Molecular Biology and Genetics
Statistics of barcoding coverage
|Specimen Records:||432||Public Records:||207|
|Specimens with Sequences:||454||Public Species:||15|
|Specimens with Barcodes:||312||Public BINs:||11|
|Species With Barcodes:||16|
The crocodile icefish or white-blooded fish (Channichthyidae) are a family of perciform fish found in the cold waters around Antarctica and southern South America. Water temperature can drop to -1.9 °C (the freezing point of seawater) in the Antarctic sea but stays rather constant. There are about 25 currently recognized species of crocodile icefish. They feed on krill, copepods, and other fish.
Icefish reach total length of 25-75 cm.
Respiratory and circulatory system
Their blood is yellow because it contains no hemoglobin. Red blood cells 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 because of their low metabolic rates and the high solubility of oxygen in water at the low temperatures of their environment. However, the oxygen carrying capacity of their blood is less than 10% that of their relatives with hemoglobin.
To compensate for the loss of hemoglobin, they have larger blood vessels (including capillaries), greater blood volumes (four times that of other fish), bigger hearts and greater cardiac outputs (fivefold greater) compared to other fish. Their hearts lack coronary arteries and the ventricle muscles are very spongy, enabling them to absorb oxygen directly from the blood they pump. 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 loss of hemoglobin. In the past, it has been widely supposed that their scaleless skin helps 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. The little extra oxygen absorbed by the skin may play a part in supplementing the oxygen supply to the heart which receives venous blood from the skin and body before pumping it to the gills.
Channichthyidae are the only known vertebrates without hemoglobin, an oxygen transport protein in the blood. 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). Almost all of the alpha and beta subunit genes have been lost from the genomes of 15 of the 16 icefish species. In only one of the icefish species, Neopagetopsis ionah, there is a more complete, but still nonfunctional hemoglobin gene.
Myoglobin is an oxygen transport protein used in muscles, but is also absent from all icefish skeletal muscles. In ten species, myoglobin is found in the heart muscle, specifically ventricles. Myoglobin has been lost[clarification needed] in icefish heart ventricles at least four separate times and by four different mechanisms.
The loss of hemoglobin was initially supposed to be an adaptation to the extreme cold; the higher solubility of O2 reduces the demand on hemoglobin and the lack of RBCs decreases the blood viscosity. However, current analysis has shown that the lack of hemoglobin—though not lethal—is still maladaptive. The fish have evolved fairly drastic changes to their physiology to compensate. These compensations include spending twice as much energy pumping blood compared to other fish.
These fish have descended from a sluggish demersal ancestor. The cold, well mixed, oxygen rich waters of the Antarctic ocean provided an environment where a fish with a low metabolic rate could survive even without hemoglobin—albeit less efficiently. 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. This gave the opportunity for four lines of fish to even lose their myoglobin genes by a similar process.
- Froese, Rainer, and Daniel Pauly, eds. (2013). "Channichthyidae" in FishBase. February 2013 version.
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- Zhao, Y. Q.; Ratnayake-Lecamwasam, M.; Parker, S. K.; Cocca, E.; Camardella, L.; Detrich, H. W. (1998). "The major adult alpha-globin gene of antarctic teleosts and its remnants in the hemoglobinless icefishes-calibration of the mutational clock for nuclear genes". Journal of Biological Chemistry 273: 14745–14752.
- Di Prisco, G.; Cocca, E.; Parker, S. K.; Detrich, H. W. (2002). "Tracking the evolutionary loss of hemoglobin expression by the white-blooded antarctic icefishes". Gene 295: 185–191.
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- Near, T. J.; Parker, S. K.; Detrich, H. W. (2006). "A genomic fossil reveals key steps in hemoglobin loss by the antarctic icefishes". Molecular Biology and Evolution 23: 2008–2016.
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