FISH: CHARACTERISTICS, CLASSIFICATION, SYSTEMS, REPRODUCTION - BIOLOGY - 2025

The fish are a group of aquatic vertebrates with gills, appendages forms fins and usually cover skin structures called flakes. With more than 28,000 living species, they have been able to colonize all kinds of aquatic ecosystems.

Historically, the word "fish" has been used without taxonomic value, as it does not describe an actual grouping. The first taxonomists called "fish" any organism that lived in water. Thus, jellyfish, starfish, crabs, amphibians, seals, and whales were considered fish. Over time, the definition began to become more and more refined.

Source: pixabay.com

Today, the term is used to describe vertebrates that are not tetrapods. However, it is not a monophyletic group, because the ancestor of land vertebrates is found within a group of fish - the sarcopterygians.

Fish have a number of adaptations that are associated with aquatic life. Most have a spindle-shaped appearance to move efficiently through the water, a swim bladder, organs that mediate salt and water exchange, gills, an optimal chemoreceptor system, and a lateral line system.

Within living species, fish are divided into two large groups: non-jaw and jaw. The former are witch fish and lampreys, while in the jawed group we find the species with which we are most closely related: sharks, rays, and ray-finned and lobed fish.

General characteristics

Fish are part of a large group of organisms that breathe through gills and have modified fin-shaped appendages. Within the group of vertebrates, fish are the oldest and most diverse members.

Anatomy of a of an osteictium. (1) - Operculum, (2) - Lateral line, (3) - Dorsal fin, (4) - Fatty fin, (5) - Caudal peduncle, (6) - Caudal fin, (7) - Anal fin, (8) - Photophore, (9) - Pelvic fin, (10) - Pectoral fin. Author: GrahamBould. Wikimedia Commons.

All its members are poikilothermic, that is, they do not have the ability to regulate their body temperature, as we mammals do.

One of the most prominent evolutionary events of the group was the appearance of the jaws. These structures managed to expand the range of available dams, promoting the diversification of the group.

This group of aquatic animals arose in the Cambrian period from an unknown ancestor. Today, there are five kinds of living fish that you will explore in the next section.

Classification (types)

The fish are divided into three groups: Agnatos (Agnatha), Gnathostomata and Osteictios (Osteichthyes). In turn, each of these groups is divided into classes.

-Agnatha (agnatos): fish without a jaw

Mouth of Petromyzon marinus (lamprey) in the Maremagnum Room of the Aquarium Finisterrae (Casa de los Peces), in La Coruña, Galicia, Spain. By Drow_male, from Wikimedia Commons

Currently, there are about 180 species of fish that lack jaws. This group has vertebrae in a rudimentary state. Despite this, they are considered vertebrates, thanks to the presence of a skull and other structures homologous to the rest of vertebrates.

Agnatos are divided into two classes: Myxini, which includes the popularly called witch fish, and Petromyzontida, whose representatives are lampreys.

A grouping of both groups has been proposed, based on their morphological characteristics. This group is called “Cyclostomata”, and it appeared to be paraphyletic when analyzed following the cladistic methodology, since lampreys have many characteristics shared with mandibulated organisms.

Thanks to the application of molecular methods, it has been concluded that, indeed, lampreys and witch fish form a monophyletic group. However, this phylogenetic hypothesis needs more evidence, as most zoologists tend to reject it.

Myxini class

The mixins or witch fish are a group of almost 70 species, made up of scavengers and predators. Although they are practically blind, they manage to catch their prey following chemical stimuli. Its habitat is completely marine.

Morphologically, they resemble an eel. Its body is naked, without even appendages (fins), the notochord is persistent and the skeleton is cartilaginous.

One of the most striking and peculiar characteristics of witchfish is their ability to produce significant amounts of milky mucus when disturbed. When the fluid combines with seawater, the animal takes on such a slimy consistency that it is almost impossible to grasp.

The internal fluids of the mixines are in osmotic equilibrium with the sea water, a typical characteristic of invertebrates and not vertebrates.

Class Petromyzontida

This class is made up of 38 species of lampreys. Like witchfish, lampreys have an eel or vermiform body. These do not have even appendages, but one or two dorsal fins.

Regarding their life habits, there are parasitic species and non-parasitic species. They inhabit fresh water ecosystems and also salt water bodies.

