Distributed in oceans, rivers, lakes, lagoons, and streams, these eel-like specimens with a feared oral cavity have very different characteristics from other fish. We will tell you about jawless fish if you want to know more about them.
What are agnathans or jawless fish?
The jawless fish are a group of approximately 100 living species distributed in two classes.
- Mixins
- Hyperoartia.
Jawless fish Characteristics
- Jawless fish has an eel-shaped body with a circular section (eel-like appearance).
- Fish have pore-shaped gill openings.
- Jawless fish have a mucous, slippery skin, free of scales.
- They lack paired fins and a single continuous odd fin.
- They have a light-sensitive pineal eye (except hagfish, whose eyes have degenerated).
- The notochord is persistent (an embryonic structure in all chordates that later degenerates).
- Its digestive system does not have a stomach .
- Some species have a small cerebellum.
Jawless fish species
Lampreys
All species are divided into three families.
Lampreys have a sucker-shaped oral disk that has well-developed teeth along with their tongue. The shape of their oral cavity allows them to attach themselves to the prey they feed on.
There are parasitic species that tear the flesh with their mouths and suck the body fluids and inject an anticoagulant, increasing the blood flow. When satisfied, they release their prey, leaving a large wound.
Non-parasitic species do not feed when they become adults, as their digestive system degenerates, so they reproduce and die shortly after.
Lampreys can live in both oceans and freshwater bodies. Marine species leave the oceans and travel up rivers and freshwater.
Males build the nest using oral discs to transport stones into depressions. Females deposit eggs in the nest; the male fertilizes them. Adults die after spawning. From the eggs emerge a larva called an ammonite. After a while, it is carried to slow-moving waters, where it remains for three to seven years, becoming an adult.
Examples of lampreys
Sea lamprey ( Petromyzon marinus ):
Sea lamprey is distributed mainly in the Northern Hemisphere’s temperate zones, Europe, and North America. Lamprey can reach up to one meter in length.
River lamprey ( Lampetra fluviatilis )
Dark grey on the upper part of its body. It lives in shallow waters throughout almost the entire European continent.
Silver lamprey ( Ichthyomyzon unicuspid )
Silver lamprey is a parasitic species distributed in the northern and central United States and southern Canada.
Argentine lamprey ( Geotria macrosomia )
lives in lakes and rivers such as the Patagonian rivers of South America and then migrates to the sea, where it is a parasite of bony fish.
Mixins
They feed on dead fish and crustaceans. They can easily find their prey and attach themselves to it with their mouths, tearing off pieces of tissue. For greater strength, they can tie a knot in their own body, which leaves them anchored to the prey.
Females lay a few large eggs with a lot of yolk, and development is direct; there is no larval stage as in lampreys.
Examples of mixins
Hagfish ( Myxine glutinosa ): lives on the coasts of the Atlantic Ocean at depths of up to 600 meters. It is carnivorous and nocturnal.
Purple hagfish ( Eptatretus stout )
Purple hagfish is a living fossil in the Pacific Ocean’s depths.
Slimy eel ( Myxine affinis ):
Slimy eel lives in the southwestern Atlantic Ocean, in the Strait of Magellan, southern Chile, and southern Argentina.
Notomyxine tridentiger inhabitant of muddy bottoms of the Atlantic Ocean on the coasts of South America (Argentina and southern Chile).
Exactly nine of the eleven extant species are no longer with us. One of the best-known species is the ostracoderm, which lived during the Devonian and had paired fins and a characteristic bony armor. This is found in saltwater and freshwater environments and always amazes us.