How do fish breathe?
No animal can live without oxygen. It is in the air and dissolved in water. Terrestrial vertebrates breathe oxygen from the air; their respiratory organ is the lungs. Fish extract oxygen from water; for this they have gills.
The fish breathes by taking water into its mouth. Through the pharynx, in which there are rows of paired openings - gill slits, water flows to the gills located on both sides of the head and, washing them, flows out from under the gill cover. At the same time, oxygen dissolved in it penetrates into the blood through the thinnest covers of the gill filaments, penetrated by blood capillaries, and the circulatory system delivers oxygen to the cells. When exposed to air, the fish suffocates as soon as the gill filaments dry out and become impermeable to oxygen.
When fish breathe, they “inhale” and “exhale” not air, like people, but water. Watch a fish in an aquarium: its mouth and gill covers open and close, providing the body with fresh aqueous oxygen.
However, from this general rule there is an exception. In Africa, South America and Australia, lungfish live, which breathe not only with gills, but also with a swim bladder connected by a duct to the pharynx. However, the structure of their cellular swim bladder is not much different from real lungs. In most modern species it is even a paired organ, as in all higher vertebrates. Lungfishes draw air into their “lungs” through their nostrils with their mouths closed, like all terrestrial vertebrates, but in addition, they can also breathe through their gills, like fish. All of them are inhabitants of fresh water bodies, which partially or completely dry up during the dry season. Then the lungfish lie down in holes dug in the ground and hibernate. African protopters can live without water for 9 months, and one experimental protopter set a record for more than four years!
Both protopterans and South American lepidoptera from the Amazon breathe air during hibernation. The Australian cattail does not hibernate and survives if at least a fetid puddle remains from its pond. Even then, breathing with his unpaired “lungs,” he feels good, but without any water he quickly dies.
Lungfish feed on invertebrates, fish and amphibians. They spawn during the rainy season.
Previously, scientists believed that land vertebrates descended from ancient lungfishes. But now it is firmly established that the connecting link between fish and amphibians were animals from the class of almost completely extinct lobe-finned fish, and lungfish, also extinct, except for the modern six species, is a lateral, dead-end branch of evolution.
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Includes 11 families, 3 of which (Protopteridae, Lepidosirenidae and Ceratodontiformes) contain fish that have survived to this day. Lungfishes are contemporaries of lobe-finned fishes. Known from the Middle Devonian, they were numerous until the Permian period. Modern lungfishes are represented by 6 species, united in 2 orders. They live in the fresh tropical waters of Africa, America and Australia, and are adapted to life in drying up water bodies. In addition to gills, they have lungs, formed from a swim bladder and similar in structure to the lungs of terrestrial vertebrates.
The structure of their systems and organs is changed due to pulmonary respiration. The conus arteriosus is partially divided and resembles the same section in amphibians, which as adults breathe only through the lungs. Due to specialized feeding on vegetation and invertebrates, the teeth of lungfish have the shape of plates. Probably, lungfishes could be a lateral branch of lobe-fins. A number of scientists suggest that lungfish were the common ancestor of all terrestrial vertebrates, and propose to distinguish them into a separate subclass or even class.
In bipulmonates, order Lepidosireniformes, the two lungs connected to the esophagus have pouches and alveoli that increase inner surface. The body is elongated, the scales are small, buried deep into the skin. Paired fins are flagellated. During drought (up to 9 months), they completely switch to pulmonary breathing and hibernate. The order includes the families Protopteridae and Lepidosirenidae and 4 species of African protoptera and 1 species of South American lepidoptera.
Protoptera differ in their habitats, coloration, a number of anatomical features and sizes: Protopterus amphibius is 30 cm long, P. aethiopicus - 2 m. They feed on invertebrates and fish. Most active at night. When drought approaches, protopters dig holes, gnawing out pieces of soil, crushing them with their jaws and throwing them out through their gill covers. The stroke, round in cross-section, has a diameter of 5-70 mm and goes vertically down. At a depth of 50 cm, the passage expands, forming a “sleeping” chamber, where, curled up almost in half, the protopter waits out the dry period. Before hibernating, it seals the entrance with a cap of clay and is covered with a thin cocoon of hardened mucus. During hibernation, the protopter loses up to 20% of its mass, and uses muscle tissue as a source of energy necessary to maintain life. This energy is used not only for survival, but also for gonad maturation.
