Fish is the largest group of aquatic chordates in terms of species diversity, which is also the most ancient. Fish inhabit almost all fresh and s alt water bodies. All their organ systems are adapted to living in the aquatic environment. According to the accepted scientific classification, Pisces are assigned to the domain Eukaryotes, the kingdom Animals and the type Chordates. Let's take a closer look at the superclass.
Body covers
The outer covering of the body of a fish is skin and scales. There are rare exceptions when the scales are missing or modified. The skin is divided into dermis and epidermis. The epidermis of the superclass Pisces is not keratinized.
It is the dermis that plays the main role in the formation of scales. The scales are different depending on the class of fish they belong to.
- Placoid scales are found in the Cartilaginous fish class. It consists of dentin covered with enamel. It is this kind of scales that in the course of evolution turned into the teeth of sharks and rays. Losing a Scale Link will not restore it.
- Ganoid scales are characteristicfor the sturgeon order. It is a bone plate coated with ganoin. Such a shell perfectly protects the body.
- Cosmoid scales are observed in lobe-finned and lungfish individuals. It consists of cosmine and dentin.
The coloration of individuals of the superclass Pisces can be very diverse. Representatives of the fauna can be either painted in one color, or be variegated, they can have a dull or, conversely, color that warns of danger.
Musculoskeletal system
The musculoskeletal system allows the fish to move and change position in the environment. The skeleton of a fish is different from that of a land animal. Her skull has over forty elements capable of moving independently. This allows the animal to stretch and spread its jaws, 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 of the fish. They are divided into paired (thoracic, abdominal) and unpaired (dorsal, anal, caudal). In bone representatives of the superclass, the fin consists of bone rays, which are united by a membrane. Muscles help to unfold, fold and fold it as the fish wants.
Swimming of the inhabitants of the aquatic environment is possible thanks to the muscles. They contract and the fish move forward. Musculature is divided into "slow" and "fast" muscles. The first are needed for calm swimming, drifting. The second is for fast and powerful jerks.
Nervous system of fish
The brain of fish is divided into sections. Each of them performs a specific function:
- The forebrain consists of the intermediate and final. The olfactory bulbs are located in this section. They receive signals from external organs of smell. Fish that actively use the scent while hunting have enlarged bulbs.
- The midbrain has optic lobes in its cortex.
- 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 circulation and a two-chambered heart. The circulatory system is closed, it conveys blood from the heart through the gills and body tissues. The fish heart does not at all separate oxygen-rich arterial blood from poor venous blood.
The chambers of the heart in fish follow each other and fill with venous blood. This is the venous sinus, atrium, ventricle, arterial cone. Blood is able to move only in one direction - from the sinus to the cone. Special valves help her with this.
Gas exchange organs in fish
Gills in fish are the main organ of gas exchange. They are located on the sides of the oral cavity. In bony fish, they are covered with a gill cover, in others they can freely open outward. When ventilation of the gills occurs, water passes into the mouth, then into the gill arches. After that, it again comes out through the holes in the gills of the fish.
The structure of the gills is as follows: they have semi-permeable membranes, penetrated by blood vessels, and are located on the bone arches. Gill filaments, pierced by the smallest network of capillaries, help the fish to feel even more freely under the water column.
Besides gill breathing, fish can use another method of gas exchange:
- Fish larvae can exchange gases through the surface of the skin.
- Some species have lungs that store humidified air.
- Some fish species can breathe air on their own.
How is the digestive system of fish?
Fish grab and hold food with their teeth, which are located in the mouth (like most vertebrates). Food enters the stomach through the pharynx through the esophagus. There it is processed by gastric juice and the enzymes contained in it. The food then moves into the intestines. The remains of it 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 also 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 during their life, for example, eating only plankton at a young age, and small fish when they are mature. There are also predatory fish that feed only on ectoparasites. They choose places where "cleaners" gather to hunt and eat them from the bodies of parasitized fish.
Excretory system of fish
The characterization of the superclass Pisces cannot be complete without a description of the excretory organ system. Life in the water leads fish to a number of problems with osmoregulation. Moreover, these problems are typical for freshwater and marine fish equally. Cartilaginous fish are isosmotic. The concentration of s alt in their body is lower than in the environment. Osmotic pressure levels out due to the high content of urea and trimethylamine oxide in the blood of fish. The cartilaginous class maintains a low concentration of s alt due to the work of the rectal gland and the excretion of s alts by the kidneys.
