Sharks - cartilaginous fish with a streamlined body. It is believed that they evolved about 420 million years ago before the dinosaurs. Currently, 440 species of sharks are known in size from a dwarf lantern shark only 17 cm long to the largest whale shark that reaches up to 12 meters and is a filter. They are completely sea located at a depth of 2000 meters and are absent in fresh waters. Bull sharks and river sharks are exceptions, as they can grow well in both freshwater and seawater. Sharks have seven pairs of gill breathing holes. The skin is supplied with dermal teeth that protect the fish from parasites, damage and improve the fluid dynamics for fast swimming. There are several sets of teeth that can be replaced. The great white shark, tiger shark, hammerhead sharks are the best predators of the underwater food chain. They also scare people with their amazing views and activities.
Until the 16th century, sharks were known as sea dogs. Shark teeth are deeply embedded in the gums, and not attached to the jaws and replaced during life. Some sharks are known to lose over 30,000 teeth during their lifetime. Teeth are replaced every 8-10 days. The shape of the teeth depends on the feeding habits, since sharks feeding on mollusks and crustaceans flatter their teeth for crushing, while those who eat fish have pointed teeth to grip and cut. The teeth of a basking shark feeding on plankton have small and non-functional teeth. The skeleton of sharks is different from the skeleton of fish and other vertebrates. Sharks and other cartilaginous fish have a skeleton consisting of cartilage and connective tissue. Cartilage is flexible and less dense than bone. It reduces body weight and offers fast swimming.
Like rays and skates, the jaws in a shark are not attached to the skull. Jaws provide support for gill arches and reduce stress. The jaws have a hexagonal layer consisting of calcium salts located in a mosaic called tessera. They provide support for a bone-like body in other vertebrates. As a rule, one layer of tessera is present in sharks, but bull sharks and tiger sharks have two layers. In the stands, the cartilage is soft and spongy and acts as a shock absorber. The fingers are elongated and supported by soft, non-segmented rays of the ribs, called ceratotrihies, which are actually strands of protein and tumor keratin in hairs and feathers. Chips do not allow sharks to move in the tail direction. Sharks have a complex corset, consisting of flexible collagen fibers arranged in a spiral. It acts as an exoskeleton that supports the muscles of swimming and thus saves energy. In the past, shark skin has been used as wallpaper. Dermal teeth help maintain hydrostatic balance.
Shark tail shape varies according to habitat. The tail provides stress and helps to swim fast. The tail is characterized by the presence of a heterocercal tail fin, in which the dorsal part is somewhat larger than the ventral part of the fin. This is due to the fact that the vertebral column extends to the dorsal part, providing a large surface area for attaching muscles. This feature helps maintain buoyancy while swimming. Bony fish, on the other hand, have homocercal tails. Sharks, like bony fish, do not have a swim bladder to maintain buoyancy. For shark buoyancy, they depend on a large liver, which is filled with oil containing squalene. The liver is responsible for 30% of shark's body weight. The liver also contains a dynamic lift, which helps in swimming. Like other fish, sharks extract oxygen from seawater and pass it through the gills. The gills are not covered by any coating, but are present in the row immediately behind the head. Spiracle is present right behind the eyes and helps the water during breathing and is important in benthic species. Respirations are either absent or reduced in pelagic forms. While floating water moves through the mouth, it is then passed through the gills through a process called ventilating ventilation. At rest, sharks pump water over the gills to ensure a constant supply of oxygen. The heart consists of two chambers in sharks. The heart pumps blood into the gills through the ventral aorta, and then flows into afferent gill arteries. Re-oxygenation of the blood occurs through the efferent gill arteries. Then deoxygenated blood penetrates through the cardinal veins, and then into the cardinal sinus and finally into the ventricle. The whole cycle repeats.
Sharks - poikilothermic animals. Their blood and other tissues are isotonic for their marine environment due to the presence of urea and trimethyl N-oxide, helping them maintain osmotic balance in seawater. Digestion takes time. Food moves to a J-shaped stomach, where food is stored and initial digestion takes place. The intestine is short and equipped with a spiral valve that provides a large surface area for digestion. Artificial meaning in them is very developed. The eyes are similar to those of other vertebrates, consisting of the cornea, lens and retina, as well as a layer of tapetum lucidum, which helps in clear vision. The sense of hearing is also well developed, and they can be heard from considerable distances. Ampoules of Lorenzini are electroreceptors. The lateral linear system is well developed in sharks, and it helps in detecting vibrations. Sharks have an average lifespan of 20-30 years.
Sharks under internal fertilization. In males, the pelvic fins are transformed into clusters, which act as intracratic organs to transfer sperm in females. Mating is rare in sharks. The man tightly holds the female and transmits sperm. Parthenogenesis has been observed in some species. Sharks show three ways of breeding broods, such as the ovipositor, liveliness and agility.
Currently, they are at risk due to unlawful killing for commercial production of leather and other fancy products. Some species have become endangered. To save them, you must take the necessary steps.
We will save our valuable sharks.
