Osteichthyes – Characteristics, Classification and Examples

  • Osteichthyes, also known as bony fish, are a diverse group of vertebrates characterized by skeletons primarily composed of bone tissue. They form the largest class of vertebrates, with over 28,000 known species. Bony fish can be found in both freshwater and marine environments. Examples of freshwater bony fish include carp, perch, bass, trout, and catfish, while marine bony fish include tarpon, mackerel, tuna, sailfish, and barracuda.
  • The class Osteichthyes originated in the early Silurian period and experienced a significant adaptive dispersion during the middle Devonian period. They exhibit a remarkable diversity of species, with adaptations suited for life in various aquatic environments. Bony fish have undergone a remarkable evolutionary radiation, resulting in their current abundance and variety.
  • The classification of Osteichthyes has undergone revisions over time. The class is now typically divided into two subclasses: Sarcopterygii (lobe-finned fish) and Actinopterygii (ray-finned fish). Sarcopterygii includes more primitive fish groups like the coelacanths and lungfish. Actinopterygii comprises the majority of bony fish species and represents the true bony fish.
  • Within the class Osteichthyes, there are four main categories recognized by fishery scientists: lungfish or dipnoans, crossopterygians, actinopterygians, and bichir fish or brachiopterygians. However, some taxonomists disagree on the classification of these categories, and there is ongoing debate regarding their legitimacy.
  • Bony fish exhibit a wide range of adaptations and ecological roles. They have adapted to various feeding strategies, habitats, and reproductive modes. Bony fish possess well-developed gills for respiration, scales covering their bodies, and fins for propulsion and maneuvering in water.
  • The subclass Sarcopterygii includes lobe-finned fish, such as the coelacanths and lungfish. These fish have fleshy, lobed fins, which are considered more primitive. The subclass Actinopterygii consists of ray-finned fish, which have fins supported by bony rays. Ray-finned fish represent the majority of bony fish species and have evolved diverse forms and adaptations.
  • Bony fish have a significant impact on ecosystems and human societies. They serve as a vital food source for many communities around the world and play important ecological roles as predators, prey, and contributors to nutrient cycling in aquatic environments.
  • In summary, Osteichthyes, or bony fish, are a diverse and abundant group of vertebrates with skeletons primarily composed of bone tissue. They encompass a vast array of species adapted to different habitats and display remarkable diversity in form, function, and behavior.

General Characteristics of Osteichthyes

Osteichthyes, or bony fishes, exhibit a range of general characteristics that define this diverse class of vertebrates:

  1. Size and Diversity: Osteichthyes is the largest class of vertebrates, comprising over 29,000 species. They display a wide variety of sizes, from the tiny Paedocypris progenetica (less than 8 mm) to the massive ocean sunfish (4 m, weighing about 1500 kg).
  2. Endoskeleton: Bony fishes have an endoskeleton composed of bones, providing support and structure to their bodies.
  3. Fins: They possess both paired and median fins. These fins are supported by long rays of cartilage or bone, allowing for swimming and maneuverability. Sarcopterygians, a subclass of bony fish, have fleshy lobed fins adapted for bottom-dwelling.
  4. Homocercal Tail: The majority of bony fishes have a homocercal tail, meaning the upper and lower lobes of the tail fin are symmetrical.
  5. Swim Bladder: Bony fishes possess a swim bladder or air bladder, which aids in buoyancy control and facilitates gaseous exchange.
  6. Terminal Mouth: Bony fishes have a terminal mouth, positioned at the front of the head.
  7. Gills and Operculum: Bony fishes typically have four pairs of gills, which are protected by a bony flap called the operculum.
  8. Lungs in Sarcopterygians: Some bony fish groups, such as lungfishes and lobe-finned fishes, have lungs in addition to gills, allowing them to breathe air.
  9. Skin and Scales: The skin of bony fishes is covered by bony dermal scales, which can be ganoid, cycloid, or ctenoid in shape.
  10. Poikilothermy: Bony fishes are poikilothermic or cold-blooded animals, lacking the ability to regulate their internal body temperature. However, some larger marine species exhibit limited endothermy.
  11. Circulatory System: Bony fishes have a two-chambered heart with one auricle and one ventricle. Lungfishes possess a three-chambered heart with two auricles and one ventricle.
  12. Nervous System: The brain of bony fishes includes a small olfactory lobe and cerebellum. They have ten pairs of cranial nerves.
  13. Lateral Line Organ: Bony fishes possess a lateral line organ consisting of hydrodynamic receptors called neuromasts. This sensory system helps them sense vibrations, water pressure, and aids in navigation and locating prey.
  14. Reproductive System: Most bony fishes have separate sexes, although some species are hermaphrodites. They are predominantly oviparous, laying a large number of eggs. Fertilization is typically external, and direct development occurs.
  15. Parental Care: While most bony fishes do not exhibit parental care, some species display courtship behaviors and provide care for their eggs or offspring.

