Agnathus are fish that began to exist 470 million years ago. Although most have become extinct, some types still survive that feed in a unique way. They can be found in waters around the world and share few characteristics with other marine animals.
In this article on our site we introduce you to the agnate or jawless fish, we talk about their characteristics with examples and images. Find out what they are!
What is an agnathus (Agnatha)?
Before showing you the species that exist, it is necessary to define what agnates (Agnatha) are. They are jawless fish from the Paleozoic era They appeared on the planet 470 million years ago and became extinct 370 million years ago, although some species still survive.
Agnathians are the third group included in the common classification of fish. Along with them are the cartilaginous or chondrichthyan fish (Chondrichthyes), such as rays, sawfish and torpedo fish, and sharks, and the bony or osteichthyan fish (Osteichthyes), a group that includes all those species that have gills and a bladder. swimming.
When the first vertebrates began to appear, initially the fish we know today as agnathus developed. Over time, they disappeared to make way for species with more defined and resistant bone formations. The first of these were the chondrichthyans and, later, the bony fishes appeared.
Characteristics of agnathic fish
We have already seen that the group that includes the jawless fish is the one called "agnate", whose scientific name is Agnatha. Now, let's see its main features. In ancient times, the morphology of these species was very varied, although they shared the fact that they were among the first vertebrates, possessing external structures similar to armour, as well as fins in some species. The characteristics of modern agnathic fish are similar, since they share something more than the fact that they do not have a jaw.
A common trait is an eel-like appearance, i.e., elongated body without scales or fins, as well as a skin mucosa and light-sensitive eyes. The skeleton is cartilaginous and lacks an occipital region, while the gills are sac-shaped.
Agnathans parasitize other species of fish or feed on carrion with particular methods such as suction. Today, two kinds of agnathans or jawless fish survive: lampreys (41 varieties) and the haggars (31 varieties).
Lampreys: what they are and characteristics
Hyperoartios (Hyperoartia) are jawless fish commonly known as lampreys. The body is similar to that of eels: elongated, flexible and thin. They can live in fresh or s alt water, depending on the subspecies.
The lamprey's mouth is circular and full of suckers, with which it adheres to the species it parasitizes, such as sharks and marine mammals. Inside the mouth, the lamprey has conical teeth and a tongue adapted to scrape tissues; these structures allow it to make a wound in the skin of its prey and suck the blood that constitutes its food.
A curious feature is that pouched or wide-mouthed lampreys develop a pouch under their eyes during mating season. Although the exact function is still unknown, it could be due to a structure needed to stir the seabed and prepare the nest.
Within the group of lampreys there are two types: sea lampreys and river lampreys.
Sea Lampreys
They are those that inhabit the seas and oceans around the world. Among them it is possible to mention these species:
Chilean lamprey
The Chilean lamprey (Mordacia lipicida) is an agnathic fish endemic to the coast of Chile. It measures up to 54 cm and has a pouch from the neck to the gills, as well as a large eye located in the lateral-dorsal region.
During the winter, the Chilean lamprey moves away from the cold coasts and migrates towards the sea. The image shows a Chilean lamprey.
Pouched or Widemouth Lamprey
The wide-mouthed lamprey (Geotria australis) inhabits the Indian, Pacific, and Atlantic oceans, as well as being found around the countries that make up the Ring of Fire. It measures up to 60 cm and develops a bag under the eyes, apparently designed to build the nest.
This species of lamprey feeds on teleost fish. Despite living in the sea, during the mating season it approaches the rivers to lay its eggs.
River Lampreys
Various species of lampreys carry out part of their life cycle in rivers, while others only inhabit these fresh waters. These are the river lamprey species:
River Lamprey
Called Lampreta fluviatilis, it is a species that measures 40 cm and is distributed in the rivers of Europe, where it spends most of its his life. However, it also lives in the sea when it reaches adulthood.
Features 7 gill holes and two well-developed eyes. Its teeth are sharp and allow it to feed on the fish it parasitizes.
Brook Lamprey
The creek lamprey (Lampreta planeri) is similar to the river lamprey but smaller. It is found only in the Spanish community of Narrava and in some rivers in Portugal.
A curiosity about this species of jawless fish is that the larvae take 6 years to reach sexual maturity. During this period, they feed on algae and detritus found in the river beds.
Other lamprey species
There are other species of lampreys distributed in seas, rivers and oceans around the world. These are some of them:
- Australian Brook Lamprey (Mordacia praecox).
- Short-headed Lamprey (Mordacia mordax).
- Marine lamprey (Petromyzon marinus).
- Pacific Lamprey (Lampetra tridentata).
- Ohio lamprey (Ichthyomyzon bdellium).
- Caspian Lamprey (Caspiomyzon wagneri).
- Carpathian Lamprey (Eudontomyzon danfordi).
- Danube Lamprey (Eudontomyzon vladykovi).
Mixines: what they are and features
Also called hagfish (Myxini), they are the other class of agnathans or jawless fish that exist. Like lampreys, they have a long, circular body covered with a mucous layer. The general appearance is very primitive, since they do not have a sense of taste in their mouths, but instead have receptive cells in their skin and simple eyes.
Hagfish feed on the carrion and viscera of larger animals, which they can gnaw on their live prey as they enter their bodies. As they do not have a jaw, they have a rudimentary mouth with which they can adhere to them, as well as a tongue capable of scraping the skin.
Hagfish species
Among the species of hagfish that currently exist, are the following:
Goliath Hagfish
Its scientific name is Eptatretus goliath and it is known from a single sighting in New Zealand, where this species is endemic. is known to live at a depth of 811 meters and is 1 meter long. Other details about their habits are unknown.
Slug Eel
Also called sea slug or mucus fish (Myxine glutinosa), it lives in the waters that surround the Iberian Peninsula, Norway, Canada, Mexico and the United Kingdom, where it is found between 40 and 1100 meters deep.
The species reaches up to 1 meter in length and is nocturnal. It consumes dead or dying animals, entering their bodies to devour their entrails.
Other hagfish species
In addition to those mentioned, there are other species of hagfish such as the following:
- Sea slug (Myxine australis).
- Whitehead Hagfish (Myxine ios).
- Cape booger fish (Myxine capensis).
- Mekura eel (Myxine garmani).
- Dwarf mucus fish (Myxine pequenoi).
- Slug Lamprey (Myxine circifrons).
- Jespersen's mucus fish (Myxine jespersenae).
- Caribbean hagfish (Myxine mcmillanae).
Ostracoderms: extinct jawless fishes
When it comes to extinct Agnatha-class fish, the ostracoderms (Ostracodermi) are among the best known. They were between 50 and 60 cm long and extinct 350 million years ago.
Among the agnathic fish, the ostracoderms were different, since they had thick scales that formed a bony shield that fulfilled the role of protecting them from predators. They are among the first vertebrates to appear on Earth, although current agnates have a cartilaginous skeleton and not a bony one, which is why they are also considered chondrichthyans.
