Blood and ice: how Antarctic fish lost hemoglobin but were able to survive

In 1928, a biologist named Ditlef Rustad caught an unusual fish on the shores of Bouvet Island in Antarctica. The “white crocodile fish,” as Rusted described it, had large eyes, a voluminous mouth full of teeth, and transparent fins that looked like long feathers. The fish was very pale, in some places its body was as white as snow, while other parts of the body were almost transparent. Even the Norwegian whalers in the 19th century said that in the faraway Antarctic, off the South Georgia island in the southwestern Atlantic Ocean, strange fish with colorless blood live, which they christened “bloodless” and “ice”. And now these stories, considered by scientists to be fiction, turned out to be true.

When Rusted conducted a studyfish, he realized that her blood is almost colorless - there was not a drop of red anywhere. Her gills were also strange: painted white, they resembled yogurt in consistency, if such a comparison can be made. In the same cod, the gills are red, as in most other fish. All thanks to the large number of blood vessels that permeate this organ. In white fish, the network of vessels is generally thicker than in ordinary fish, especially in the gills.

Lack of red blood cells? How is this possible?

In the gills, as you know, blood is saturated with oxygen, after which oxygen is distributed throughout the body. Therefore, whatever the color of the fish, its gills should be red, or at least dark - and not completely white, like a strange fish from the Antarctic.

Later, Johan Ruud and other researchers explained why this is the case. The fact is that this fish, which was called “ice fish” (Champsocephalus gunnari), turned out to be a representative of the fish of the Channelichthyidae family, almost all of which are deprived of hemoglobin and red blood cells. It is hemoglobin and red blood cells that we owe to the fact that our body receives oxygen, capturing it from atmospheric air.



Initially, scientists decided that the absence of hemoglobin is an adaptation to ultra-low temperatures and oxygen-rich supercooled water in this region. Indeed, there is a lot of oxygen in the water of the fish habitat region - it is absorbed almost by itself. Why, one wonders, to fish these blood cells, when oxygen enters the gills without problems? And experts came to the conclusion that these are all evolutionary changes that allowed the fish to adapt to extreme temperatures.

But it turned out that everything is a bit wrong. Some time later, scientists found that the loss of hemoglobin is not an adaptation at all. Rather, it is a genetic mutation with not too positive changes for the species or even the whole family. Since the blood of a fish without hemoglobin is able to carry only 10% of the oxygen that is usually transported by the blood red blood cells of tropical fish, the "ice" fish had to completely redraw their body in order to survive.

This species managed to survive in spite of everything due to a number of circumstances. Ice fish lives in the South Arctic Ocean, washing the Antarctic. Currents block the region from getting more warm water here. For this reason, the water here is always cold. Its temperature ranges from 1.5 degrees Celsius in the summer to - 1.8 degrees Celsius in the winter (as you know, sea and ocean water freeze at temperatures well below zero).

And the fish had to develop a special non-freezing protein that protects animals from the formation of ice crystals in their blood when the water temperature drops below zero. 16 species of fish in the Antarctic belong to the family Channichthyidae, which, in turn, is part of Notothenioidei. Moreover, among all other notothenia, only this fish does not have hemoglobin in the blood. By the way, icefish and nototheniae prevail in the Southern Arctic Ocean - they make up 35% of all fish species and form 90% of the biomass in the region.

Genetics analyzed the DNA of icefish with the DNA of representatives of fish species with red blood. This study was conducted by William Detrich of Northeastern University.. Detrich and colleagues discovered genes that led to specific mutations. In fact, one of the genes that is responsible for the “assembly” of the hemoglobin molecule was lost by icefish.

Along with hemoglobin, during the evolution, proteinaceae lost myoglobin, which carries oxygen in the muscles of the skeleton.

As mentioned above, until recently, the loss of hemoglobin and red blood cells by icefish was considered adaptation to cold water. Since no vertebrate in the world lost hemoglobin and red blood cells anymore, scientists concluded that the loss of red blood cells was a response to the changing environment. When the temperature drops (meaning a long-term process), animals with "liquid blood", which is easier to circulate through the vessels, get an advantage. And since red blood cells are quite large cells, they in some sense interfere with this process. Most fish species that live in cold climates have much less red blood cells in their blood than their relatives living in warm waters. Plus, some fish species can lower red blood cells during the winter in order to conserve energy.

