Interesting Facts About the Antarctic Icefish

True to their name, the Antarctic Icefish lives in the icy cold waters of the Arctic —and has icy-looking blood to match. Their cold habitat has given them some interesting features. 

Most animals, like people, have red blood. The red of our blood is caused by hemoglobin, which carries oxygen throughout our body. Icefishes don't have hemoglobin, thus they have a whitish, nearly transparent blood. Their gills are also white. Despite this lack of hemoglobin, icefish can still get enough oxygen, although scientists aren't sure quite how — it could be because they live in already oxygen-rich waters and might be able to absorb oxygen through their skin, or because they have large hearts and plasma which may help transport oxygen more easily.

The first icefish was discovered in 1927 by zoologist Ditlef Rustad, who pulled up a strange, pale fish during an expedition to Antarctic waters. The fish he pulled up was eventually named the blackfin icefish (Chaenocephalus aceratus). 

Description

There are many species (33, according to WoRMS) of icefish in the Family Channichthyidae. These fish all have heads that look a little like a crocodile — so they are sometimes called crocodile icefishes. They have grayish, black or brown bodies, wide pectoral fins, and two dorsal fins that are supported by long, flexible spines. They can grow to a maximum length of about 30 inches. 

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Another fairly unique trait for icefish is that they don't have scales. This can aid in their ability to absorb oxygen through the ocean water. 

Classification

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Superclass: Gnathostomata
• Superclass: Pisces
• Class: Actinopterygii
• Order: Perciformes
• Family: Channichthyidae

Habitat, Distribution, and Feeding

Icefish inhabit Antarctic and subantarctic waters in the Southern Ocean off Antarctica and southern South America. Even though they can live in waters that are only 28 degrees, these fish have antifreeze proteins that circulate through their bodies to keep them from freezing. 

Icefish don't have swim bladders, so they spend much of their lives on the ocean bottom, although they also have a lighter skeleton than some other fish, which allows them to swim up into the water column at night to capture prey. They may be found in schools.

Icefish eat plankton, small fish, and krill. 

Conservation and Human Uses

The lighter skeleton of icefish has a low mineral density. Humans with a low mineral density in their bone have a condition called osteopenia, which may be a precursor to osteoporosis. Scientists study icefish to learn more about osteoporosis in humans. Icefish blood also provides insights into other conditions, such as anemia, and how bones develop. The ability of icefish to live in freezing water without freezing can also help scientists learn about the formation of ice crystals and storage of frozen foods and even organs used for transplant. 

Mackerel icefish are harvested, and the harvest is considered sustainable. A threat to icefish, however, is climate change — warming ocean temperatures could reduce the habitat that is suitable for this extreme cold water fish.