The circular structure present in its mouth allows it to anchor itself to rocks and adhere to other fish. Parasitic lampreys are capable of feeding on the bodily fluids of their prey. In contrast, the characteristic larvae of this group feed on particles that are suspended in the aquatic environment.

-Gnathostomata: jawed fish

White shark in the waters of Guadalupe Island, Mexico.

Chondrichthyes class - chondrichthyes

Chondrichthyans are made up of more than 970 living species of cartilaginous fish. This small class of fish is characterized by its sensory organs that are exquisitely adapted to predation in aquatic environments, strong jaws and powerful musculature.

Its habitat is predominantly marine, although there are about 30 species that live mainly in freshwater bodies.

The cartilage that characterizes the group comes from ancestors with a bony skeleton - a curious evolutionary event. Part of the transition has been observed in the fossil record, as specimens of sharks with bony parts have been found.

Although bone was lost in the chondrichthyans (possibly in a process of neoteny), tissues with phosphate minerals were still present, including teeth and scales.

After the huge whales, sharks are among the largest vertebrate species in the world. The largest specimens can measure more than 12 meters in length.

Sharks and rays belong to the Elasmobrinchii subclass. The morphology ranges from fusiform bodies to flattened variants in the dorsal ventral plane. The tail fin is hetero close and has even pectoral and pelvic fins. The mouth is located in the ventral region. The skin may be bare or have placoid scales.

-Osteichthyes (Osteichthyes): bony fish

Generalized skeleton of an osteictium.

1 Maxilla, 2 Hyoid arch, 3 Dental, 4 Ocular orbit, 5 Bone of the ocular orbit, 6 Preopercular, 7 Subopercular,

8 Interopercular, 9 Opercular, 10 Bones of the shoulder girdle, 11 Pectoral fins, 12 Bones of the pelvic girdle, 13 Ventral fins, 14 Spinal column, 15 Upper vertebral process, 16 Lower vertebral process,

17 Ventral ribs, 18 Dorsal ribs, 19 Pterygophores of the dorsal fins, 20 Pterygophores of the anal fin,

21 Spines of the dorsal fin, 22 Rays of the dorsal fin, 24 Caudal plate, 25 Caudal fin. '

The bony fish are grouped under the name of Osteichthyes. These fish and tetrapods are usually united in a group by the presence of the endochondral bone; a type of bone that replaces cartilage during body development.

Although it is traditionally used, the Osteichthyes group does not describe a clade (monophyletic group). Therefore, most classifications do not recognize it as a valid taxon. Instead, it is used as a "convenience" term to describe vertebrates with endochondral bone.

Different adaptations have contributed to the extensive radiation that this group has suffered in the course of evolution. One of them was the appearance of the operculum on the gills; in this way increases the efficiency of respiration. In addition, the development and specialization of the elements of the jaw, expanding the range of possible trophic habits.

Class Actinopterygii: ray-finned fish

The Actinopterygii class consists of about 27,000 species. The earliest forms were very small fish with large eyes and a straight tail - these features are considered "primitive".

The main characteristic of this class of bony fish is the presence of fins with rays, which have an internal support formed by fines and numerous stripes or lepidotrichia.

The muscles that control the movement of the fins are found within the body wall; in contrast to sarcopterygian fish, where the musculature is located outside the body, along the fin.

Some taxonomists divide the Actinopterygii class into three groups: chondrostes, holosteums, and teleostos, attempting to represent "primitive", "intermediate", and "advanced" forms respectively. These groups progressively increase the degree of ossification.

Teleostos

Teleostos represent about 96% of all living species of fish, and about half of vertebrates, so they deserve to be treated separately. The shapes and sizes vary widely, from which we find tiny fish up to species that can reach 4.5 meters in length.

Their habitats are as varied as their morphologies. They are capable of living in temperatures close to 50 degrees, or in seas with temperatures of -2 degrees Celsius.

This group presents scales of the cycloid and ctenoid type, replacing a heavy armor with a light variant that facilitates movement. In some species the scales are absent.

The type of tail in teleostos is symmetric and is called a homo-fence tail. The change in the class of the fins improved the mobility of the animals, making swimming a more efficient activity. Some species have modified their dorsal fin for various purposes - like vein spines, for example.

This line of fish has developed a control in the swim bladder that allows them to control the flotation, and together with the modifications in the fins, improve the efficiency of the feeding.