With the onset of the rainy season, the protopter prepares for spawning - it digs a brood hole in shallow water, which has two entrances. The brood chamber is located at a depth of 40 cm. The male guards the clutch and takes care of the offspring. At the age of one month, the larvae, 30-35 mm long, leave the nest.
Lepidosiren paradoxa lives in central South America. Body length is 130 cm. It differs from protopterans in its more elongated body, more reduced paired fins, smaller scales that sit deeper in the skin, and in that it consumes fat during hibernation. Unlike protopters, which spawn at the bottom of the brood chamber, the lepidoptera makes a bedding from pieces of vegetation. Successfully kept in aquariums.
The order of horn-toothed, or one-lunged (Ceratodontiformes), represents the only modern look- horntooth, or barramunda (Neoceratodus forsteri). Lives in slow, vegetated rivers of north-eastern Australia. Length 175 cm, weight 10 kg. Their elongated, laterally compressed body is covered with large scales and ends in a diphycercal caudal fin. Unlike two-legged fish, it has one lung, paired fins are more powerful, flipper-shaped, and a non-ossified cartilaginous skull. In search of food (bottom animals and plants), it crawls along the bottom, relying on its fins. Swims quickly when necessary, bending its body. Every 40-60 minutes it rises to the surface of the water for a portion of air. Exhalation and inhalation are accompanied by a loud sob. During periods of drought, when the rivers of Australia are filled with liquid mud, the cattail completely switches to pulmonary breathing. However, complete drying out of the reservoir is dangerous for it, since it does not hibernate.
Reproduction from early spring to late autumn. It does not build a nest, lays eggs on aquatic vegetation and does not take care of it. Eggs 7 mm in diameter contain large number yolk and surrounded by a gelatinous membrane. Development of eggs within 1.5 weeks. Newborn cattails do not have paired fins; thoracic ones appear after two weeks, abdominal ones - after 2.5 months.
Fish can be found in swamps, lakes, seas and rivers of all geographical zones of the planet. They spend their entire lives underwater without experiencing any difficulty breathing. Most of them do not need to float to the surface to swallow another portion of air. What do fish breathe? What mechanisms help them survive in an aquatic environment? We will talk about the internal structure of fish and the natural tricks of these aquatic animals in our article.
Oxygen requirement
In the aquatic environment, fish are the predominant group of animals. In rivers and oceans they go through all stages of their biological development - from eggs to adults. At the same time, only a few species can emerge from time to time and inhale atmospheric air, while the majority have adapted to live without it.
But what do fish breathe when they are constantly in the water? Like other vertebrates, they require oxygen to function normally. They “extract” it not from the air, but directly from the water, literally filtering it. To get enough gas, they have to “process” a huge amount of liquid.
The oxygen content in a reservoir is extremely important for their normal functioning, and a deficiency causes oxygen starvation and death in animals. However, each type has its own gas concentration standards. For example, tench and carp live in stagnant bodies of water and are able to survive even in the weak presence of oxygen (from 4 cm 3 / l to 0.5 cm 3 / l). Trout, salmon, and pike perch, on the contrary, are very demanding. They need a gas concentration of more than 7 cm 3 / l.
The perception of fish changes with their age, with the transition from season to season, and also depending on their activity. So, the younger and more active the individual, the more oxygen it needs. The needs increase greatly before spawning, when the fish need a lot of strength and energy. In the heat and when the pond freezes in winter, there is a lack of oxygen, causing animals to have difficulty breathing.
What do fish breathe? Devices for gas exchange
Just like ours, gas exchange in fish is carried out using the circulatory system. To do this, most of them have only one circle of blood circulation and a two-chambered heart; in lungfish species there are two such circles. Oxygen enters the heart through the vessels, and enters them through the gills, which filter gas from the water.
The respiratory system of fish is, in fact, more efficient than the human one. It is capable of filtering two to three times more oxygen from water than the lungs remove from the atmosphere. Basically, fish breathe with gills, but sometimes their work is not enough or conditions do not allow them to be used normally. In this case, other special bodies are connected to them.
