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These cold water fish live in the Southern ocean, where they are important to commercial fisheries. They have transparent blood, lacking red blood cells, which gives them their pale coloration.
They are closely related to the blackfin icefish.
They reach a maximum length of about 65cm, and a maximum weight of 2kg.
These are solid, bottom-dwelling fish with enlarged head and toothy mouths, which leads to their alternative name, crocodile icefish. They are unique among vertebrates in lacking the oxygen-binding protein haemoglobin, that gives blood its red colour. The blood is translucent, giving them a pale complexion and the gills are white.
Without haemoglobin, the blood has a low oxygen-carrying capacity. To compensate for this, they have particularly large hearts which beat twice as fast as other fishes'.
Mackerel icefish are found in the Southern Ocean and the southern Atlantic. Localities include the islands of the Scotia Sea, the northern Antarctic Peninsula, Kerguelen, Heard and Bouvet Islands, South Georgia, the South Orkney Islands and South Shetland.
They are pelagic (live in the open sea) and swim at depths of 0-700m.
They feed on fish and krill.
Very few fishes can cope with the icy conditions of the Antarctic. Icefish cope by having special 'antifreeze' in the blood that prevents ice crystals building up in the body. This allows them to live in conditions that are below the body freezing point. They have a very low metabolic rate and are very sluggish, spending much of the time resting on the bottom.
They live in well-oxygenated waters, and may absorb some oxygen directly through the skin.
Mackerel icefish are not listed as endangered.