Channichthyidae

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 average of 1,735 eggs in each nest.

Genera

The following genera have been classified within the family Channichthyidae:

Chaenocephalus Richardson, 1844
Chaenodraco Regan, 1914
Champsocephalus Gill, 1861
Channichthys Richardson, 1844
Chionobathyscus Andriashev & Neyelov, 1978
Chionodraco Lönnberg, 1905
Cryodraco Dollo, 1900
Dacodraco Waite, 1916
Neopagetopsis , 1947
Pagetopsis Regan, 1913
Pseudochaenichthys Norman, 1937

Diet and body size

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 at one time. Maximum body lengths of have been recorded in these species.

Respiratory and circulatory system

thumb|upright|[[Champsocephalus gunnari on a 1978 Soviet postage stamp]]

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 hemoglobin beta subunit, but a still nonfunctional, hemoglobin gene.

Red blood cells (RBCs) are usually absent, and, if present, are rare and defunct. Without a hemoglobin protein, oxygen is transported throughout the body by using oxygen dissolved in their plasma. The fish can live without hemoglobin due to low metabolic rates and an environmental condition of high oxygen solubility in the low temperature waters in which they live (the solubility of a gas tends to increase as temperature decreases).

Myoglobin, the oxygen-binding protein used in muscles, is also absent from all icefish skeletal muscles. However, in 10 species, myoglobin is found in 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 adapted

larger blood vessels (including capillaries) and low-viscosity (RBC-free) blood to enable very high flow rates at low pressures.
greater blood volumes (four times those of other fish).
larger hearts with greater cardiac outputs (five times greater) compared to other fish.
larger cardiac mitochondria and increased mitochondrial biogenesis to facilitate enhanced oxygen delivery by increasing mitochondrial surface area, and reducing distance between the extracellular area and the mitochondria in comparison to red-blooded notothenioids.
antifreeze glycoproteins (AFGPs) that prevent intracellular ice formation.

In the past, their scaleless skin had been widely thought to help absorb oxygen. However, current analysis shows that the amount of oxygen absorbed by the skin is much less than that absorbed through the gills. 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 further allowed for the loss of myoglobin. 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, a hemoglobin gene fragment does not contain any iron binding sites.

Loss of myoglobin

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 due to limited iron availability in ocean environments. Through hemoglobin loss, icefish may minimize their iron requirements, which could have helped the icefish survive 8.5 million years ago when Antarctic 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 traits 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.