Fish have quite a lot of additional or alternative methods of breathing. Absolutely all species help themselves, partially carrying out gas exchange through the skin. Some also use the swim bladder, others use the intestine or caeca in the stomach. Some species have adapted to breathing atmospheric air; for this they use labyrinthine or epibranchial organs.
Internal structure of fish: how gills are arranged
Fish respiration begins with swallowing water through the mouth. In their pharynx there is a gill apparatus, in which the further process takes place. The apparatus consists of gill arches located on the sides of the animal. They are supported by gill filaments and stamens. On the outside, the arches of bony fish are covered with covers.
The gills of fish are connected to numerous blood vessels. Once in the pharynx, water passes through the gill arches, washes the petals and releases oxygen to the arteries attached to them. The enriched blood is directed to the heart and tissues, and from there it returns to the pharynx, where it gives off carbon dioxide to water and removes it through the gill slits to the outside.
Lungfish
As mentioned above, the main instrument for gas exchange is the gills. However, what do fish, which are called “lungfishes”, breathe? These animals are now represented by only one order, which includes six species. They live near Australia, Africa and South America.
Of all fish, they are the closest relatives of tetrapods. Another feature of them is that in addition to gills, they have simplified lungs. This adaptation allows them to live in bodies of water with a very small amount of oxygen, and, if necessary, obtain it from the atmospheric air by emerging to the surface.
Labyrinth or creeper fish
Labyrinth fishes represent the order of ray-finned fish. These include many aquarium species, such as lapius, gourami, Siamese cockerels, macropods, labtosis and others. In nature, they live in the fresh waters of Africa and Asia.
They all also know how to breathe air. They do not have lungs, but they have a special organ in the form of a pocket, consisting of many plates. Capillaries approach its walls, with which gases are exchanged. The labyrinth organ is located above the gills of the fish. Thanks to it, animals can survive for several days without water. At the same time, the “second wind” is not a convenient addition to the gills. They cannot avoid using the labyrinthine organ, so they are forced to periodically emerge from the water, otherwise they risk suffocation.
When, during a six-month drought, Lake Chad in Africa reduces its area by almost one third and the muddy bottom is exposed, local residents go fishing, taking with them... hoes. They look for mounds on the dry bottom that resemble molehills, and from each they dig out a clay capsule with a fish folded in half, like a hairpin.
This fish is called Protopterus ( Protopterus) and belongs to subclass 1 lungfish ( Dipnoi). In addition to the usual gills for fish, representatives of this group also have one or two lungs - a modified swim bladder, through the walls of which gas exchange occurs, entwined with capillaries. The fish capture atmospheric air for breathing through their mouths, rising to the surface. And in their atrium there is an incomplete septum, which continues in the ventricle. Venous blood coming from body organs, enters the right half of the atrium and the right half of the ventricle, and blood coming from the lung enters left side hearts. Then the oxygenated “pulmonary” blood enters mainly those vessels that lead through the gills to the head and body organs, and the blood from the right side of the heart, also passing through the gills, largely enters the vessel leading to the lung. And although poor and oxygen-rich blood partially mix both in the heart and in the vessels, we can still talk about the rudiments of two circulatory circles in lungfish.
Lungfishes are a very ancient group. Their remains are found in sediments of the Devonian period of the Paleozoic era. For a long time, lungfish were known only from such fossilized remains, and only in 1835 was it established that the protoptera living in Africa was a lungfish. In total, as it turned out, representatives of six species of this group have survived to this day: the Australian cattail from the order of monopulmonates, the American lepidoptera - a representative of the order of bipulmonates, and four species of the African genus Protopterus, also from the order Dipulmonates. All of them, like, apparently, their ancestors, are freshwater fish.
Australian horntooth(Neoceratodus forsteri) is found in a very small area - in the Burnett and Mary river basins in north-eastern Australia. This is a large fish with a body length of up to 175 cm and a weight of over 10 kg. The massive body of the horntooth is laterally compressed and covered with very large scales, and its fleshy paired fins resemble flippers. The horntooth is painted in uniform tones - from reddish-brown to bluish-gray, the belly is light.
