What is an Icefish?

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.

Icefish possess a unique circulatory adaptation; their blood lacks hemoglobin, the protein that typically gives blood its red color and carries oxygen. This adaptation allows their blood to flow more freely in frigid Antarctic waters, reducing the energy required for circulation. Instead, they absorb oxygen directly from the water through their skin, according to research published in polar biology journals.

The icefish has evolved several adaptations to thrive in sub-zero temperatures. Its antifreeze proteins prevent the formation of ice crystals in its blood and tissues, ensuring survival in waters that can reach -2°C. Additionally, a large heart and increased blood volume facilitate oxygen distribution throughout the body, compensating for the lack of hemoglobin, as noted by marine biologists.

Icefish are predatory creatures, feeding primarily on krill, small fish, and plankton. They have developed keen senses to locate prey in the dark, murky waters of the Antarctic. Their large eyes are particularly adapted to the low-light conditions, enabling them to spot bioluminescent prey, as observed in ecological studies of Antarctic marine life.

While not currently classified as endangered, icefish face potential threats from climate change and overfishing. Warming ocean temperatures can disrupt their habitat and food sources. Additionally, commercial fishing practices may indirectly affect their population by targeting krill, a primary food source for icefish, as reported by conservation organizations monitoring Antarctic ecosystems.

Icefish play a crucial role in the Antarctic food web. They serve as both predators and prey, maintaining the balance of marine life. As predators, they control the populations of krill and smaller fish, while as prey, they provide sustenance for seals, penguins, and larger fish, thus supporting biodiversity, as detailed in Antarctic ecosystem studies.

Due to their specialized habitat requirements, icefish are rarely kept in aquariums outside of research facilities. Scientists study them in controlled environments to understand their unique physiological traits and their responses to changing temperatures, which can offer insights into the effects of global warming on marine species, as indicated by polar research institutes.