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What is an Icefish?

By Jason C. Chavis
Updated Jun 04, 2024
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An icefish is an animal that exists around the continental shelf of Antarctica in the Southern Ocean. Scientifically, they are referred to as Notothenioidei. Icefish are found in no other location of the planet, making them endemic. In the entire region, these types of fish are the most prominent, representing 95 percent of all marine life.

Icefish are perciformes, a type of vertebrate that accounts for 40 percent of all fish. There are eight distinct families with 43 different genus types. In all, the icefish accounts for 122 species, all found in the Southern Ocean. Although new ones are discovered all the time.

One of the unique components of Notothenioidei is that they have proteins flowing through their bloodstream that allow them to not freeze in the frigid antarctic waters. Other fish can only survive in temperate and tropical climates and would die in the Southern Ocean. When the water temperature drops below the freezing point of their blood, the fish are killed. Icefish have what scientists refer to as “antifreeze proteins.” These glycoproteins bind to the ice crystals that form in their bloodstream, preventing the animal from dying.

Although most of the animals live in the temperature zone of 28.4°F (about -2°C) and 39.2°F (about 4°C), some species within the icefish family can survive in hotter climates. Examples of Notothenioidei have been located near South America and New Zealand where the water temperature reaches 50°F (about 10°C). Salt water fails to freeze until it gets 28.4°F (about -2°C), which means that the animal dies below these temperatures despite its “antifreeze proteins.”

Due to the fact that icefish have no swimbladder, an organ that fills with gas and allows a fish to rise and fall in the ocean, they are relegated to the world of bottom-dwelling fish. This means they are known as benthic fish. Some of the fish have adapted through evolution, however, by developing larger deposits of lipids, types of fats. In addition, Notothenioidei have also lost some of their bone structures and thinned the density of these bones. This helps reduce the weight of the fish, prompting them to have a better ability to control their buoyancy.

Icefish are also unique in their use of oxygen within their blood. Most fish and other animals require approximately 45 percent of hemoglobin in their blood. This is the material that binds with oxygen and provides energy for the animal. Due to the cold waters tendency to contain high oxygen levels, many Notothenioidei have as little as one percent hemoglobin. Instead, the oxygen is spread through the bloodstream directly by plasma.

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