But Christine O'Brien from the University of Alaska at Fairbanks and colleagues decided to test the assumption of adaptive changes in icefish. In the results of the study, scientists indicated that icefish have a larger heart and blood vessels than other nototenoid fish. Despite the fact that blood circulates through the vessels of icefish more actively, since it is deprived of red blood cells, the fish has to pump huge volumes of blood to deliver enough oxygen to all tissues and organs.

As a result, icefish, according to scientists, spends twice as much energy on the circulatory process than its relatives. About 5% of the energy spent by the whole organism is spent on the work of the heart at rest in ordinary Arctic fish. In ice fish, this indicator increases up to 22%. Some organs of transparent fish are penetrated by a denser network of blood vessels than other fish. In particular, this applies to the eyes of icefish.

As you can see, this can hardly be called a profitable adaptive acquisition aimed at saving energy. On the contrary, it leaves more than fish species with red blood. Here is the enlargement of the heart and the expansion of the network of blood vessels; yes, it is already a consequence of the evolutionary process that allowed this strange fish to survive. Energy savings are achieved through other mechanisms. For example, fish have no kidney bodies. Removal of toxic substances in icefish is performed by special secretory cells of the renal tubules.

In addition, icefish have a higher mitochondrial volume, with their number similar to other related fish species. In the mitochondrial membranes of proteinacea, a higher ratio of lipids to proteins. This is probably due to a specific protein regulator of mitochondrial biogenesis PGC-1α *. PGC-1α is a transcriptional coactivator and a central element in the formation of mitochondria in cells. It has recently been discovered that PGC-1α regulates the composition and functions of individual mitochondria and their oxidative metabolism. An increase in oxidative metabolism is associated with increased activity of PGC-1α, which is accompanied by an increase in reactive oxygen species (ROS) in mitochondria. But this protein is also a powerful regulator of ROS removal, because a high level of PGC-1α triggers the expression of numerous antioxidant enzymes.

Most of the time, white blood birds have to spend motionless. These fish can also absorb oxygen from the water through the skin.

Ice in the blood - and no problem

About 25 million years ago, the Southern Arctic Ocean began to cool. By the way, the Southern Arctic Ocean is a conditional name for the waters of three oceans (Pacific, Atlantic and Indian) surrounding Antarctica and often distinguished as the “fifth ocean”, which, however, does not have a clearly defined northern border on islands and continents.

So, when the ocean’s waters cooled in this region, a large number of animal species died that could not adapt and develop special antifreeze proteins or adapt to cooling in any other way. Those species who were able to do this survived.

Ice crystals form in the same ice fish in the blood - this is deadly for many other animal species, but not for this species. The fact is that a special protein does not allow ice flakes that have already appeared to become a crystallization center, which would lead to complete freezing of the fish. Blood and intercellular fluid remain fluid. It is this feature that allows icefish to feel good at the South Pole.

The antifreeze protein is called AFGP (antifreeze glycoprotein). It is likely to have evolved from pankeratic trypsinogen-like protease. The protein is able to bind to microscopically small ice crystals, preventing their growth.

Belokrovki (also called ice fish) completely freeze only at - 6 degrees Celsius.

Man creates problems for white blood

Global warming of the Earth’s climate leads to the fact that the waters of the Southern Arctic Ocean are becoming warmer and acidic. Food for white fish (usually detritus that appears in the water during the melting of pack ice) becomes smaller. Belokrovki are more sensitive to climate change than their relatives with red blood. Ichthyologists believe that this species of fish can exist only in cold water in the polar regions and only at a certain temperature range. In any other regions, the features of icefish will lead to its rapid death.

If the warming continues - and this is most likely true, then the white blood ones will either have to adapt again, again "inventing" red blood cells, or completely die out, leaving nothing behind. Of course, I would like to hope that this unusual and strange fish can survive even in the conditions of an ever changing climate.