Classification of FISH: agnate, cartilaginous and bone

Fish are vertebrate animals that live in aquatic environments whether they are salt water (seas, oceans, etc.) or fresh water (rivers, lakes, etc.) Within the group of fish we can find a great variety of animals, from small fish that inhabit high mountain rivers, such as the goby to large fish that inhabit the Pacific seabed such as demon fish (stomped). These animals have been, like the vast majority of living beings on our planet, classified throughout history taking into account different criteria.

In this lesson from a TEACHER we will see the fish classification most commonly used, the one that takes into account your anatomy and way of life. If you want to know more about the different groups of fish, keep reading this lesson!

Index

1. What is a fish? What classification of fish is the most used?
2. The agnate fish or cyclostomata
3. Chondrichthyan fish or cartilaginous fish
4. The octectians or bony fishes

The group of fish is a very particular group since it has been "created" by researchers, that is, it is a artificial or paraphyletic group. It is said that a group is artificial when the living beings that make up said group do not have a known common ancestor, that is, they have diverse origins and they are grouped "for convenience" of the researchers taking into account other characteristics different from their origin evolutionary.

In this case, the group of fishes includes all those animals vertebrates, aquatic, which are generally ectothermic and breathe through gills. In addition, they usually have other external characteristics that make us classify them within the same group: they have scales and fins. Another definition is any vertebrate animal that is not a tetrapod nor does it possess the common derived characteristics (apomorphies).

There are two types of classifications widely used to group fish. In the first place, the simplest is the classification of the fish. attending to its habitat. Fish can be classified into freshwater fish (eels or aquarium fish such as cichlids) and saltwater fish (sharks, rays, etc).

Classification of fish according to anatomical characteristics

This classification is too broad and generic in some cases, so the most widely used is undoubtedly the classification carried out. according to anatomical characteristics such as the presence or absence of bones or protective structures such as the operculum, which protects the gills. According to the anatomical characteristics we can find three main groups of fish classification:

• Agnate fish. It is a "mixed bag" within this classification in which all those fish that lack jaws are included. This group has few representative species alive today, but the best known are the lampreys, along with the mixines.
• Chondrichthyan fish or cartilaginous fish. The greatest characteristics of these fish is that their body is made up of cartilage and they do not have a single bone. In addition, they have the outer part of the gills (gill slits) exposed. Some examples of chondrichthyan fish are rays, sharks, and chimeras.
• Pbony or osteitic organs. Osteictian fish have a skeleton made up of bones and covering the external part of the gills they have a structure called the operculum, which serves to protect them. Some examples of bony fish are European eel, pike, trout, or hake.

Within the classification of fish that we have proposed we find the group of fish agnate What are all those fish that they do not have jaws. This makes it something that in biology is called a "disaster drawer", that is, they are a group that encompasses animals because they do not have a certain characteristic, with which the animals of that group usually have little to do each. The main current representatives of the agnates are the lampreys and the mixins or witch fish.

In this case, the current agnates have the following features:

• Anguilliform appearance and mucous skin with absence of scales. All agnates have a tube-like appearance, very similar to that of eels, and their skin lacks scales and is protected by a somewhat sticky substance.
• Absence of even fins. Agnatos only have an odd fin at the end of their body.
• Hematophagous feeding (lampreys) or ghoul (mixines).
• Presence of notochord. They have a nerve tube both in larval and adult stages
• Presence of seven or more gill sacs.
• Presence of a pineal eye sensitive to light. This "third eye" is homologous to the pineal gland of mammals and controls many circadian and endocrine rhythms in these and other animals.
• Skeleton cartilaginous
• Heart with two cameras

The chondrichthyans are those fish whose skeleton is made up of cartilage, in addition to having the following characteristics:

• Body flattened dorso-ventrally. Unlike bony fish, which are flattened laterally (and the eyes are left and right of the line media of the animal), the cartilaginous fish are flattened dorso-ventrally and both eyes are on one side of the line half. An example of this symmetry is the one we can see in the stingray or the blankets.
• Body covered with cartilaginous scales and mucus. They are scales similar to small teeth that, in addition to protecting them against predators because they are very sharp, they serve to make them more hydrodynamic.
• Giantism. Another of the adaptations of chondrichthyans, which lack true teeth, are the exaggerated dimensions of their body, which make them appear more dangerous than they really are.
• Presence of 5 to 7 pairs of gills, in direct contact with the outside world. Later we will see how bony fish have a chamber and a structure (operculum) that protect the gills.
• Swimming bladder deficiency. The swim bladder is an organ that fills with air and keeps fish afloat. This structure does not appear in agnate fish or chondrichthyans, which have to keep swimming continuously to float.

Chondrichthyans are classified into elasmobranchs (sharks, rays, mantas, sawfish, etc) and holocephalos, with the chimeras as the only living representatives.

Image: Pinterest

And we finish this classification of fish by talking about the group of osteichthians what are fish with him partially ossified skeleton. When we think of the appearance of a typical fish, we usually think of bony fish: salmon, trout, tuna, etc. they are all bony fish. In addition to their skeleton, other characteristics of bony fish are:

• Body flattened laterally. Unlike cartilaginous fish, in bony fish the midline of symmetry usually divides them into the right half and the left half.
• Presence of bony scales coating the body.
• Presence of true teeth. Unlike cartilaginous fish, which lack true bony teeth, octeitians have true teeth, attached to the jaw.
• Possess more fins than cartilaginous fish. The vast majority of osteictia have: two pairs of thoracic fins, two pairs of pelvic fins, two pairs of dorsal fins, two pairs of anal fins and a single tail fin.
• Presence of swim bladder en any of their stages of life. The swim bladder is an organ that fills with air and allows fish to float without the need to swim. Some osteictia have lost this structure or have transformed it (such as lungfish, which create a "false lung" from the swim bladder).
• Appearance of a gill chamber, in which the gills are located separated from the rest of the body and protected by a structure similar to a flap called the operculum.

Osteitians are classified as: actinopterygians (moray eels, eels, etc) Y sarcopterygii (coelacanths, scaly salamander fish, etc).

Image: Slideshare

If you want to read more articles similar to Fish classification, we recommend that you enter our category of biology.

• Hickman, C. P., Ober, W. C. and Garrison, C. W., 2006. Comprehensive Principles of Zoology, 13th Edition. McGraw-Hill-Interamericana, Madrid
• Burnie, D (2005). "Cartilaginous fish". Animal. The definitive and impressive visual guide to the wildlife on our planet. Madrid: Pearson Educación, S.A.
• Wikipedia (March 25, 2020). Agnatha. Recovered from https://es.m.wikipedia.org/wiki/Agnatha
• Gil Recio, C (May 20, 2016). Cartilaginous fish, characteristics and examples. Recovered from https://peces.paradais-sphynx.com/actualidad/peces-cartilaginosos.htm