This fish lives in slow-flowing rivers, heavily overgrown with aquatic and surface vegetation. Every 40 - 50 minutes, the cattail emerges and noisily exhales air from the lung, while emitting a characteristic groaning-grunting sound that spreads far across the surrounding area. After inhaling, the fish sinks to the bottom again.
The horntooth spends most of its time at the bottom of deep pools, where it lies on its belly or stands, leaning on its flipper-like fins and tail. In search of food - various invertebrates - it slowly crawls, and sometimes “walks”, relying on the same paired fins. It swims slowly, and only when startled does it use its powerful tail and show the ability to move quickly.
The cattail survives the period of drought, when the rivers become shallow, in preserved pits with water. When fish die in overheated stagnant and practically oxygen-deprived water, and the water itself turns into a fetid slurry as a result of putrefactive processes, the cattail remains alive thanks to its pulmonary respiration. But if the water dries out completely, these fish still die, since, unlike their African and South American relatives, they cannot hibernate.
The horntooth spawns during the rainy season, when the rivers swell and the water in them is well aerated. The fish lays large eggs, up to 6–7 mm in diameter, on aquatic plants. After 10–12 days, the larvae hatch, which lie on the bottom until the yolk sac is absorbed, only occasionally moving a short distance. On the 14th day after hatching, the fry develop pectoral fins, and from this time the lung probably begins to function.
The cattail has tasty meat and is very easy to catch. As a result, the number of these fish has greatly decreased. Now the horned teeth are under protection and attempts are being made to acclimatize them in other bodies of water in Australia.
The history of one of the most famous zoological hoaxes is connected with the horntooth. In August 1872, the director of the Brisbane Museum was touring north-eastern Australia, and one day he was informed that a breakfast had been prepared in his honor, for which the natives had brought very rare fish, which they had caught 8-10 miles from the place of the feast. And indeed, the director saw a fish of a very strange appearance: a long, massive body was covered with scales, the fins looked like flippers, and the snout resembled a duck’s beak. The scientist made drawings of this unusual creature, and after returning, he handed them over to F. De Castelnau, a leading Australian ichthyologist. Castelnau was quick to describe a new genus and species of fish from these drawings - Ompax spatuloides. A rather heated discussion ensued about the relationships of the new species and its place in the classification system. There were many reasons for disputes, since in the description Ompax much remained unclear and there was no information on anatomy at all. Attempts to obtain a new specimen were unsuccessful. There were skeptics who expressed doubts about the existence of this animal. Still mysterious Ompax spatuloides For almost 60 years, it continued to be mentioned in all reference books and reports on Australian fauna. The mystery was resolved unexpectedly. In 1930, a note appeared in the Sydney Bulletin, the author of which wished to remain anonymous. This note reported that an innocent joke was played on the simple-minded director of the Brisbane Museum, since the “Ompax” served to him was prepared from the tail of an eel, the body of a mullet, the head and pectoral fins of a horntooth and the snout of a platypus. From above, this entire ingenious gastronomic structure was skillfully covered with scales of the same horned tooth...
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African lungfishes - protopters - have thread-like paired fins. The largest of the four species is large protopter(Protopterus aethiopicus) can reach a length of more than 1.5 m, and the usual length small protopter(P.amphibius) – about 30 cm.
These fish swim, bending their bodies like snakes like eels. And along the bottom, with the help of their thread-like fins, they move like newts. The skin of these fins contains numerous taste buds - as soon as the fin touches an edible object, the fish turns around and grabs the prey. From time to time, the protopters rise to the surface, swallowing atmospheric air through their nostrils.
