How aquatic creatures survive under a frozen surface

Biological antifreeze, behavioural adaptations and more Over the winter months, many ponds, lakes and rivers freeze over, leaving creatures trapped beneath the ice. Despite these harsh conditions, aquatic species have developed remarkable adaptations which enable them to survive under frozen surfaces.

Firstly, it’s important to note that aquatic creatures aren’t cryogenically frozen over the winter only to thaw out when the ice melts and keep going on as usual - we’re not dealing with a Captain America type of situation. When the top portion of bodies of water freezes over, the bottom half will almost always remain in a liquid form. This is because only surface water is exposed to cold air. Once a layer of ice has formed, although chilly, the water below is protected from the sub-zero temperatures above. This holds even for shallow pools of water such as puddles.

The trapped creatures hence do have an environment to roam in, but they still need to contend with the blistering cold. Many have adapted abilities to adjust their body temperature. Cold-blooded animals like fish and amphibians slow their metabolic rate when temperatures drop, allowing them to conserve energy and survive the colder waters. Some species, like the northern pike, even actively regulate and adjust their body-temperatures, keeping their core above freezing by generating heat through their muscles. The ‘just keep swimming’ method if you will… Many pond and river creatures also develop behavioural adaptations. Species of turtles, frogs, muskrats, clams and snails bury themselves in the mud and leaf litter, where temperatures remain above freezing. Some remain dormant for months until the ice melts in order to conserve energy. In larger bodies of water, species of fish will simply seek out deeper waters in winters where the temperature is more stable and the water less likely to freeze over. Perhaps the most fascinating adaptation developed by many living under frozen surfaces is the production of biological ‘antifreeze’. Certain insects, fish and frogs have evolved the ability to produce antifreeze proteins or excess glucose. They help organisms to prevent ice-crystals from forming and their bodily fluids from freezing solid. It enables them to continue moving below the ice, even when the water temperature drops well below freezing. Others manage by simply tolerating the freezing temperatures effect on their insides. The wood frog for example, has been shown to survive despite 70% of its internal fluids freezing – how cool is that? Pardon the pun.

Aside from the cold, the main problem for pond-dwellers is low oxygen levels. As the surface freezes over, the ice layer effectively seals off the water from the air above, stopping the oxygen exchange with the atmosphere, and reducing the amount available to aquatic organisms. The other key adaptations that many pond creatures have developed as a result, are abilities and strategies to tolerate minimal oxygen. Many species cope with this, through reducing their metabolic rate. This slows down their bodily functions during the winter months, helps them conserve energy and reduces their oxygen requirements. Akin to hibernation, snails and turtles are able to enter a state known as ‘aestivation’ which allows them to survive with an extremely reduced metabolic rate in a dormant-like state. ‘Air-breathers’ are able to extract oxygen from the air and store them in specialised lungs or swim bladders which can be used as reserves in periods of low-oxygen levels. Some species of fish, such as catfish can also switch to ‘anaerobic’ respiration, which does not require oxygen - allowing them to survive for a short time without it.

The abilities of pond and river creatures to survive under ice is a testament to the remarkable adaptability and resilience of nature. By developing a range of specific physiological and behavioural adaptations, organisms can survive even in the most extreme environments.