned animals in the deepest trenches, living down to about 8,100 metres.11eLife. Deep-Sea Adaptation: Surviving under pressure Other snailfish live in coastal waters, which gives the group the widest depth range of any sea fish.12eLife. Deep-Sea Adaptation: Surviving under pressure One trench snailfish has see-through skin, thin soft bones and an open skull, adapted to crushing pressure and cold.13PubMed. Morphology and genome of a snailfish from the Mariana Trench provide insights into deep-sea adaptation Deep-sea snailfish live in cold that is mostly stable and ice-free, so their main challenges are pressure and scarce food, not ice.
How Antifreeze Proteins Keep Fish From Freezing
Polar sea fish make antifreeze proteins, molecules that stick to tiny ice crystals that get into their bodies.14PubMed Central. Antifreeze protein-induced superheating of ice inside Antarctic notothenioid fishes inhibits melting during summer warming The crystals cannot grow, which prevents the fish from freezing from the inside out.15PubMed Central. Antifreeze protein-induced superheating of ice inside Antarctic notothenioid fishes inhibits melting during summer warming Some of these proteins work by shaping the crystal’s sides so its tips stay small rather than coating the whole crystal.16PubMed. When are antifreeze proteins in solution essential for ice growth inhibition? The protection has a side effect: ice inside Antarctic fish also resists melting when summer water warms.17PubMed Central. Antifreeze protein-induced superheating of ice inside Antarctic notothenioid fishes inhibits melting during summer warming
Fish swallow seawater and food carrying small ice crystals, and ice can touch skin and gills in icy surface water. Without protection, one crystal can seed freezing through the body fluids, because ice grows by adding water molecules to a seed crystal. The proteins fasten onto the crystal’s surface and leave little room for more water molecules to join. The blood then stays liquid below its normal freezing point, though not without limit. Since the proteins act on crystals already present, they stop growth rather than preventing ice from ever appearing. Unlike car antifreeze, which dissolves in large amounts, fish proteins work in tiny amounts by blocking crystal growth directly.
Antarctic fish gained their antifreeze between 42 and 22 million years ago, a span that included major global cooling about 35 million years ago.18PubMed Central. Ancient climate change, antifreeze, and the evolutionary diversification of Antarctic fishes Its arrival is what let notothenioids survive in sub-zero water.19PubMed Central. Genomics of cold adaptations in the Antarctic notothenioid fish radiation In Antarctic fish, the antifreeze gene grew out of a digestive enzyme gene from the pancreas.20PubMed. Convergent evolution of antifreeze glycoproteins in Antarctic notothenioid fish and Arctic cod Over time, the stretch of DNA holding the antifreeze genes expanded into many copies.21PubMed Central. Genomics of cold adaptations in the Antarctic notothenioid fish radiation
Antarctic notothenioids and northern cods are distant relatives, yet they make nearly identical antifreeze.22PubMed. Convergent evolution of antifreeze glycoproteins in Antarctic notothenioid fish and Arctic cod Their antifreeze genes are built differently and came from different starting DNA.23PubMed. Convergent evolution of antifreeze glycoproteins in Antarctic notothenioid fish and Arctic cod
Keeping Cells, Nerves and Muscles Working in the Cold
A fish’s body temperature follows the water, so a fish in near-freezing water has a near-freezing body, unlike a warm-blooded animal. Chemical reactions slow sharply as temperature falls, so muscles, nerves and digestion would all run sluggishly without adaptations. Cell membranes behave like fat that stiffens, as butter does in a fridge, and a stiff membrane cannot let nutrients and signals pass properly.
Antarctic fish pack their cell membranes with fats that stay liquid in the cold, similar to the oils in fish oil.24PubMed Central. Effect of elevated temperature on membrane lipid saturation in Antarctic notothenioid fish Enzymes, the proteins that run chemical reactions in the body, also tend to stiffen and slow in the cold. Yet a muscle enzyme in Antarctic fish works three to four times faster at 0°C than the same enzyme in a dogfish.25PubMed Central. Principles of Cold Adaptation of Fish Lactate Dehydrogenases Revealed by Computer Simulations of the Catalytic Reaction Its surface is softer and more flexible, so it changes shape with less effort.26PubMed Central. Principles of Cold Adaptation of Fish Lactate Dehydrogenases Revealed by Computer Simulations of the Catalytic Reaction The flexibility is concentrated in small parts of the enzyme, not the active core.27PubMed. Hot spots in cold adaptation: localized increases in conformational flexibility in lactate dehydrogenase A4 orthologs of Antarctic notothenioid fishes That looseness makes the enzyme grip its targets less tightly.28PubMed. Hot spots in cold adaptation: localized increases in conformational flexibility in lactate dehydrogenase A4 orthologs of Antarctic notothenioid fishes Flexibility that helps in the cold makes proteins fragile when warm.
Not everything is cold-adapted, though. Unlike cold-loving bacteria, Antarctic fish have not rebuilt most of their proteins for the cold.29PubMed Central. Adaptation of Proteins to the Cold in Antarctic Fish: A Role for Methionine? In Arctic fish, nerve signals travel about half as fast as in temperate fish at their usual temperatures.30Italian Journal of Zoology. Conduction of nervous impulse at low temperature in Polar fishes
Icefish, the Fish With Clear Blood
Icefish are the only backboned animals without working hemoglobin, the red protein that carries oxygen in blood.31PubMed Central. Genomics of cold adaptations in the Antarctic notothenioid fish radiation Hemoglobin normally picks up oxygen at the gills and carries it around the body. It lets blood hold far more oxygen than plain water-based blood could, which is why nearly all backboned animals have it. Without it, icefish blood is nearly clear and their gills are pale.
