Amphibians. Difference between amphibians and other animals Stages of development of amphibians

Reproduction and development of amphibians happens in water. Amphibians are dioecious animals. Fertilization in most species is external.

As soon as spring comes, aquatic animals awaken from winter torpor and begin to look for bodies of water warmed by the sun's rays. Males of some species of frogs develop special sacs called resonators in the corners of the mouth. (Fig. 192). They can swell and amplify sounds. In this way, males announce their presence by loud croaking. Therefore, females easily find spawning sites and soon appear there.

During the mating season, males of some species change the color of their entire body or individual parts of it. For example, a male sharp-faced frog, during this period acquires a bluish color, while the female remains brown (like the male after breeding) (Fig. 196, 2). This indicates that amphibians have developed color vision.

Female frogs lay ripe eggs in the water, and the male releases a liquid containing sperm onto them. The shells of the eggs are usually glued together (Fig. 193), Only fire-bellied toads lay eggs separately.

After some time, the outer shell of the egg swells and increases in volume. This helps the eggs stay near the surface of the water, where the temperature is higher. At night, when the water temperature drops, the temperature in the clutches, thanks to the swollen shells, remains several degrees higher. The upper part of the egg contains a dark pigment. It captures sunlight better, the embryo receives more heat and develops faster. In addition, this pigment blocks ultraviolet rays from the sun, which in large doses have a harmful effect on all living things. And finally, when the larvae hatch, for the first hours of their lives they stay on floating gelatinous shells at the surface of the water, where the temperature is higher.

Frog larva - tadpole (rice.193) — At first it looks like a fish larva. At first, she breathes with the help of external gills, which look like small tufts located on the sides of the head. A lateral line is visible on the skin. Amphibian larvae have many common characteristics with bony fish. For example, a two-chamber heart and one circulation.

During the first days, the tadpoles live off the yolk of the eggs. Over time, they develop a mouth and begin to feed on their own. First, they eat the gelatinous shells of the eggs, and then move on to feeding on small organisms (algae, protozoa, etc.). Teeth hidden under the fleshy lips help tadpoles obtain food (scrape from underwater objects). Then they move on to larger prey, catching small invertebrate animals in the water column. Material from the site

The tadpole grows rapidly, external gills are replaced by internal ones (Fig. 193). After some time, the tadpole’s limbs begin to develop. At first, only the hind limbs are noticeable, while the front limbs are hidden under a fold of skin covering the gill slits. Even later, tadpoles develop lungs, the heart becomes three-chambered, and two circulation circles are formed. The tail gradually shortens, and the forelimbs become noticeable. The tadpole turns into a frog. The period of development in water lasts 2-3 months, after which the frogs settle in their usual habitats on land. Thus, amphibians have indirect development, during which the body undergoes complex restructuring.

Amphibians- dioecious animals.

They are characterized by the following signs:

• fertilization is usually external;
• development is indirect, accompanied by significant restructuring of the body; Amphibian larvae have a number of features common to fish: gills, a two-chambered heart, one circle of blood circulation, a lateral line.

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Amphibians, as a rule, are characterized by a change in environment during development, since most of them retain the method of reproduction in water characteristic of fish by laying eggs and external fertilization. This most characteristic feature of amphibians leads to the fact that they go to a body of water during the breeding season.

The eggs of amphibians, like fish, have only mucous membranes that protect them from drying out only for a very short time, and the water necessary for the embryo to develop enters the developing egg from the environment. However, in amphibians, external fertilization has a number of features. The male always clasps the female in one way or another. How do males develop adaptations to a tighter grip?various outgrowths, spines, “calluses,” etc., most often smoothed out or disappearing at the end of the breeding season. Males, compared to females, have more developed muscles of the front legs and a heavier skeleton. Such mating of amphibians during external fertilization ensures the simultaneous release of eggs and sperm, which increases the percentage of egg fertilization.

Metamorphosis, like development in the aquatic environment and external fertilization, is to a certain extent inherited by amphibians from their fish-like ancestors. But, unlike fish, the respiratory, locomotion and digestive organs of amphibian larvae are different from those of adults. This is due to the fact that in fish the adult organism and the fry live in the same environment, while in amphibians the habitat changes during the development process.

In this regard, in the larvae of amphibians, which have a number of characteristics of aquatic animals, already in the early stages of development, long before the end of metamorphosis, characteristics characteristic of a terrestrial organism begin to develop. In frogs, about a month before the end of metamorphosis, the rudiments of the fore and hind limbs appear. At the same time, the tadpoles already have internal nostrils breaking through, there is a glottis covered by the epiglottis, a larynx and paired thin-walled vascular sacs - the lungs. Soon a septum appears in the atrium and a pulmonary circulation is formed, etc. In other words, the completely aquatic larva already has the features of an adult terrestrial organism.

Among living amphibians, due to the peculiarities of their organization and way of life, there is significant diversity in the biology of reproduction.