Bony fish are not isosmotic. In the course of evolution, they were able to develop a mechanism that traps or removes ions. Biology of the Chordata type helps the fish to bring the s alts out into the sea. This is because the fish are losing water. Chloride and sodium ions are excreted by the gills, while magnesium and sulfates are excreted by the kidneys.
Freshwater fish have exactly the opposite mechanism. The concentration of s alt 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 space by the gills.
Pisces superclass: how does reproduction work?
Fish have several types of reproduction. Let's consider each of them in more detail.
- Bisexual reproduction is the most common form. In this case, the two sexes of fish are clearly separated. Often this can be seen even by external signs (for example,color). Most often, males have secondary sexual characteristics. They can manifest themselves in the difference in the size of the body of the male and female, the difference in body parts (for example, a longer fin). Males in bisexual reproduction can be monogamous, polygamous, or have promiscuity.
- Hermaphroditism - in such fish, the sex can change during life. Protoandria are males at the beginning of life, then after the restructuring of the body they become females. Protogyny is a form of hermaphroditism where all males are transformed females.
- Gynogenesis is a breeding method for fish species represented only by females. It is rarely found in nature.
Fish can reproduce by viviparity, oviparous and ovoviviparous.
Class Bony fish
Superclass Fish are divided into two classes: Cartilaginous and Bony fish.
Bone fish are the most numerous group of vertebrates. They number more than 19 thousand species. Their skeleton is bony. In some cases, the skeleton may be cartilaginous, but then it is additionally strengthened. Bony fish have a swim bladder. There are over 40 squads in this class. Let's talk more about the most numerous.
- Sturgeon order includes ancient bony fish such as sturgeon, beluga, sterlet. They are distinguished by the presence of a snout and a mouth on the ventral side of the body. The mouth looks like a transverse slit. The basis of the skeleton is cartilage. Sturgeons live only in the Northern Hemisphere.
- Squad Herrings are marine schooling fish,feeding on plankton. Herring, herring, sardines, anchovies are commercial fish. They lay their eggs on the ground or algae.
- Squad Salmonformes - freshwater fish that lay their eggs on the bottom. They are found in the Northern Hemisphere. They are valuable commercial fish with tasty meat and caviar. The main representatives are salmon, chum salmon, pink salmon, trout, brown trout.
- Squad Cypriniformes are freshwater fish without jaw teeth. They crush their food with their pharyngeal teeth. The order includes commercial fish (roach, bream, tench, ide) and fish artificially bred in reservoirs (carp, white carp, silver carp).
- The lungfish detachment is the oldest detachment. They can breathe with gills and lungs (hollow outgrowths on the wall of the esophagus). They have adapted to life in hot countries and drying up water bodies. The prominent representatives of the order are the Australian horntooth and the American flake.
Cartilaginous fish
The main difference between cartilaginous and bony fish lies in the structure of the skeleton, the absence or presence of gill covers and a swim bladder. The class Cartilaginous fish is represented by the inhabitants of the seas, which have a cartilaginous skeleton throughout their life. Since there is no swim bladder, representatives of this class swim actively so as not to go to the bottom. As in sturgeons, the mouth looks like a transverse slit, there is a snout.
Cartilaginous fish include only two orders. These are Sharks and Rays. Sharks have a torpedo-shaped body, they are active swimmers and fearsome predators. Their powerful jaws are studded with sharp teeth. AtThis is where the largest sharks feed on plankton.
Stingrays have a flattened body with gills at the belly. 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 staple foods. About 60 million tons of fish are caught every year around the world. At the same time, herring, cod and mackerels are caught the most.
Recently, the fish catch has been noticeably declining. This is due to the deterioration of the environmental situation in the world. Stocks are being depleted due to overfishing, the destruction of some fish species, pollution of their spawning grounds, poisoning with s alts of heavy metals. Gradually, humanity is moving from unmanaged fishing to growing fish as a commercial object.
The best success in raising fish are farms that are rooted far back in history. They exercise full control over the cultivation of products from larvae to marketable products. Fish are bred in artificial ponds for various purposes: feeding, rearing, wintering, and so on. There are also special ponds for spawning. They are always small and warm well.
