Fish is the largest group of aquatic chordal animals in terms of species diversity, which is also the oldest. Fish inhabit almost all fresh and salty water bodies. All their organ systems are adapted to live in the aquatic environment. According to the accepted scientific classification, Pisces belong to the Eukaryotic domain, the Animal Kingdom, and the Chordata type. Let's take a closer look at the superclass.
Integument
The outer cover of the body of the fish is the skin and scales. Rare exceptions occur when the scales are missing or mutated. The skin is divided into dermis and epidermis. The epidermis of the superclass Pisces is not keratinized.
In the formation of scales, the main role is played by the dermis. Scales vary depending on the class of fish to which it belongs.
- Placoid scales are found in the class Cartilaginous fish. It consists of enameled dentin. It is this kind of scale that has evolved into shark and stingray teeth during evolution. If the link of the scale is lost, it will not be restored.
- Ganoid scales are characteristic of the sturgeon order. It is a bone plate coated with ganoin. Such a shell perfectly protects the body.
- Cosmoid scales are observed in cysterae and double-breathing individuals. It consists of cosmine and dentin.
The coloring of individuals of the superclass Pisces can be very diverse. Representatives of the fauna can either be painted in the same color, or be variegated, can have a dull or, conversely, warning of danger color.
Musculoskeletal system
The musculoskeletal system allows the fish to move and change position in the environment. The skeleton of a fish is different from the skeleton of a land animal. Her skull has more than forty elements that can move independently. This allows the animal to stretch and extend its jaw sometimes very widely.
The spine consists of individual vertebrae that are not fused together. It is divided into the trunk and tail sections. When swimming, the driving force is created by the fin in the fish. They are divided into paired (thoracic, abdominal) and unpaired (dorsal, anal, caudal). In bony representatives of the superclass, the fin consists of bone rays that are joined by the membrane. Muscles help to expand, fold and collapse it, as you like fish.
Swimming inhabitants of the aquatic environment is possible thanks to the muscles. They contract and the fish moves forward. The muscles are divided into “slow” and “fast” muscles. The first ones are needed for calm swimming, drifting. The second - for fast and powerful jerks.
Fish nervous system
The brain of fish is divided into departments. Each of them performs a specific function:
- The forebrain consists of the intermediate and final. The olfactory bulbs are located in this section. It is they who receive signals from the external organs of smell. Fish that actively use the scent while hunting have enlarged bulbs.
- The midbrain in its cortex has optical lobes.
- The hindbrain is divided into the cerebellum and medulla oblongata.
The spinal cord of representatives of the superclass Pisces runs along the entire length of the spine.
Circulatory system
Most representatives of the superclass have one circle of blood circulation and a two-chamber heart. The blood supply system is closed, it transfers blood from the heart through the gills and body tissues. The heart of fish does not completely separate oxygen-rich arterial blood from poor venous blood.
The heart chambers of fish go one after another and are filled with venous blood. This is a venous sinus, atrium, ventricle, arterial cone. Blood is able to move in only one direction - from the sinus to the cone. In this, special valves help her.
Gas exchange organs in fish
Gills in fish - the main organ of gas exchange. They are located on the sides of the oral cavity. In bone fish, they are covered by a branchial lid, in others, they can freely open outwards. When the ventilation of the gills occurs, water passes into the mouth, then into the gill arches. After that, she again goes outside through the gills of the fish.
The structure of the gills is as follows: they have semipermeable membranes penetrated by blood vessels and are located on the bone arches. Gill petals, pierced by the smallest network of capillaries, help fish to feel even more freely under the thickness of the water.
In addition to gill breathing, fish can use another gas exchange method:
- Fish larvae can exchange gas through the surface of the skin.
- Some species have lungs that retain moist air.
- Some species of fish can breathe air on their own.
What is the digestive system of fish?
Fish grab and hold food with their teeth, which are located in the oral cavity (as in most vertebrates). Through the throat through the esophagus, food enters the stomach. There it is processed by gastric juice and the enzymes contained in it. Then the food moves into the intestines. Its remains are thrown out through the cloaca (anus).
What do the inhabitants of the aquatic environment eat? The choice is very wide:
- Herbivorous fish eat algae and aquatic plants. Some of them can feed on plankton (for example, silver carp).
- Predatory fish can feed on plankton, various worms, mollusks, crustaceans, and, of course, other, smaller fish.
- Some fish can change their taste preferences throughout life, for example, at a young age, eat only plankton, and in mature ones, small fish. There are also predatory fish that feed only on ectoparasites. They choose for hunting places of accumulation of "cleaners" and eat them from the bodies of parasitized fish.
