Channichthyidae
Encyclopedia
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 below 0°C (the freezing point of freshwater) in the Antarctic sea but stays rather constant. There are sixteen known species of crocodile icefish. They feed on krill
, copepod
s, and other fish.
Icefish reach length of 75 cm.
is transparent because it contains less than 10% hemoglobin
and/or only defunct red blood cell
s. Oxygen
is absorbed directly through their scaleless skin from the water. It is then dissolved in the plasma and transported throughout the body without the hemoglobin protein. The loss of hemoglobin is not fatal because of the cold environment in which channichthyids live. Cold water has a much higher dissolved oxygen content than warmer water.
s 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.
Hemoglobin loss was probably an adaptation for dealing with cold water temperatures, though it was not a very efficient one. Icefish had to develop many more adaptations to compensate for the loss of the oxygen transporting protein. They increased their blood volume and heart size, and lowered their metabolic rates and their oxygen demand in tissues.
. Myoglobin has been lost in icefish heart ventricles at least four separate times and by four different mechanisms.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of perciform fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
found in the cold waters around Antarctica and southern South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. Water temperature can drop below 0°C (the freezing point of freshwater) in the Antarctic sea but stays rather constant. There are sixteen known species of crocodile icefish. They feed on krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
, copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, and other fish.
Icefish reach length of 75 cm.
Blood
Their bloodBlood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
is transparent because it contains less than 10% hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
and/or only defunct red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is absorbed directly through their scaleless skin from the water. It is then dissolved in the plasma and transported throughout the body without the hemoglobin protein. The loss of hemoglobin is not fatal because of the cold environment in which channichthyids live. Cold water has a much higher dissolved oxygen content than warmer water.
Hemoglobin
Channichthyidae are the only known vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s 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
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
. 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.
Hemoglobin loss was probably an adaptation for dealing with cold water temperatures, though it was not a very efficient one. Icefish had to develop many more adaptations to compensate for the loss of the oxygen transporting protein. They increased their blood volume and heart size, and lowered their metabolic rates and their oxygen demand in tissues.
Myoglobin
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 ventriclesVentricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
. Myoglobin has been lost in icefish heart ventricles at least four separate times and by four different mechanisms.