Protoptera live in Central Africa, in lakes and rivers flowing through swampy areas that are subject to annual flooding and dry out during the dry season. When the reservoir dries out, when the water level drops to 5–10 cm, the protopters begin to dig holes. The fish grabs the soil with its mouth, crushes it and throws it out through the gill slits. Having dug a vertical entrance, the protopter makes a chamber at its end, in which it is placed, bending its body and sticking its head up. While the water has not yet dried, the fish rises from time to time to take a breath of air. When the film of drying water reaches the upper edge of the liquid silt lining the bottom of the reservoir, part of this silt is sucked into the hole and clogs the exit. After this, the protopter no longer appears on the surface. Before the cork completely dries, the fish, poking its snout into it, compacts it from below and slightly lifts it in the form of a cap. When dry, such a cap becomes porous and allows enough air to pass through to support the life of a sleeping fish. As soon as the cap hardens, the water in the burrow becomes viscous due to the abundance of mucus secreted by the protopter. As the soil dries, the water level in the hole drops, and eventually the vertical passage turns into an air chamber, and the fish, bent in half, freezes in the lower, expanded part of the hole. A mucous cocoon tightly adjacent to the skin is formed around it, in the upper part of which there is a thin passage through which air penetrates to the head. In this state, the protopter awaits the next rainy period, which occurs in 6–9 months. In laboratory conditions, the protopters were kept in hibernation for over four years, and at the end of the experiment they woke up safely.
Protopter buried in mud during a drought
During hibernation, the metabolic rate of protopterans sharply decreases, but nevertheless, over 6 months the fish loses up to 20% of its original mass. Since energy is supplied to the body through the breakdown of not fat reserves, but mainly muscle tissue, nitrogen metabolism products accumulate in the fish’s body. During the active period, they are excreted mainly in the form of ammonia, but during hibernation, ammonia is converted into less toxic urea, the amount of which in the tissues by the end of hibernation can be 1–2% of the fish’s weight. The mechanisms that ensure the body's resistance to such high concentrations of urea have not yet been elucidated.
When reservoirs are filled with the beginning of the rainy season, the soil gradually becomes wet, water fills the air chamber, and the protopter, having broken through the cocoon, begins to periodically stick its head out and inhale atmospheric air. When water covers the bottom of the reservoir, the protopter leaves the burrow. Soon, urea is eliminated from his body through the gills and kidneys.
A month and a half after emerging from hibernation, protopterans begin to reproduce. In this case, the male digs a special spawning hole at the bottom of the reservoir, among thickets of vegetation, and lures one or several females there, each of which lays up to 5 thousand eggs with a diameter of 3–4 mm. After 7–9 days, larvae appear with a large yolk sac and 4 pairs of feathery external gills. Using a special cement gland, the larvae attach to the walls of the nesting hole.
After 3–4 weeks, the yolk sac is completely absorbed, the fry begin to actively feed and leave the hole. At the same time, they lose one pair of external gills, and the remaining two or three pairs can remain for many months. In the small protoptera, three pairs of external gills are retained until the fish reaches the size of an adult.
Having left the spawning hole, the protoptera fry swim only next to it for some time, hiding there at the slightest danger. All this time, the male is near the nest and actively protects it, even rushing at an approaching person.
Protopter dark(P. dolloi), found in the Congo and Ogowe river basins, lives in swampy areas where a layer of underground water persists during the dry season. When surface waters begin to decrease in summer, this fish, like its relatives, buries itself in the bottom mud, but digs down to a layer of liquid silt and underground water. Having settled there, the dark protoptera spends the dry season without creating a cocoon and rising up from time to time to breathe fresh air.
The burrow of the dark protopter begins with an inclined passage, the expanded part of which serves as a spawning chamber for fish. According to local fishermen, such holes, if they are not destroyed by floods, serve the fish for five to ten years. Preparing the hole for spawning, the male builds up a mud mound around it year after year, which eventually reaches 0.5–1 m in height.
Protopters have attracted the attention of scientists involved in the creation of sleeping pills. English and Swedish biochemists tried to isolate “hypnotic” substances from the body of hibernating animals, including from the body of the protoptera. When an extract from the brains of sleeping fish was injected into the bloodstream of laboratory rats, their body temperature began to drop rapidly, and they fell asleep as quickly as if they were fainting. The sleep lasted 18 hours. When the rats woke up, no signs were found that they were in artificial sleep. The extract obtained from the brains of awake protopters did not cause any effects in rats.
American scalefish(Lepidosiren paradoxa), or lepidosiren,- a representative of lungfishes living in the Amazon basin. The body length of this fish reaches 1.2 m. The paired fins are short. Lepidosirens live mainly in temporary reservoirs, filled with water during rains and floods, and feed on a variety of animal foods, mainly mollusks. Maybe they eat plants too.