Reproduction and development of amphibians

Reproduction of amphibians

Amphibians breed in shallow, well-warmed areas of water bodies. On warm spring evenings, loud croaking sounds can be heard from ponds and rivers. These "concerts" are staged by male frogs to attract females.

The reproductive organs in male amphibians, like in fish, are the testes, and in females the ovaries. They are located in the body cavity, and by the time of reproduction they increase many times over. Eggs that mature in the ovaries enter the oviducts. While moving through the oviduct, the eggs are covered with a transparent mucous membrane and are released through the cloaca. The clutch of a grass frog contains up to 1.5 thousand eggs. In males, the testes, which are oval in shape, secrete many motile sperm. Seminal fluid, rich in sperm, enters the cloaca through the vas deferens and is released out. Fertilization in amphibians is external. Clusters of fertilized eggs attach to aquatic plants or float in separate clumps at the surface of the water. Ribbons of fertilized eggs toads, like individual eggs newts, stick to the leaves of aquatic plants.

Amphibian development

Let's look at the development of amphibians using the example frogs . The development of its embryo in the egg lasts about one and a half weeks. Then the embryo breaks the egg shell and comes out. By appearance and lifestyle tadpole looks like a fish. It has gills, a two-chambered heart and one circulatory system, and lateral line organs.

During development, important changes occur in the tadpole. The hind limbs develop first, and then the front limbs. Lungs appear, and the tadpole rises more and more often to the surface of the water to breathe. In connection with the development of the lungs, a second circle of blood circulation is formed, the heart becomes three-chambered. The tail gradually becomes smaller. The tadpole becomes like an adult frog. The little frog switches from plant food to the consumption of animal food (becomes carnivorous ) and leaves the pond. From the time the eggs are laid until the tadpole turns into a frog, 2-3 months pass.

Adult frogs no longer have a tail. They begin to reproduce at the age of 3-4 years.

Caring for offspring in amphibians

Most frogs and toads lay their eggs in water and leave them unattended. However, many species of amphibians exhibit interesting forms of caring for their offspring. Yes, female Surinamese pipa hatches eggs in skin cells on the back. Male Darwin's rhinoderma stores eggs in a special throat pouch. Woody snapping frogs , living in Africa, lay eggs in a foamy secretion on the leaves of trees located above the water. The hatched tadpoles fall straight into the water. Female dart frog bicolor lays eggs in water-filled leaf axils. The larvae emerging from the eggs stick to the back of the male, who carries them to “individual” reservoirs.

The influence of seasonal changes in nature on the life of amphibians

The annual life cycles of amphibians are well expressed in temperate latitudes with sharp seasonal changes in living conditions.

When the average daily temperature drops to +12 ... +8 °C, amphibians move to their wintering places, and when the temperature drops further in September - early October, they hide in shelters. In search of wintering sites, individual individuals move many hundreds of meters.

Ozernaya, pond And grass frogs They winter in reservoirs, gathering several dozen individuals together, hiding under stones, among aquatic plants, and burying themselves in silt. They choose the deepest areas where reservoirs do not freeze to the bottom.

Toads, toads, newts, salamanders winter on land: they climb into holes, rodent holes, hide in the dust of rotting stumps, under stones, etc. In severe winters with little snow, when the ground freezes to great depths, more amphibians wintering in it die than those wintering in reservoirs, since When body temperature drops below -1 °C, amphibians die.

Wintering amphibians are in daze: their metabolism sharply decreases, oxygen absorption decreases by 2-3 times, and the number of respiratory movements and heart contractions decreases.

In the spring, with the onset of warmth, at the end of March and in April, amphibians switch to an active lifestyle, leave their wintering places and head to their breeding grounds. These spring movements take place quite amicably, the animals overcome hundreds of meters, reaching shallow, well-warmed reservoirs by the sun.

After breeding brown frogs, toads, tree frogs move to their usual summer habitats in meadows, fields, vegetable gardens, orchards, etc. Tritons And fire toads spend another 2-3 months in reservoirs, and then move to land.

Interactive lesson-simulator (Go through all pages of the lesson and complete all tasks)

Amphibians are dioecious animals that reproduce in water. Females lay eggs, males secrete seminal fluid. Fertilization is external. Development with metamorphosis: the eggs hatch into fish-like tadpoles, which develop into adult amphibians during development. The lifestyle of amphibians changes depending on seasonal changes in living conditions.

Reproductive organs of amphibians

The reproductive organs of amphibians are very similar in structure to the reproductive organs of fish. All amphibians are dioecious.

Spawning of amphibians

After spending the winter in a state of torpor, amphibians wake up with the first rays of the spring sun and soon begin to reproduce. Males of some species of frogs croak loudly. Special bags help to amplify sounds - resonators, which swell on the sides of the male’s head when croaking. When breeding, animals split into pairs. The germ cells enter the cloaca through tubular ducts and are thrown out from there. Female amphibians lay eggs in the water, similar to fish eggs. Males release fluid containing sperm onto her.