Class Sarcopterygii: lobe-finned fishes

The first sarcopterygii were characterized by presenting lungs and a gill system. The tail is of the hetero close type, that is, with one of the lobes larger than its partner. With the passage of time, the tail took symmetry and became difficult.

The ancestor of tetrapods is found within this class of fish, specifically in a group called ripidistians. The characteristic genus is Eusthenopteron, of which the cylindrical body, its large head, its fleshy fins and possibly lungs stand out.

Sarcopterygians have powerful jaws and scales of a dentin-like material called cosmina. The fins are strong and paired, allowing these organisms to walk on the bottom of the water.

Although it is true that sarcopterygii do not represent an abundant or diverse group, they are of immense interest to biologists, since they help to elucidate the origin of tetrapods.

Today there are only eight species alive: six species of lungfish and two species of coelacanths.

Dipnoos: lungfish

The most prominent genus of lungfish is Neoceratodus, which inhabits Australian bodies of water. In South America we find Lepidosiren and in Africa Protopterus. This last genus has the peculiarity of surviving during the dry season buried in the mud as a kind of hibernation.

Coelacanths are characterized by inhabiting deep regions of salt water, a prominent notochord, and a fat-filled swim bladder.

Digestive system

Organs of a cod (Gadus morhua): 1. Liver, 2. Swim bladder, 3. Roe, 4. Duodenum, 5. Stomach, 6. Intestine. H. Dahlmo, from Wikimedia Commons

The digestive system of witchfish and lampreys is quite simple. They lack a stomach, spiral valve, and cilia in the intestinal tract. Lampreys that do not exhibit a parasitic lifestyle degenerate the digestive system into adult form; they no longer feed.

In chondrichthyans, the digestive system is more complex. There is a J-shaped stomach and the intestine has a spiral valve. In chimeras, the stomach is absent.

The digestive system of bony fish is made up of a stomach and the rest of the typical components of a digestive system. The range of foods is very wide, there are carnivorous, herbivorous, plankton consuming, detritivore species, among others.

Circulatory system

Heart model of a fish. Wagner Souza e Silva / Museum of Veterinary Anatomy FMVZ USP

In witchfish, the circulatory system consists of a heart with a venous sinus, an atrium, and a ventricle. There are accessory hearts.

Sharks and related have a circulatory system made up of several pairs of aortic arches. The heart has a venous sinus, an atrium, ventricle, and a venous cone.

Fish circulatory system. Pedro D. Ponce, from Wikimedia Commons

In the class Actinopterygii the system consists of a heart and a venous sinus, with an undivided atrium and ventricle. There are generally four aortic arches. Unlike mammals, these organisms have red blood cells with nuclei.

In this class the circulation is single, while in the Sarcopterygii class the circulation is double, with pulmonary and systemic circuits.

Nervous system

Schematic view of the brain of a trout. (Oncorhynchus mykiss). Fish_brain.png: The original uploader was Neale Monks at English Wikipedia.derivative work: Furado, via Wikimedia Commons

Mixins have a nerve cord with a differentiated brain, but no cerebellum. They have 10 pairs of cranial nerves, and ventral and golden nerve cord units. The eyes have degenerated, they have a pair of semicircular canals and senses of taste and smell.

Similarly, lampreys have a cord and a differentiated brain. In this class a small cerebellum can be seen and, as in the previous group, there are 10 pairs of cranial nerves. The organs of vision are well developed, as are the senses of taste and smell.

Chondrichthyans have a brain with two olfactory lobes, two cerebral hemispheres, two optic lobes, a cerebellum, and a medulla oblongata. There are 10 pairs of cranial nerves, three semicircular canals, and well-developed organs for smell, vision, and electroreception.

Sharks are able to perceive vibration stimuli thanks to the lateral line system.

Hearing system

Like all vertebrates, fish have the ability to detect sounds in their environment. Logically, being immersed in a body of water implies a specialized hearing system.

In water, the vibrations that occur are almost at the same density as the bodies of animals. This is a considerable inconvenience, since the waves could pass almost unnoticed.

Weber apparatus

An effective solution to counteract the problem of densities is the Weber ossicle system or Weber apparatus. This mechanism has been reported in a group of teleost fish and consists of a system of small bones that improves the hearing system.

