Wild fish are overwhelmingly bony fish, while sharks are cartilaginous fish. In addition to bony fish and cartilaginous fish, there is a small population of primitive fish, and that is jawless fish. This population includes hagfish and lamprey eels, both of which are the most primitive fishes in existence.
They are slender, scaleless, and have mucus glands attached to them. The skeleton is made up of cartilage and has a chordochal cord, and the heart is visible to the atrium and ventricles. Lampreys have 7 pairs of holes, with the gills of the hagfish between 1-16 pairs.
Lamprey eels have no stomach, a spiral-shaped intestine can be seen in the body, they are very old vertebrates, their ancestors can be traced back hundreds of millions of years, belong to very low fish.
Jawless fish with no vision at all – the hagfish
This fish is visionless, very gluttonous, and in today's class of jawless vertebrates, it exhibits the following characteristics: only one nostril, no organs are not paired, no sympathetic nervous system, and no spleen and scales. Its tail is nearly rounded , and there are many finger-like sensory tentacles around its mouth.
There are triangular teeth side by side in the mouth, very sharp (one tooth is located at the base, the rest of the teeth grow on the tongue). The ventral side of the body is covered with an undivided oblique muscle, which gives them a strong torsion ability to twist the body at will. The circulatory system has a spacer system that connects the main heart organs and various adnexal organs with arteries and veins. The genus of this fish can be distinguished by differences in the number of teeth and gill sacs.
In general, their gill sacs do not connect directly to the outside of the body, but into a long tube, with separate openings on each side connecting the outside of the body. The Japanese species of the genus Parablypter eel did not have this tube, replaced by 16 pairs of gill fissures distributed on both sides. Similarly, the number of gill fissures on both sides of the genus Sticky hawks ranges from 5 to 14 pairs. The longest hagfish recorded is up to 80 cm long (Atlantic hagfish).
Hagfish eat uniquely
The plate at the base of the mouth of the hagfish can be moved back and forth by the movement of the traction and traction muscles. As soon as they come into contact with food, the gum plates begin to retract, driving the teeth to chew the food up and deliver it to their mouths. The movement is very rapid and continues throughout the feeding process. A knot is tied at the back of the body with each bite, and as a fulcrum, the mucus wrapped around the body is used to slide it to the height near the mouth, and then the knot is broken up along the body. These are repetitive actions, and they are very fast.
The habitat and habits of hagfish
Hagfish inhabit temperate or boreal brackish waters, with active water depths ranging from 30 to 1000 m. They are a very rare animal breed, and when they are threatened, many pores on the side of the body secrete a large amount of sticky substances, so they were also called "sticky eels". In some areas, hagfish are abundant, preferring to stay at the bottom of the mud or clay and bury their bodies in them, exposing only their mouths or tentacles. They look for food only through their sense of smell.
Once the food in front of you is locked, it will jump out of the mud and rampage like a snake towards its prey. They are greedy and fast,and feed mainly on carrion, dying fish, and benthic creatures. When they fall into the net, they burrow into the bodies of other fish and devour their internal organs, causing a very large loss to humans.
The large amount of mucus secreted by the body of a hagfish can cause the victim to suffocate, in addition to helping it to invade smoothly. It is worth noting that because hagfish are in vitro fertilized fish, the number of eggs and sperm produced during their reproduction is very small. In the same species of hagfish, we can see both females and males, as well as hermaphrodite individuals and even infertile individuals.
Lamprey with a strange mouth
The eel has a funnel-shaped mouth with short legs around it, a cylindrical tongue that relies on powerful muscular tissue for flexible movements, teeth and glands in its mouth that prevent blood from clotting in its prey. When they eat food, their mouths are like suction cups, their tongues make telescopic movements, and they use their teeth to scrape in order to suck.
As the lamprey swims, water is poured from the mouth down the pharynx into the sac and then discharged from the fissure. As they hold their prey with their suction cup-like mouths, water flows in and drains from the gill fissures to the body under pressure from the muscle tissue. Their gills are independent, unbouncted, and sac-shaped like gills of bony fish.
The mucosal cells of the lamprey eel are distributed under the skin and are relatively small in size and numerous. Inside the body of the lamprey, muscle cells are distributed in various positions throughout the body, overlapping and arranged in a "W" shape, and moving in a wave-like manner when swimming. The unpaired fins on the outside of the body increase the area of the body surface of the lamprey eels that are hermaphroditic. Males of some species are the first to reach spawning sites (located upstream of rivers) to build nests, where they move the stones to form a nest where the female can enter vertically and lay eggs.
When the nest of the lamprey is invaded, it is usually invaded by a "couple", and the male fish stationed there will drive the intruder out. During spawning, the male will hold the female with his head or a whip and wrap his tail around the female's body so that the cloaca are juxtaposed, followed by a period of intense vibration before spawning and fertilization begins.
The jawless fish, which dates back 400 million years to date, is considered a living fossil of fish, and this species of fish is now only a few species of hagfish and lamprey, most of which are extinct.