When the reservoir begins to dry up, lepidosiren digs a hole at the bottom, in which it settles in the same way as protopters, and clogs the entrance with a plug of soil. This fish does not form a cocoon - the body of the sleeping lepidosiren is surrounded by mucus moistened by groundwater. Unlike protopters, the basis of energy metabolism during the hibernation period in lepidoptera is stored fat reserves.
2–3 weeks after the new flooding of the reservoir, lepidosirens begin to reproduce. The male digs a vertical burrow, sometimes bending horizontally towards the end. Some burrows reach 1.5 m in length and 15–20 cm in width. The fish drags leaves and grass to the end of the hole, onto which the female lays eggs with a diameter of 6–7 mm. The male remains in the hole, guarding the eggs and hatched young. The mucus secreted by its skin has a coagulating effect and purifies the water in the burrow from turbidity. In addition, at this time, branching skin outgrowths, 5–8 cm long, abundantly supplied with capillaries, develop on its ventral fins. Some ichthyologists believe that during the period of caring for offspring, lepidosirene does not use pulmonary respiration and these outgrowths serve as additional external gills. There is also the opposite point of view - rising to the surface and sipping fresh air, the male lepidosiren returns to the burrow and, through capillaries on the outgrowths, releases part of the oxygen into the water in which eggs and larvae develop. Be that as it may, after the breeding period these growths resolve.
The larvae hatched from the eggs have 4 pairs of highly branched external gills and a cement gland, with the help of which they attach to the walls of the nest. About a month and a half after hatching, when the fry reach a length of 4–5 cm, they begin to breathe using the lungs, and the external gills dissolve. At this time, the lepidosiren fry leave the hole.
The local population appreciates the tasty meat of lepidoseren and intensively exterminates these fish.
Diagram of the arterial circulation of lungfish:
1–4 – first to fourth pairs of branchial arterial arches; 5 – dorsal aorta;
6 – abdominal aorta; 7 – pulmonary artery; 8 – pulmonary vein.
Literature
Animal life. Volume 4, part 1. Pisces. – M.: Education, 1971.
Science and life; 1973, no. 1; 1977, no. 8.
Naumov N.P., Kartashev N.N. Zoology of vertebrates. Part 1. Lower chordates, jawless fish, amphibians: A textbook for biologists. specialist. univ. – M.: Higher School, 1979.
1 According to other ideas, lungfish ( Dipneustomorpha) – superorder in the subclass lobe-finned ( Sarcopterygii).
2 In most fish, the nostrils are blindly closed, but in lungfish they are connected to the oral cavity.
It all started the day William Forster decided to take a walk around the city. Previously, he raised sheep and lived on a farm, far from the civilized world, on the Benet River in Queensland. Then he got tired of this business, and he came to Sydney to settle there. One day in 1869, Forster decided to explore the city. Of course, I went to the museum.
Here I met Gerard Kreft, the curator of the museum, and they started talking. Forster asked casually:
“Sir, why aren’t there any of those big fish that live in Benet River in your museum?”
- Big fish? What are these big fish?
- And barramunda. We also call them Benet salmon.
-Where is Benet River? I don't know.
- In the north, sir. In Queensland. There are so many of these fish. They look like oily blackheads. Green, about five feet long. Their scales are thick and large. And imagine - this barramunda has only four fins! Everything is on the belly. Yes, only four, I remember well: I caught them more than once myself.
“You know, Forster, I have no idea what kind of fish you’re talking about.” I haven't heard anything about your barramuida. Maybe this is some species still unknown to science? It would be nice to get us a couple of barramundas for the museum.
“Oh, of course,” Forster agreed kindly. - It can be done. My cousin still lives on the farm. I'll write to him.
And a few weeks later a barrel was brought to the Sydney Museum, and in the barrel there were fish, very strongly salted.
Kreft was literally dumbfounded when he saw them. Forster was not mistaken: the fish are completely unprecedented. Yes, they only have four fins. Everything is on the belly. And they all look more like short legs, but without fingers. And the tail is very special: not forked, like many fish, but as if feathered, like a bird’s feather. Zoologists call this type of tail diphycercal. This is perhaps the most ancient form of all fish tails.
