Baby it’s cold outside: Freezing in marine animals

Written by Rebecca Greatorex

Freezing of animals is probably a concept you haven’t given much thought to. As humans, we can put on a hat and coat and survive out in the snow for hours on end. This is because we are endothermic, which means we produce our own heat. Some arctic animals like whales and seabirds are also endothermic and have insulation to help them retain their heat. But most organisms on Earth don’t have this luxury and temperatures below freezing represent a global ecological challenge.

In the ocean, pressure (depth) and salinity (saltiness) both influence the freezing temperature of water. In the right conditions, freshwater from lakes, bottled water and rain freezes at 0oC. Due to the greater salinity of seawater, it freezes at the surface at -2oC. The depths of the ocean never freeze as the temperature doesn’t often drop below 0oC, but theoretically the bottom of the ocean (on average, approximately 3686 metres deep) has a freezing temperature of -4.7oC. This is due to the greater pressures at this depth.

A seal swimming near Rothera at the British Antarctic Survey base. As mammals, seals are capable of maintaining their body temperature which reduces the risk of freezing

Ectothermic marine animals — which can’t regulate their own body temperatures — that find themselves in temperatures outside those which they are adapted for suffer physiological injury. The internal fluid or blood in most marine animals is very similar to seawater and during freezing maintaining cell structure proves difficult. Water expands during freezing and sharp ice crystals cause damage to cells, often leading to death. When freezing occurs, the concentration of salts and molecules in the internal fluid of the organism also become highly concentrated, this process is similar to extreme dehydration (as the water is no longer accessible to use by the animal as ice). Incidentally, organisms in tropical environments which are adapted to dehydration and hypersaline (very salty) environments are surprisingly well adapted to freezing, despite never being exposed to sub-zero temperatures in their recent evolutionary history.

Habitats in polar regions can go through freeze/thaw cycles practically every day. Even with a relatively low mortality of approximately 5% after every cycle, the population would be reduced to less than 10 % after 50 days. So how have animals adapted to survive these extreme conditions?

Adaptations to freezing

Fish living near ice have proteins in their blood which essentially act as anti-freeze. The accepted mechanism for how these proteins work is adsorption-inhibition.  Adsorption refers to the proteins attaching onto new, very small ice crystals in the body immediately after they form. They then inhibit further growth of the crystal. There are multiple different proteins which have this role in many different fish species in the Arctic and the Antarctic suggesting that these anti-freeze molecules have evolved more than once in multiple different species.

An icefish from the Antarctic. Well-known for its anti-freezing abilities

Arthropods — invertebrates with segmented body parts and exoskeleton, such as crabs and lobsters — which are intolerant to freezing, can make antifreeze proteins like fish too. They can physiologically prepare for freezing temperatures by synthesising ‘cryoprotectants’. For example, molecules like glycerol, a simple compound, when produced in high enough concentrations in the internal fluid, provides significant protection against freezing. These anti-freeze molecules have been found in multiple arthropod species.

Other invertebrates have different ways of dealing with the cold. Some freeze-intolerant invertebrates can move to a more stable climate, for example, they can burrow into the ground. However, this isn’t an option for all species because of various feeding requirements and morphological constraints.

Unexpected freezing

Over an evolutionary timescale of millions of years, Arctic and Antarctic sea creatures have adapted to the freezing temperatures in which they spend their entire life cycle. But freakishly cold weather in other parts of the world exposes animals with no such adaptations to uncommonly low temperatures. The Guardian, three years ago, published shocking images of the effects of cold weather on invertebrates in the North East of the UK. Cold waters reduced activity levels and mobility in the invertebrates and particularly violent weather washed the animals ashore, leading to mass deaths. Unexpected weather events like this cause population-level damage and are likely to increase in frequency due to climate change producing unpredictable weather in many places around the world.

Washed up crabs. lobsters and starfish after a freak cold spell in the North East of England March 2018

This article is based off a book chapter, ‘Freezing’ from Principles of Thermal Ecology by Andrew Clarke. To find this book, search ISBN: 978-0199551675.

To read a similar post about animal’s adaptions to cold and coping with cold temperatures, see:

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