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The icefish, or Antarctic icefish, is a white blooded fish found in the Southern Ocean and belongs to the family Channichthyidae. There are several genera and species of icefish. They inhabit the cold and oxygen-enriched Antarctic waters around Antarctica, with this marine animals' range expanding to the southern parts of the Indian and Atlantic Oceans. This fish is commonly known as the crocodile icefish due to the shape of its face.
The bodies of crocodile icefishes are provided with antifreeze proteins that prevent them from freezing in ice-cold waters. These fishes are the only known vertebrates that do not have red blood cells and hemoglobin. This modification has significantly affected the oxygen-carrying capacity in them, and so, they have several adaptations including modified gills, spongy ventricles from which oxygen is directly absorbed in the plasma, and larger blood vessels. Interestingly, the hemoglobin genes can still be found in these fishes, suggesting the loss of hemoglobin was part of an evolutionary process. Climate change and habitat loss pose a serious threat to crocodile icefishes as they are adapted to the cold water in their habitat. These fishes enrich the field of science with their unique traits.
The crocodile icefish, also called the Antarctic icefish, is a species of white-blooded fish lacking red blood cells and the protein known as hemoglobin. This fish is also special due to the antifreeze proteins in its body that provide it protection from the ice-cold waters in the southern part of the world.
Crocodile icefish belong to the class Actinopterygii and they are a part of the notothenioid fish group. They are a member of the family Channichthyidae. Under the family Channichthyidae, there are 11 genera which are further classified into several species.
The exact population of crocodile icefishes is not known. However, the population trend of one species, known as the pike icefish, has been recorded as decreasing by the International Union for Conservation of Nature or IUCN. These icefishes are considered to be rare.
Crocodile icefishes are endemic to the Southern Ocean, encircling Antarctica. Their populations can be found in the southern parts of the Indian Ocean and the Atlantic Ocean. The different species have different geographical ranges within this vast area surrounding Antarctica. For example, mackerel icefish live in the southern Atlantic and northern Antarctic peninsula.
The habitat of the crocodile or Antarctic icefish is characterized by the extreme cold and freezing temperatures of the Southern Ocean. In fact, they were named icefish due to the ice-cold waters they reside in. They are generally found towards the bottom of the ocean. Icefishes are adapted to survive in the oxygen-rich Antarctic waters and other parts of the Southern Ocean with cold temperatures. They are not tolerant of drastic changes to their habitat.
Antarctic icefishes might be found in a group or school while feeding. However, the pike icefish is observed to display solitary behavior while hunting for food.
The lifespan of mackerel icefish has been recorded to be 15 years. It can be assumed that the other species of Antarctic icefishes also have a similar lifespan.
Most crocodile icefishes spawn during the late summer, early fall, or early winter months. Mackerel icefishes and blackfin icefishes travel closer to the shore in order to spawn. Female crocodile icefishes lay 10,000-20,000 eggs which are fertilized externally. In spiny icefishes, the males are noted to guard the eggs and the incubation period lasts between two to six months.
The conservation status of only one kind of crocodile icefish has been recorded by the International Union for Conservation of Nature or IUCN, and that icefish is called the pike icefish. These icefishes are listed as Vulnerable with habitat loss and fluctuating temperatures playing a major part in their endangerment.
Although there are multiple species of crocodile icefish, they all have some features in common. These fishes lack scales and the skin is usually brown, black, or gray in color and appears transparent due to the lack of hemoglobin. They have two dorsal fins and quite wide pectoral fins. The shape of their heads resembles that of a crocodile's, earning it the name of crocodile icefish. In Chaenocephalus aceratus, commonly called the blackfin icefish, the members have vertical stripes on their bodies. In spiny icefishes (Chaenodraco wilsoni), the second dorsal fins possess stripes.
The crocodile or Antarctic icefish is more fascinating than cute. The mechanism through which these fishes have evolved and their special adaptations enrich the field of science.
Not much is known about the methods of communication in crocodile icefishes. However, in general, members of the order Perciformes to which crocodile icefishes belong primarily communicate through visual and chemical cues.
The length of different species of icefish ranges between 9.8-19.6 in (25-50 cm). Icefishes are generally larger than the blobfish, a red-blooded fish of the class Actinopterygii which measures less than 11.8 in (30 cm).
The exact swimming speeds in icefishes are not known. However, some icefishes, like the long-fingered icefish, tend to become more sedentary as they grow older, whereas the mackerel icefish becomes more active and regularly migrates vertically in the waters in order to feed.
The different species of crocodile icefishes have varied weights. The blackfin icefish has a maximum recorded body weight of 8 lb (3.7 kg), while mackerel icefishes weigh a maximum of 4.4 lb (2 kg). Both of these species are not as heavy as the various catfish species, which can exceed 220 lb (997 kg).
The male and female fishes of the various crocodile icefish species are known as male crocodile icefishes and female crocodile icefishes.
A baby crocodile icefish is known as a larva.
Crocodile icefishes mainly feed on small fishes, crustaceans, and a type of zooplankton known as krill. They are capable of preying on fishes with body lengths that are half as much as their own. The blackfin icefish (Chaenocephalus aceratus) are described as ambush predators as they wait for their prey instead of chasing it.
There are no instances to suggest that crocodile icefishes are dangerous.
Given the natural adaptations of crocodile icefishes, it would be difficult to fulfil their requirements if they are kept as pets.
Theories about crocodile icefish ancestry suggest they evolved 6 million years ago, whereas other theories suggest it was 15-20 million years ago.
Thornfishes are relatives of crocodile icefishes.
Crocodile icefishes lack swim bladders.
Gentoo penguins prey on crocodile icefishes.
It is quite interesting to note that icefish do not have any hemoglobin or red blood cells, meaning they have white or clear blood. They are the only vertebrates to lack both of these. Early research suggested that crocodile icefish blood lacks red blood cells and hemoglobin as they had already absorbed enough oxygen from the extremely low-temperature but oxygen-rich waters of their surroundings. It was thought that this adaptation made their blood thinner which helped the blood in their bodies to circulate faster. This also aided in adapting to an extreme environment. However, new developments in science have revealed that the lack of hemoglobin and red blood cells might be accidental, as the absence of the protein hemoglobin along with red blood cells has reduced the oxygen-carrying capability of their blood.
The most significant adaptation that helps crocodile or Antarctic icefishes to survive in the extremely cold waters of the Antarctic Ocean is the presence of antifreeze proteins, which circulate in their blood and prevent it from being frozen. The antifreeze gene evolved from an existing gene for an enzyme known as trypsinogen in their body. Since icefishes evolved to lose hemoglobin and red blood cells, the oxygen-carrying capacity of their blood was negatively affected. To overcome this, they also have spongy ventricles in their heart which absorb oxygen into the plasma. Additionally, they have much higher volumes of blood and larger blood vessels when compared to red-blooded fishes and larger hearts to pump out all that blood efficiently. The blood vessels in their gills and smooth skin also aid in the sufficient uptake of oxygen. All these mechanisms help icefishes to live and survive in the cold, freezing temperatures of the ocean.
Here at Kidadl, we have carefully created lots of interesting family-friendly animal facts for everyone to discover! For more relatable content, check out these milkfish facts and fluke fish facts for kids.
You can even occupy yourself at home by coloring in one of our free printable crocodile icefish coloring pages.
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