But then Kreft saw on the sky and lower jaw of the fish four large plates of teeth fused together, similar to cockscombs - this was completely unexpected.
The same grater teeth have long been encountered by paleontologists among ancient fossils, but they have not yet been found in any living fish. Professor Agassitz, a great expert on fossil fish, called the owners of these strange teeth ceratodes, that is, horned teeth. Countless flocks of them inhabited the fresh waters of our planet 70 and 100 million years ago.
And now Kreft held this very ceratod in his hands! This is what he decided after carefully examining the teeth of the barramunda, and therefore, without hesitation, he christened the “Benet salmon” ceratodes. But later paleontologists found not only teeth, but also skeletons of real fossil ceratodes, and they were not quite similar to the skeleton of the Benetian “ceratod”. Therefore, some ichthyologists have proposed adding the prefix “neo” (that is, “new”) or “epi” (which means “after”) to the scientific name of barramunda. But often it is still called simply ceratod, without any prefixes.
While examining the fish, Kreft cut one of them and found something else amazing - light! Real lung in fish! She also had gills, but she also had a lung. This means that barramunda breathed with both gills and lungs, which means it is a lungfish!
Before this, zoologists knew only two daoiak-breathing fish: lepidosiren, or in local karamuru, living in South America, and Protopterus (aka Comtok), which is common in Central Africa. They have two lungs, while neoceratodes have only one. Lepidosiren and Protopterus live in swampy pools overgrown with grass and algae, which are filled with water only during rainy periods. But a drought sets in and the water runs out. River oxbows and swamps dry up, and in order not to die, fish, which nature has endowed with lungs in addition to gills, bury themselves in the silt and hibernate, like a bear in a den.
The neoceratod found in Australia differs from its lungfish relatives not only in that it has one lung. He is more of a “vegetarian” than they are: true to the traditions of his ancestors, he also eats plants that other lungfish now refuse. The barramunda does not lay its very large eggs in burrows and holes at the bottom - each egg in a thick gelatinous shell is attached to underwater plants. And most importantly, during drought, when rivers dry up, neoceratodes do not bury themselves in silt. The fish simply gather in the puddles and “breathe with their lungs here.
They crawl to where, under the thick shade of bushes, the sun is not so hot and drops of moisture remain. There they lie motionless. And they breathe, and they breathe. And they are waiting for rain. But, of course, they can’t hold out like this for long. During severe droughts, many neoceratodes die. Therefore (and also because they are very tasty) these fish are now very rare, they survived only in the Benet and Mary Rivers.
By the time the barrel of salted neoceratodes arrived from the Benet River at the Sydney Museum, Ernst Haeckel and Franz Müller had already formulated their famous biogenetic law: phylogeny repeats itself in ontogeny. These few words mean a lot. Biologists call the secular evolution of plants and animals phylogeny. And ontogenesis is the embryonic and post-embryonic development of each individual organism.
So, according to the biogenetic law, every animal, developing from egg to newborn, at an accelerated pace goes through the main stages of the evolution of its species, in a few weeks repeating in general terms the key phases of phylogenetic metamorphosis, which lasted hundreds of millions of years. This is why the embryos of birds, frogs, fish, animals and people at certain stages of development are similar to each other. Human embryos several weeks old clearly indicate that our distant ancestors were once... fish.
With the discovery of the biogenetic law, Darwin's theory received powerful reinforcement. Further evidence was obtained that all vertebrates descended from fish.
But from what fish? And who gave birth to the fish themselves?
This is what the famous German biologist and Darwinist Ernst Haeckel wanted to establish when he equipped an expedition to Australia to collect neoceratod embryos. After all, this ancient fish, as it was then decided, is closest to those mysterious creatures that became our ancestors three hundred million years ago.
In August 1891, Haeckel's student Richard Semon arrived in Australia. Dr. Kreft, describing the neoceratod, assured that it lives in brackish water, eats plants and buries itself in silt during drought. Everything turned out wrong. And Semon wasted his time by trusting Kreft and hunting for fish at the mouths of the Benet and Mary Rivers, where the water was brackish. No one there had ever heard of such a fish.
