How are fishes able to survive in sub-zero conditions?
In the deep, dark, cold waters of the Antarctic, beneath sheets of ice you would expect to find nothing but microbial life. When in fact, when you look underneath the masses of ice you can find fish that can withstand these freezing conditions. These fish are known as Antarctic ice fish and are found under the suborder Notothenioidei, of the order Perciformes. Waters in the Southern Ocean have temperatures between -2 and -4 ⁰C, so how exactly do the Antarctic ice fish manage to survive at these temperatures?
Researchers say that up until about 30 million years ago, the waters around the Antarctic were warmer than they are today. This meant that there was a greater diversity of fish species in the Antarctic than there are today and this can be seen through fossil evidence. Scientists say that there was only one species of Nototheniod, compared to the number of different species that are found today. Through the years, many Antarctic marine fish species that didn’t have the anti-freeze protein died off, while the fish that did have the anti-freeze protein continued to thrive. This gave them a great advantage because the Antarctic seas were mostly empty, and therefore this gave them the chance to find optimum habitats and have minimal competition for food. Over time, these optimum conditions allowed for the Notothenioids to grow massively in population size and this gave them the chance to create subpopulations in different habitats in the region. Each subpopulation adapted different features that allowed them to survive in their habitats and therefore becoming a new species. As a result of this, there are more than 90 different Notothenioid species in the Antarctic today.
Surviving in the cold
This group of fish managed to develop a protein anti-freeze in their blood which allowed them to survive whilst other fishes died out. They were able to gain an advantage and live with it. The anti-freeze protein is a substitute for the fish’s lack of haemoglobin and myoglobin, if you were to extract the blood from an ice fish you would find that their blood is almost translucent (Figure 1). Where most fish have red gills because of the presence of red blood cells, ice fishes present almost clear gills. Ice fishes are known to be somewhat of a phenomenon, nowhere else on earth would you be able to find a fish like this. The anti-freeze glycoprotein protects their blood from temperatures where their blood should normally freeze.
Near et al (2006), states that despite their loss in haemoglobin and myoglobin, the exceptionally cold and oxygen-saturated waters of the Southern Ocean are able to provide an environment the ice fish species are able to flourish without any sort of oxygen binding proteins. This is because they have larger hearts than most fish species and this allows for larger volumes of blood to be pumped around the body. As well as this, they have other adaptations:
- The capillary beds in their body are much denser and this allows for the tissues in their body to be properly oxygenated.
- Increased blood flow due to having a larger heart
- Due to their lack in red blood cells, they have low viscosity blood
- They lack scales, which allows them to absorb oxygen through their skin
- Evolution of the anti-freeze glycoprotein
These adaptations are advantageous to ice fish because oxygen dissolves better in colder waters than in warm waters. This is an advantage to them because, their habitat, the coastal waters of the Antarctic are well oxygenated.
Ice fish don’t have swim bladders so they spend most of their time on the benthos, however they do have a lighter skeleton than most fishes and this allows them to swim up into the water column to feed. Ice fish don’t have very strong jaws, so what they can eat is limited to plankton, small fish and krill.
What is the anti-freeze protein?
As humans, we are familiar with antifreeze. To us it is a fluid that is used in car’s motors to avoid it from freezing during the winter months. The anti-freeze in ice fish works in a similar way. The anti-freeze that we use is made from alcohol, however the antifreeze used by ice fish is biological and is made up of proteins and sugars. The anti-freeze is circulated through the blood of the fish and they keep the ice crystals small and in compartments away from the blood so that they have no negative impacts on the fish, essentially the anti-freeze keeps the blood as a liquid.
There are four classes of anti-freeze proteins (Figure 2) and one class of anti-freeze glycoprotein. Each of these can be found in different species of ice fish, but the most common one is the anti-freeze glycoprotein. This is because this anti-freeze is found in suborder Notothenioidei, and the Nototheniods make up most of the ice fish species.
The Antarctic ice fishes has gone through many evolutionary adaptations that make them the animals that they are today. Many fish would freeze if they were to find themselves at temperatures where ice fishes are found. Microscopic ice crystals would infiltrate their body, finding their way through the gills and skin, and would start growing inside the fish’s body. Nerves would soon be severed and tissues would be damaged, and soon enough the fish would die. The Antarctic ice fishes have been able to prevent this by attaining the anti-freeze protein and this allows them to thrive in such cold waters, where many others would die.