Amphibian development

After some time, the shell of each egg swells and turns into a gelatinous transparent layer, inside of which the egg is visible. The upper half is dark and the lower half is light: the dark part of the egg uses the sun's rays better and heats up more. Clumps of eggs in many frog species float to the surface where the water is warmer.

Figure: Frog Development

Low temperature retards development. If the weather is warm, the egg divides repeatedly and develops into a multicellular embryo. After one or two weeks, the frog larva hatches from the egg - tadpole. Outwardly, it resembles a small fish with a large tail. The tadpole first breathes through external gills (in the form of small tufts on the sides of the head). Soon they are replaced by internal gills. The tadpole has one circulation and a two-chambered heart; a lateral line is visible on the skin. Thus, amphibian larvae have some structural features of fish.

During the first days, the tadpole lives off the nutritional reserves of the eggs. Then a mouth appears, equipped with horny jaws. The tadpole begins to feed on algae, protozoa and other aquatic organisms. The hotter the weather, the faster the tadpole changes. First his hind legs appear, then his front legs. The lungs are developing. The tadpole begins to rise to the surface of the water and swallow air. The tail gradually shortens, the tadpole becomes a young frog and comes ashore. From the moment the eggs are laid until the end of the transformation of the tadpole into a frog, about 2-3 months pass. Baby frogs, like adult frogs, eat animal food. They can reproduce from the third year of life.

Origin of amphibians

The close connection of amphibians with water, as well as the structure and lifestyle of their larvae, indicate the origin of these animals from fish. It was possible to find the fossilized remains of extinct amphibians. Their skin had scales, and their skull resembled the skull of lobe-finned fish.

Figure: Ancient lobe-finned fish and amphibian

Scientists have found that the first amphibians appeared more than 300 million years ago. Their ancestors were freshwater lobe-finned fish. Comparison of the skeleton of the fins of extinct lobe fins with the skeleton of the limbs of amphibians indicates their great similarity. It is believed that extinct freshwater lobefingers had lungs. They lived in small lakes and rivers and could crawl from one body of water to another with the help of their muscular fins. From these fish came the first land vertebrates - ancient tailed amphibians. Tailless amphibians appeared later and descended from ancient tailed amphibians.

200 million years ago, the Earth was covered with huge swamps. This period was the most favorable for the development of amphibians. Many of them reached a length of 5-6 m (the largest modern amphibian, 1.5 m long, is the giant salamander living in Southeast Asia).

REPEATING THE NECESSARY KNOWLEDGE

What allows amphibians to live on land? (§ 41–42)

The following adaptations help amphibians live on land: body shape, lever-like limbs attached to the spine, pulmonary and cutaneous respiration, glandular skin.

How do fish reproduce? (§ 37)

Fish are characterized by sexual reproduction. Fertilization is often external, less often internal (cartilaginous fish). During spawning, females spawn and males release milk. Fertilization occurs in the aquatic environment, after which embryos form in the eggs.

APPLYING KNOWLEDGE

1. Where and how do amphibians reproduce?

Amphibians reproduce in an aquatic environment. The ovaries and testes of amphibians and fish are similar. External fertilization occurs in water. The male immediately waters the eggs with milk, so almost all the eggs are fertilized. Amphibian eggs have large reserves of nutrients compared to fish.

2. What conditions do amphibians prefer?

Amphibians are cold-blooded animals. Therefore, their life depends on the ambient temperature. Amphibians prefer shady places and warm, food-rich water bodies. The active life of frogs on land is limited to periods with temperatures above +100 C. For the winter, they hide at the bottom of deep, frost-free ponds and streams, where they spend many months in a stupor, without food and pulmonary respiration.

3. What does the annual cycle of amphibians known to you consist of?

At the beginning of autumn, when average daily temperatures drop to +12 ... +8 °C, amphibians move to their wintering places, and with a further drop in temperature in September - early October, they hide in shelters.

In the spring, with the onset of warmth, at the end of March and in April, amphibians switch to an active lifestyle, leave their wintering places and head to their breeding grounds. These spring movements take place quite amicably, the animals overcome hundreds of meters, reaching shallow, well-warmed reservoirs by the sun. After breeding, brown frogs, toads, tree frogs move to their usual summer habitats in meadows, fields, vegetable gardens, orchards, etc.

4. What role do amphibians play in ecosystems?

Amphibians are links in food chains. Amphibians are food for many birds and fish. Amphibians themselves regulate the number of insects and slugs by eating them.

6. What reproductive traits of amphibians limit their distribution?

Amphibians breed only in the warm season, since reproduction depends on water temperature. This limits the distribution of amphibians in cold latitudes.

7. What amphibians live in your area?

8. In what ways and why does the development of a tadpole deviate from the course of evolution?

The emerging tadpole is very different from the adult frog; he has predominantly fish-like features. The tadpole in its early stages develops like a fish. It has a flattened body, a caudal fin, gills, and a lateral line. With further growth, the tadpole deviates from the development of fish characteristics. It begins to develop the features of an amphibian - the body changes shape, the gills become overgrown, lungs are formed, and limbs are formed.