Then Richard Semon went into the interior of the country. He knew that neoceratodes lay eggs on plants. The caviar is large, almost a centimeter in diameter. It would seem that it is not difficult to notice her. But Semon did not find her. Day after day, week after week, he searched for algae and underwater grasses, but there was no caviar. But Semon stubbornly climbed the reeds in waist-deep water. And finally - oh luck! Three eggs! Here they are - three matte beads on a green stem! At first he couldn't believe his eyes. But there was no doubt: this is barramunda caviar!
- Barramundas? No, mister, - dyelle. The Australians who helped the possessed stranger look for a needle in a haystack shook their heads in unison.
- No, not barramundas. This is dyelle caviar.
Semon's hands dropped. But then he thought - and he was not mistaken - if Kreft had made a mistake here too: maybe Neoceratoda in his homeland is called not barramunda, but dyelle?
- What is he like - dyelle?
They told him which one. His gnawed bones were also shown, and Semon realized that he had found what he was looking for.
Now that everyone knew that the foreigner was looking for djelle caviar, things immediately went smoothly. Semon preserved and brought seven hundred neoceratod eggs to Europe. The embryos contained in them were of different ages. And when Semoy began to study them, all phases of the ontogenesis of the most ancient of fish were revealed to his eyes.
Many zoologists believe that the ancient ancestors of fish and all vertebrates in general (including humans), the so-called chordates *, descended from some polychaete worms - polychaetes. Lancelet, a small, lily of the valley leaf-like "fish" with no fins, no bones, no teeth and no jaws (but with a chord, которая, зарывшись в песок, процеживает ртом воду, выуживая детрит и планктон, представляет собой, пожалуй, наименее искаженный живой "портрет" давно вымерших наших предков, когда они не были уже червями, но не стали еще и рыбами.!}
Behind the lancelet-like creatures, jawless “protofish” appeared, from which only petrified skin teeth, and then jawed fish, have survived.
Then there was a great migration of fish from the seas to the rivers. It is possible that they fled to fresh waters from predatory crustacean scorpions, which multiplied immensely in the seas.
The first four-legged creatures came out of rivers and lakes onto land. The fish that lived here three hundred and fifty million years ago breathed with both gills and lungs. Without lungs, they would suffocate in the musty, oxygen-poor water of primeval lakes.
Some of them chewed plants with millstone teeth (the so-called real daisy-breathers). Others, lobe-finned ones, ate everyone they could catch.
A great future awaited the cross-finned creatures: they were destined by fate to give birth to all the four-legged and feathered inhabitants of the land.
The ancient fish with lungs had amazing paw-like fins with a hand-like skeleton, very mobile and muscular. On these fins they crawled along the bottom. They probably climbed ashore to breathe and relax here calmly. Gradually, the stilt fins turned into real paws. The fish came out of the water and began to live on land.
But what reason prompted lobe-finned fish, which, presumably, felt quite good in the water, to leave their native element? Lack of oxygen? No. Even if there was not enough oxygen, they could rise to the surface and breathe clean air. After all, they also had lungs.
Maybe they were driven to land by hunger? No, either, because the land at that time was more deserted and poorer in food than the seas and lakes.
Perhaps there is danger? No, and not a danger, since lobe-finned fish were the largest and most powerful predators in the lakes of that era.
The search for water is what prompted the fish to leave the water! This sounds paradoxical, but this is exactly the conclusion that scientists came to after carefully studying everything possible reasons. The fact is that in that distant time, shallow freshwater reservoirs often dried up. Lakes turned into swamps, swamps into puddles. Finally, under the scorching rays of the sun, the puddles dried up. In order not to die, lobe-finned fish had to look for water. In search of water, the fish, which were able to crawl well along the bottom with their amazing fins, had to travel considerable distances on land. And those who crawled well and were better adapted to a land lifestyle survived. So gradually, as a result of harsh natural selection, the fish that were looking for water found a new home. They became inhabitants of two elements - water and land. Amphibians, or amphibians, evolved, and from them came reptiles, then birds and mammals. And finally, a man walked across the planet!
* That is, the owners of the notochord - an elastic string stretched from head to tail in the dorsal muscles of the animal. This supporting rod - the notochord - later developed into the spine. The first (still cartilaginous) vertebrae appeared in jawless fish four hundred million years ago.