Some icefish species have also lost the oxygen-holding protein in their heart muscle.32PubMed. The Antarctic hemoglobinless icefish, fifty five years later: a unique cardiocirculatory interplay of disaptation and phenotypic plasticity They make up for it with more blood, a very large heart, wide capillaries and lots of blood vessels.33PubMed. The Antarctic hemoglobinless icefish, fifty five years later: a unique cardiocirculatory interplay of disaptation and phenotypic plasticity Oxygen simply dissolves in the liquid part of the blood, and cold water allows more oxygen to dissolve, so it goes further. Because the blood carries less oxygen per unit, the heart must move a much larger volume. Without red cells the blood is thinner, which makes it easier to pump through those wide vessels. The result works, but at a high price: pumping all that blood costs icefish about 22% of their resting energy, versus about 3% in related fish.34PubMed Central. Vascular Expression of Hemoglobin Alpha in Antarctic Icefish Supports Iron Limitation as Novel Evolutionary Driver
That is also why this design does not work for most fish. Warmer water holds less oxygen, and a fish without hemoglobin would suffocate outside very cold, oxygen-rich water.
How Freshwater Fish Survive a Frozen Lake or Stream
Fish in local lakes and rivers don’t need antifreeze. Fresh water freezes at 0°C, above the freezing point of fish blood, so liquid water under ice is never cold enough to freeze them. Polar sea fish face super-chilled salty water, a harder problem.
In winter, trout slow their metabolism, which lets them go longer between meals.35Ausable River Association. What Fish Live in Cold Water and How Do They Survive? Trout and other stream fish also move to deep pools and slow currents with stable ice above.36Ausable River Association. What Fish Live in Cold Water and How Do They Survive? Deep water stays liquid because water is densest at about 4°C. Water near 4°C sinks to the bottom and colder water floats to the top, so ice forms at the surface while the depths stay a few degrees above freezing. The ice also acts as a lid that insulates the water beneath, slowing further freezing. Deep lakes therefore hold a refuge fish can use all winter.
Shallow ponds that freeze nearly solid can kill fish. Even then, winter die-offs often come from low oxygen under snowy ice rather than from cold itself.
Living Slow and Living Long in the Cold
Scientists long believed polar fish burned energy faster at rest to make up for the cold.37PubMed. Metabolic cold adaptation of polar fish based on measurements of aerobic oxygen consumption: fact or artefact? Artefact! The idea was that a fish had to run its chemistry faster to keep up, and early measurements seemed to show higher oxygen use than temperature alone would predict. But better measurements show Arctic and Antarctic fish at rest use no more oxygen than expected for their temperature.38PubMed. Metabolic cold adaptation of polar fish based on measurements of aerobic oxygen consumption: fact or artefact? Artefact! Arctic cod species likewise show no sign of a sped-up metabolism.39Journal of Fish Biology. Oxygen consumption of East Siberian cod: no support for the metabolic cold adaptation theory Polar fish cope with cold chiefly through tuned chemistry, not by running hot.
Greenland sharks grow only about 0.5 to 1 cm a year.40Wikipedia. Greenland shark They live at least 272 years, making them the longest-lived backboned animal known.41PubMed. Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus) Females are not ready to breed until about 150 years old.42PubMed. Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus) Losses from fishing therefore take generations to recover.
Their age was worked out from the eye lens, whose core forms before birth and never changes. Carbon from 1960s nuclear tests acts as a time stamp. Only the smallest sharks carried that stamp, showing the big ones were far older.
Why Warming Water Threatens Polar Fish
Antarctic fish have heart and circulation systems tuned to a very narrow temperature band.43Cambridge University Press eBooks. Fish cardio-circulatory function in the cold In an Antarctic eelpout, muscle protein building did not speed up with short-term warming.44PubMed Central. Evolutionary Adaptation of Protein Turnover in White Muscle of Stenothermal Antarctic Fish: Elevated Cold Compensation at Reduced Thermal Responsiveness Building protein is a fish’s biggest energy cost, so limits near the warm end can cut muscle growth.45PubMed Central. Evolutionary Adaptation of Protein Turnover in White Muscle of Stenothermal Antarctic Fish: Elevated Cold Compensation at Reduced Thermal Responsiveness Fish adapted to steady cold have little room to adjust as seas warm. Nor can Antarctic fish simply move: the Southern Ocean is isolated by currents, and there is no colder water further south.
In the Arctic, polar cod depend on sea ice, which is shrinking. Fewer polar cod means less food for seals and seabirds, and fish adapted to steady cold may be pushed out by warm-water species.
Fish Antifreeze in Ice Cream and Medicine
Since 2009, an ice cream maker has used an antifreeze protein first found in a North Atlantic eelpout to keep low-fat ice cream smooth.46Horizon Magazine. The antifreeze fish teaching us about cryopreservation The protein shapes ice into fine needles instead of large crunchy crystals.47Horizon Magazine. The antifreeze fish teaching us about cryopreservation It is now manufactured rather than taken from fish.48Horizon Magazine. The antifreeze fish teaching us about cryopreservation
Freezing damages food and cells because water expands into ice crystals that can grow large and sharp, piercing cell walls. When a strawberry thaws, the damaged cells leak and the fruit turns mushy. Ice cream turns gritty when its small crystals regrow into larger ones during storage and thawing. Blocking crystal growth could therefore help store organs, eggs and blood.
Researchers are testing these proteins for preserving food and living cells.49Horizon Magazine. The antifreeze fish teaching us about cryopreservation In rat studies, one type of fish antifreeze protein boosted freezing treatments for prostate and liver tumors.50PubMed Central. Antifreeze Proteins and Their Practical Utilization in Industry, Medicine, and Agriculture Most medical uses are still in lab or animal testing, and ice cream remains the main everyday product.