Fish excretory system
The characterization of the superclass of Pisces cannot be complete without a description of the excretory system of organs. Life in water leads fish to a number of problems with osmoregulation. Moreover, these problems are characteristic of freshwater and marine fish equally. Cartilaginous fish are isosmotic. The concentration of salt in their body is lower than in the environment. Osmotic pressure is equalized due to the high content of urea and trimethylamine oxide in the blood of fish. The cartilage class maintains a low concentration of salt due to the rectal gland and kidney excretion.
Bony fish are not isosmotic. During evolution, they were able to develop a mechanism that traps or removes ions. A biology such as Chordata helps fish bring salt out into the sea. This is because fish lose water. Chloride ions and sodium ions excrete gills, and magnesium and sulfates - excrete kidneys.
Freshwater fish have the exact opposite mechanism. The concentration of salt in the body of such creatures is higher than in the environment. Their osmotic pressure is equalized due to the release of a large amount of urea and the capture of the necessary ions from the water by the gills.
Superclass Pisces: How does reproduction occur?
Fish have several types of breeding. Let's consider each of them in more detail.
- Bisexual reproduction is the most common form. In this case, the two sexes of the fish are clearly separated. Often this is visible even by external signs (for example, color). Most often, males have secondary sexual characteristics. They can manifest themselves in the difference in the body sizes of the male and the female, in the difference in body parts (for example, a longer fin). Males during bisexual reproduction can be monogamous, polygamous or have random chaotic relationships (promiscuity).
- Hermaphroditism - in such fish, the sex is able to change throughout life. Protoandria at the beginning of life are males, then after the restructuring of the body they become females. Protoginia is a form of hermaphroditism when all males are transformed females.
- Gynogenesis is a breeding method for fish species represented only by females. It is rare in nature.
Fish can breed by live birth, egg production and egg production.
Bone fish class
Superclass Fish are divided into two classes: Cartilaginous and Bone fish.
Bone fish are the largest group of vertebrates. There are more than 19 thousand species. Their skeleton is bone. In some cases, the skeleton may be cartilaginous, but then it is additionally strengthened. Bone fish have a swim bladder. There are over 40 units in this class. We will tell you more about the most numerous.
- The sturgeon-like squad includes ancient bone fish such as sturgeon, beluga, and sterlet. They differ in the existing snout and mouth on the ventral side of the body. The mouth has the form of a transverse gap. The base of the skeleton is cartilage. Sturgeons live only in the Northern Hemisphere.
- The herring-like squad is a marine schooling fish feeding on plankton. Herring, herring, sardines, anchovies - commercial fish. They lay eggs on the ground or algae.
- Order Salmonid - freshwater fish that lay eggs on the bottom. They are found in the Northern Hemisphere. They are valuable commercial fish with delicious meat and caviar. The main representatives are salmon, chum, pink salmon, trout, trout.
- Order Carp-shaped - this is freshwater fish without jaw teeth. They grind food with pharyngeal teeth. The order includes commercial fish (roach, bream, tench, ide) and fish artificially bred in reservoirs (carp, grass carp, crucian carp).
- Squad Breathing - the oldest squad. They can breathe with gills and lungs (hollow outgrowths on the wall of the esophagus). They adapted to life in hot countries and drying ponds. Bright representatives of the squad are the Australian horned tooth and American flake.
Cartilaginous fish
The main difference between cartilage and bone fish lies in the structure of the skeleton, the absence or presence of gill covers and swimming bladder. The class of cartilaginous fish is represented by the inhabitants of the seas, which have a cartilaginous skeleton throughout their lives. Since there is no swimming bladder, representatives of this class actively swim, so as not to go to the bottom. As in sturgeon-like, the mouth has the appearance of a transverse gap, there is a snout.
Cartilaginous fish include only two orders. These are Sharks and Stingrays. Sharks have a torpedo-shaped body shape, they are active swimmers and scary predators. Their powerful jaws are strewn with sharp teeth. In this case, the largest sharks feed on plankton.
Stingrays have a flattened body with gills near the abdomen. The fins of the fish are greatly enlarged. Stingrays feed on bottom animals and fish.
Use and protection of fish resources
Fish is of great importance in human life, being one of the main food products. About 60 million tons of fish are caught every year around the world. At the same time, herring, cod and mackerel are most often caught.
Recently, the catch of fish is markedly reduced. This is due to the deterioration of the environmental situation in the world. Stocks are being depleted due to excessive fishing, the destruction of certain fish species, pollution of their spawning sites, and poisoning by salts of heavy metals. Gradually, humanity is moving from uncontrolled fishing to growing fish as a fishing target.
The best success in growing fish have farms rooted in history. They exercise full control over the cultivation of products from larva to commercial products. Fish are bred in artificial ponds for various purposes: feeding, growing, wintering, and so on. There are also special ponds for spawning. They are always small in size and warm well.