Ice-age seas: how marine animals adapted to cold

During glacial intervals, expanding ice sheets lowered global sea level and altered coastlines, currents and habitats. In high latitudes, seasonal sea ice shaped where light entered the water and when food became available. But a “glacial sea” was not a single frozen environment. Conditions differed between the Arctic and Antarctic, between shallow shelves and deep basins, and across seasons.

Life beneath and within sea ice

Sea ice contains brine channels and surfaces where microscopic algae and other organisms can live. When light returns in spring, ice-associated algae and open-water phytoplankton contribute to short but important pulses of production. Zooplankton graze on this growth, and small fish and larger predators feed through the food web. The timing and amount of production vary with snow cover, ice thickness, nutrients and local circulation.

Ice is also physical habitat. It can provide resting or breeding platforms for some seals and a hunting surface for polar bears. Other marine animals use open water, leads and polynyas. The same ice edge can therefore be a barrier for one species and an essential habitat for another. Arctic and Antarctic food webs differ in their species and history, so an adaptation in one pole should not be assumed for the other.

Keeping body fluids from freezing

Seawater can remain liquid below the freezing point of freshwater. Fish living in polar waters face a risk that ice crystals in the environment could trigger freezing in body fluids. Several polar and subpolar fish have antifreeze proteins or glycoproteins that bind to ice crystals and inhibit their growth. These molecules lower the temperature at which ice grows in tissues without acting as a simple chemical “antifreeze tank.”

Antifreeze systems evolved independently in different fish lineages. Arctic cod and Antarctic notothenioid fish, for example, use related but distinct solutions. These proteins address one physiological problem; they do not explain every feature that lets a fish live in cold water. Membrane function, circulation, metabolism and behaviour also matter, and not all polar species use the same mechanism.

Insulation, fat and behaviour

Marine mammals reduce heat loss with blubber, dense fur or a combination, depending on the species. Seals use sea ice in different ways for resting, moulting or giving birth, while whales and other marine mammals move through open water and ice leads. Thick fat also stores energy, which can be important during periods when food is patchy. These adaptations do not make animals independent of habitat or food supply.

Seasonal movement can reduce exposure to harsh conditions or follow prey, but migration routes are species-specific. Some animals remain in Arctic waters year-round; others move between feeding and breeding areas. The timing of ice formation and breakup can affect access to habitat and food. A change in ice can alter an entire chain, from algal production to prey fish and predators, without producing the same effect everywhere.

What fossils reveal about glacial seas

Marine mammal fossils preserve pieces of this history. Pleistocene deposits in parts of North America and northern Europe contain remains of seals, whales and walruses, sometimes in former inland seas created as ice sheets advanced or retreated. Fossils can establish that a species occurred in a region at a particular time and can help reconstruct past connections between ocean basins.

Fossil evidence is uneven. Soft tissues rarely survive, and bones may be transported or reworked from older deposits. A fossil species’ presence does not by itself reveal its exact diet, migration route or relationship to sea ice. Researchers combine anatomy, sediment, isotopes, dating and comparison with living relatives. Even then, a modern ecological analogy is a hypothesis to test, not a complete picture of ancient behaviour.

Cold was only one challenge

Polar marine life also faced darkness, seasonal light, changing salinity, food shortages and shifting ice. Some adaptations reduced freezing risk; others conserved heat, stored energy or timed reproduction to brief periods of food abundance. These traits arose in different lineages and need not have evolved together. A single list of “cold adaptations” can hide the ecological trade-offs each animal faced.

Glacial seas were dynamic environments where ice, currents and food supply changed across space and time. Their fossils preserve evidence of animals and shifting distributions, while living polar systems help researchers test how energy moves through ice-associated food webs. Together, those records show both how organisms coped with cold and why no one adaptation tells the whole story. The same caution applies when reconstructing ancient climate from fossils, as discussed in the evidence for life on the mammoth steppe.