Antarctic fishes

Though many different species comprise the Antarctic icefish cluster, there are some common characteristics between fish. They generally have a set of rounded pectoral fins and rounded pelvic fins that maximize mobility in both the water column and on the seafloor.[2][3][4] Their eyes are of medium size and are set towards the top of the head, indicating that they catch prey by moving into the water column from the seafloor.[2][3][4] The mouth is large in comparison to size of the body.[2][3][4] The spiny dorsal fin is placed halfway down the body and is detached from the soft dorsal fin.[2][3][4] The soft dorsal fin extends down the body and ends shortly before the caudal fin.[2][3][4] The anal fin mirrors the soft dorsal fin down the underside of the body before the caudal fin. The shape of the caudal fin varies based on family, but is generally either rounded, forked or truncate.[2][3][4][5][6] Only Artedidraconids have chin barbels hanging from the lower jaw that drags through the sand and a hook shaped operculum.[2] Otherwise, the operculum is rounded.[3][4][5][6]

Coloring ranges from light gray to dark gray with large spotting. Some species are tan or green or even red.

Channichthyids are the largest of these fish at a maximum of 75 centimetres (30 in),[4] with Harpagifer the smallest at 10 centimetres (3.9 in).[5]

Notothenioid fish dominate the Southern Ocean diversity and biomass largely because of the pelagization by some species.[7] Most fish are benthic and consequently, spend their lives on the seafloor. Notothenioids are found in many different niches like semipelagic, cryopelagic, pelagic, and benthic zones.[8] Species have been able to colonize the water column despite not having swim bladders like other bony fishes. Evolution that decreased the amount of minerals present in the skeleton and increased the number of lipids in the body made this possible.[7] As a result, fish can remain neutrally buoyant and decrease the energy requirement for remaining pelagic. These adaptations are most often observed in Nototheniids, the most diverse of the families.[6] Because of their movement into the water column, fish are observed to feed on both the seafloor and in the water column.[8]

Antarctic notothenioids are able to survive the freezing temperatures of −1.86 °C (28.65 °F).[9] with the use of antifreeze glycoproteins. Antifreeze glycoproteins bind to ice that enters the body through ingestion of food, water and from their environment to prevent the organism freezing internally.[10] AFGPs evolved from pancreatic trypsinogen gene for survival as Antarctic waters began to cool.[9] They consist of repeating amino acids alanine-alanine-threonine to effectively bind to ice molecules and render them as non-threatening to survival.[10] AFGPs are created in the pancreas and are released into the digestive tract to wrap around ice crystals so they can be safely excreted with excrement.[9][10] Unused AFGPs are recycled by entering the bloodstream and cycling back to the liver for storage.[10] Fish also excrete AFGPs in their mucus and on the surface of their skin to prevent external freezing.[10]

AFGPs do not eliminate all internal ice crystals and can instead fill fish with inactivated ice crystals.[11] This can pose as a danger to the organism as well. Research has shown that summer warming of waters does eliminate internal ice somewhat but it does not do so explicitly.[11]

Heat shock proteins (HSPs) are expressed during exposure to high temperatures and is a characteristic held by most organisms. In some species of nototheniids, this trait is not expressed.[9] The trait is not expressed because of the extreme cold of the Southern Oceans leading to upregulation of Hsp70.[9] HSP expression indicates that regulation of Hsp70 occurred once during speciation, showing that it is a trait of most, if not all Antarctic notothenioids possess.[9]

Some Antarctic fish are able to resist perishing when exposed to temperatures 13–18 °C (55–64 °F) higher than their environment but little research has taken place to explain how they survive.[9]

Channichthyids' most well known feature is their lack of erythrocytes. Genetic evidence shows that crocodile icefish had erythrocytes but have back evolved to not use hemoglobin, or any protein binding for oxygen transport.[12] Instead, white-blooded fish have improved heart output, higher blood volume, higher uptake of oxygen and lower metabolic rates.[13] The discovery that icefish species Neopagetopsis ionah possesses a nearly intact, but useless set of globin genes demonstrates that multiple events led to the loss of hemoglobin expression in icefish.[13]

One possible explanation for why icefish were able to survive the mutational events that removed their erythrocytes is that because iron is a limiting element in the oceans, icefish found a way to thrive by not needing it for oxygen transport.[9] One other possible explanation is that colder waters increases the viscosity of body fluids to the point that it is beneficial to eliminate erythrocytes altogether, and instead rely on adaptations.[14] Neither theories have much further research to support their points.