Most of the agnaths are now extinct, but fortunately there are still living representatives of this group: the mixines and the lampreys. Distributed in oceans, rivers, lakes, lagoons and streams, these eel-like specimens with a highly feared mouth cavity have very different characteristics from other fish. If you want to know more about them, we invite you to continue reading the following EcologíaVerde article where we tell you what the agnaths or jawless fish are and their characteristics .
What are agnaths or jawless fish?
Agnates or commonly called jawless fish, are a group of fish that have approximately 100 living species divided into two classes: Mixines and Hyperoartia . The other classes are extinct and have only fossil representatives. The agnaths together with the chondrichthyans (cartilaginous fish) and osteichthyans (bony fish) make up the fish group.
Characteristics of the agnathous fish
- They have an eel-shaped body with a circular section (the appearance of eels).
- They lack jaws and internal ossification, their structure is cartilaginous.
- They have pore-shaped gill openings.
- They have mucous , slippery, scale-free skin .
- They lack paired fins. They only have a single continuous odd fin.
- They have a pineal eye sensitive to light (except hagfish whose eyes have degenerated).
- The notochord is persistent (embryonic structure in all chordates that later degenerates).
- The digestive system does not have a stomach .
- They have a nerve cord with a developed brain. Some species with a small cerebellum.
- Hyperoartians are commonly known as lampreys and belong to the group of agnaths. Currently, all existing species belong to the order Petromyzontiformes and are grouped into three families .
- Lampreys have a sucker-shaped oral disc that along with the tongue have well-developed teeth. The shape of the oral cavity allows them to adhere to the prey on which they feed.
- There are parasitic species that tear flesh with their mouths and suck body fluids and inject an anticoagulant, increasing blood flow. When it is satisfied, it releases the prey, leaving a large wound. Those non-parasitic species do not eat when they become adults, since their digestive system degenerates, so they reproduce and die after a short time.
- Lampreys can inhabit both seas and freshwater bodies . Marine species are anadromous, that is, they leave the oceans and go up rivers and freshwater streams to spawn. The males are the ones who begin the construction of the nest using their oral discs to transport stones forming depressions. As the eggs are deposited by the females in the nest, they are fertilized by the male. The adults die after spawning.
- From the eggs emerges a larva called an ammocet that after a while is carried away towards slow waters where it remains from three to seven years becoming an adult.
Examples of lampreys
- Marine lamprey ( Petromyzon marinus ): it is distributed mainly in temperate zones of the Northern Hemisphere, in Europe and North America and can reach up to one meter in length.
- River lamprey ( Lampetra fluviatilis ): dark gray in color on the upper part of its body, it inhabits shallow waters of almost the entire European continent.
- Northern brook lamprey ( Ichthyomyzon fossor ) – is a non-parasitic filter-feeding species found in the United States and parts of Canada.
- Silvery lamprey ( Ichthyomyzon unicuspis ) – is a parasitic species distributed in the northern and central United States and southern Canada.
- Argentine lamprey ( Geotria macrostoma ): inhabits 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.
- Unlike lampreys, hagfish are exclusively marine and scavengers . They feed on dead fish, mollusks, annelids, and crustaceans. Despite having poorly developed eyes, thanks to their keen senses of touch and smell they can easily find their prey and stick with their mouths, tearing off pieces of tissue. To achieve greater strength, they can tie a knot in their own body, which leaves them anchored in the prey.
- Mixins have a large number of mucus-secreting glands around their bodies, making them completely slimy and impossible to capture.
- Its reproductive behavior is still unknown, but it is known that despite containing two gonads in a single individual , only one of them is functional.
- The females lay few large eggs with a lot of yolk and development is direct , there is no larval stage as in lampreys.
Examples of mixins
- Mixino ( Myxine glutinosa ): inhabits the coasts of the Atlantic Ocean up to 600 meters deep. It is carnivorous and nocturnal.
- Purple Hagfish ( Eptatretus stoutii ): It is a living fossil that lives in the depths of the Pacific Ocean.
- Slug eel ( Myxine affinis ): inhabits the Southwestern Atlantic Ocean, in the Strait of Magellan, southern Chile and southern Argentina.
- Notomyxine tridentiger : inhabitant of muddy bottoms of the Atlantic Ocean off the coast of South America (Argentina and southern Chile).
Extinct jawless fish
Most of the agnathus have become extinct and today we know about their existence thanks to the discovery of fossilized remains that have been found over time.
To be exact, nine of the eleven existing classes are no longer with us . One of the best-known classes is the ostracoderms , which lived during the Devonian, and had paired fins and characteristic bony armor.Fossils of these fish have been found in both saltwater and freshwater environments and they never cease to amaze us.