Reception of the stimulus begins in the swim bladder (see flotation systems). This step is logical, since vibration can easily be transmitted in an air-filled cavity. Subsequently, the stimulus is directed to the inner ear through the ossicles.

This reception system is reminiscent of our ear, which is made up of a series of ossicles that transmit the stimulus to the inner ear. However, both structures are not homologous to each other and evolved independently.

Other adaptations

In other species that lack the Weber apparatus, there are a series of adaptations that make it possible to improve the system that captures vibrations.

Some species are distinguished by having expansions of the swim bladder that allow them to establish a connection with the skull and thus transmit the stimulus.

Respiratory system

Tuna gills. WIkimedia Commons

The respiratory system of fish is made up of highly specialized structures that allow them to extract oxygen from an aquatic environment.

The gills are made up of very fine filaments rich in blood vessels. They are located within the cavity of the pharynx and are covered by the operculum. The function of this is protection, since the gills are very delicate.

Caps are not present in sharks. Instead, respiration occurs through five to seven pairs of gills. In elasmobranchs the clefts are exposed, while in chimeras they are covered by an operculum.

In sharks and bony fish, the system is responsible for pumping the water continuously through the gills. The flow of water is opposite to the direction of the blood, and in this way the maximum extraction of oxygen is achieved.

Excretory system

In vertebrates, the kidneys play a fundamental role in excretory functions. The kidneys have osmoregulation functions, which accidentally results in the removal of potentially toxic metabolites for fish.

The most primitive system is found in the embryos of the mixines, with the kidneys of the arquinephros type. The pronephrous kidneys are typical of a few bony fish in their adult state or as embryos. The latter are found functionally in the adults of witchfish.

The mesonephro renal system is in the embryo of lampreys and fish. Those of the opistonephro type are the functional forms in adult lampreys and fish.

Floatation

Due to the presence of skeletons and organs, all fish are slightly heavier than water. Each group has developed different adaptations that allow them to deal with this problem.

Flotation system in chondrichthyans

Sharks manage to stay afloat thanks to the fin system they have. The caudal fin is of the hetero-fence type (asymmetric), and the pectoral fins are flat. This combination of fins provides an ideal morphological mechanism that helps keep the individual afloat.

In addition to this system, sharks have a liver rich in a special fat called squalene. This lipid substance has a density of 0.86 grams per milliliter. This organ works by compensating for the shark's heavy body, acting as a kind of float.

Flotation system in bony fish

The most efficient flotation system consists of a gas-filled cavity. In bony fish this mechanism occurs thanks to the swim bladder. If fish did not have this organ, their heavy bodies could not stay afloat.

To maintain a natural buoyancy, individuals have a mechanism that allows regulation of gas volume. In this way, staying in the water does not translate into considerable energy expenditure for the fish.

Reproduction

Fish are characterized by exhibiting widely varied reproduction mechanisms. In general, the sexes are separate and the foundation and development occur externally, although there are a significant number of exceptions.

In agnate the sexes are separate. In the mixines, the same individual has ovaries and testes, but only one is functional. Fertilization is external. The mixins do not present in larval stage or metamorphosis.

In contrast, lampreys do present a larval stage, called the ammocete larva. In some species, the larvae can persist for up to seven years. After metamorphosis, the adult form reproduces and dies rapidly.

Chondrichthyans have separate sexes and paired gonads. In sharks, the reproductive ducts empty into a cloaca; whereas in chimeras, the urogenital apparatus is separated from the anal opening. In this group of cartilaginous fish, fertilization is internal. Some species are oviparous, viviparous, or ovoviviparous.

References

1. Audesirk, T., Audesirk, G., & Byers, BE (2003). Biology: Life on Earth. Pearson education.
2. Campbell, NA (2001). Biology: Concepts and relationships. Pearson Education.
3. Cuesta López, A., & Padilla Alvarez, F. (2003). Applied zoology. Díaz de Santos editions.
4. Curtis, H., & Barnes, NS (1994). Invitation to biology. Macmillan.
5. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
6. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
7. Llosa, ZB (2003). General zoology. EUNED.
8. Parker, TJ, & Haswell, WA (1987). Zoology. Chordates (Vol. 2). I reversed.
9. Randall, D., Burggren, WW, Burggren, W., French, K., & Eckert, R. (2002). Eckert animal physiology. Macmillan.