Антарктида мұз балығының ерекшеліктері

Family of fishes

The crocodile icefish or white blooded fish comprise a family (Channichthyidae) of notothenioid fish found in the Southern Ocean around Antarctica. They are the only known vertebrates to lack hemoglobin in their blood as adults. Water temperatures in these regions remain relatively stable, generally ranging from One icefish, Champsocephalus esox, is distributed north of the Antarctic Polar Frontal Zone. In February 2021, scientists discovered and documented a breeding colony of Neopagetopsis ionah icefish estimated to have 60 million active nests across an area of approximately 92 square miles at the bottom of the Weddell Sea in Antarctica. The majority of nests were occupied by one adult fish guarding an approximated estimate of 1,735 eggs in each nest.

All icefish are believed to be piscivorous, but can also feed on krill. Icefish are typically ambush predators; thus, they can survive long periods between feeding, and often consume fish up to 50% of their own body length. Maximum body lengths of have been recorded in these species.

Icefish blood is colorless because it lacks hemoglobin, the oxygen binding protein in blood. Channichthyidae are the only known vertebrates to lack hemoglobin as adults. Although they do not manufacture hemoglobin, remnants of hemoglobin genes can be found in their genome. The hemoglobin protein is made of two subunits (alpha and beta). In 15 of the 16 icefish species, the beta subunit gene has been completely deleted and the alpha subunit gene has been partially deleted. One icefish species, Neopagetopsis ionah, has a more complete, but still nonfunctional, hemoglobin gene. Red blood cells (RBCs) are usually absent, and if present, are rare and defunct. Oxygen is dissolved in the plasma and transported throughout the body without the hemoglobin protein. The fish can live without hemoglobin via low metabolic rates and the high solubility of oxygen in water at the low temperatures of their environment (the solubility of a gas tends to increase as temperature decreases). Myoglobin, the oxygen binding protein used in muscles, is absent from all icefish skeletal muscles. In 10 species, myoglobin is found in the heart muscle, specifically ventricles. Loss of myoglobin gene expression in icefish heart ventricles has occurred at least four separate times. To compensate for the absence of hemoglobin, icefish have larger blood vessels (including capillaries), greater blood volumes (four fold those of other fish), larger hearts, and greater cardiac outputs (five fold greater) compared to other fish. Their hearts, large blood vessels and low viscosity (RBC free) blood are specialized to carry out very high flow rates at low pressures. This helps to reduce the problems caused by the lack of hemoglobin. In the past, their scaleless skin had been widely thought to help absorb oxygen. However, current analysis has shown that the amount of oxygen absorbed by the skin is much less than that absorbed through the gills. This adaptation facilitates enhanced oxygen delivery by increasing mitochondrial surface area, and reducing distance between the extracellular area and the mitochondria.

The icefish are considered a monophyletic group and likely descended from a sluggish demersal ancestor. The ACC moves in a clockwise northeast direction, and can be up to wide. This current formed 25 22 million years ago, and thermally isolated the Southern Ocean by separating it from the warm subtropical gyres to the north. During the mid Tertiary period, a species crash in the Southern Ocean opened up wide range of empty niches to colonize. Despite the hemoglobin less mutants being less fit, the lack of competition allowed even the mutants to leave descendants that colonized empty habitats and evolved compensations for their mutations. Later, the periodic openings of fjords created habitats that were colonized by a few individuals. These conditions may have also allowed for the loss of myoglobin. Iron is a limiting nutrient in the environments inhabited by the icefish. By no longer synthesizing hemoglobin, they claim that icefish are minimizing endogenous iron use. To demonstrate this, they obtained retinal samples of Champsocephalus gunnari and stained them to detect hemoglobin alpha 3'f. They found expression of hemoglobin alpha 3'f within the retinal vasculature of Champsocephalus gunnari, demonstrating for the first time that there is limited transcription and translation of a hemoglobin gene fragment within an icefish. Because this fragment of hemoglobin does not contain any iron binding sites, the finding suggests that hemoglobin was selected against to conserve iron.

Phylogenetic relationships indicate that the nonexpression of myoglobin in cardiac tissue has evolved at least four discrete times. These cold temperatures, which allow for higher water oxygen content, combined with a high degree of vertical mixing in these waters, means oxygen availability in Antarctic waters is unusually high. The loss of hemoglobin and myoglobin would have negative consequences in warmer environments. some researchers have also suggested that the loss of hemoglobin might be tied to a necessary adaptation for the icefish. Through hemoglobin loss, icefish may minimize their iron requirements. This minimization could have aided the icefish survival 8.5 million years ago when Arctic diversity plummeted dramatically. This means that when icefish lost hemoglobin and myoglobin, it did not just mean a decreased ability to transport oxygen, but it also meant that total nitric oxide levels were elevated. The presence of nitric oxide also can increase angiogenesis, mitochondrial biogenesis, and cause muscle hypertrophy; all of these traits are characteristics of icefish. The similarity between nitric oxide mediated trait expression and the unusual cardiovascular traits of icefish suggests that while these abnormal traits have evolved over time, much of these traits were simply an immediate physiological response to heightened levels of nitric oxide, which may in turn have led to a process of homeostatic evolution. In addition, the heightened levels of nitric oxide that followed as an inevitable consequence of the loss of hemoglobin and myoglobin may have actually provided an automatic compensation, allowing for the fish to make up for the hit to their oxygen transport system and thereby providing a grace period of the fixation of these less than desirable traits.