These general characteristics contribute to the diversity and adaptability of bony fishes, allowing them to thrive in various aquatic habitats and play crucial roles in ecosystems.


Classification of Osteichthyes

Class Osteichthyes is divided into three subclasses. These are:

Subclass-1: Acanthodii

Acanthodii, also known as spiny sharks, belonged to the subclass Acanthodii and displayed several unique characteristics during their existence:

  1. Extinct Status: Acanthodii are currently extinct, meaning they no longer exist in the present-day.
  2. Geologic Range: These fascinating fish lived from the Devonian to the Permian period, spanning a considerable duration of time in Earth’s history.
  3. Habitat Range: Acanthodii inhabited various aquatic environments, including freshwater lakes, rivers, and seas.
  4. Ganoid Scales: Their skin was covered with primitive nature ganoid scales, which provided protection and resilience.
  5. Fin Structure: With the exception of the tail fins, the other fins of Acanthodii possessed fin rays, providing stability and maneuverability in the water.
  6. Example Species: Two examples of Acanthodii species are Climateus and Acanthodius, representing the diversity and uniqueness of this subclass.

Acanthodii, or spiny sharks, left a remarkable imprint in the history of aquatic life. Despite their extinction, their presence during the Devonian to Permian period highlights the evolutionary adaptations that occurred during that era. The ganoid scales and unique fin structure of Acanthodii exemplify the fascinating diversity of prehistoric fish species.

Subclass-2: Sarcopterygii (Gr., sarcos = fleshy; pterygium = fin)

Sarcopterygii, also known as lobe-finned fish, belong to the subclass Sarcopterygii and exhibit several distinctive characteristics:

  1. Skeleton Composition: The skeleton of Sarcopterygii is composed of both partial hard bones and partial cartilage, combining the two structural elements.
  2. Lung Development: In Sarcopterygii, the air sacs undergo a transformation and develop into lungs, allowing these fish to extract oxygen from the air.
  3. Tail Fin Structure: The tail fin of Sarcopterygii is of the diphycercal type, characterized by a symmetrical tail with a distinct upper and lower lobe.
  4. Lobed Paired Fins: The mid-region of the paired fins in Sarcopterygii is lobed, resembling leg-like appendages. These fins have a fleshy, bony central axis covered by scales.
  5. Fin Arrangement: Sarcopterygii have two dorsal fins and a heterocercal caudal fin, which means the upper lobe of the tail is larger than the lower lobe.
  6. Nostril Connection: The nostrils in Sarcopterygii are attached to the mouth cavity, often connected by internal nostrils or choanae.
  7. Adaptation to Air Breathing: Sarcopterygii are commonly referred to as fleshy or lobe-finned fish due to their unique fin structure and their ability to breathe air.

The subclass Sarcopterygii includes a diverse group of fish with fascinating adaptations. Their combination of hard bones and cartilage in the skeleton, development of lungs, lobed paired fins, and air-breathing capabilities make them distinct from other fish species. Sarcopterygii exemplify the evolutionary innovations that occurred in aquatic life, ultimately paving the way for the emergence of terrestrial tetrapods.

Sarcopterygii has been divided into two superorders or orders- Crossopterygii and Dipnoi.


Super order-1: Crossopterygii

Crossopterygii, commonly known as coelacanths, belong to the superorder Crossopterygii and exhibit several distinctive characteristics:

  1. Lobed Fins: Crossopterygii are lobed finned fish, characterized by paired fins that are lobed in shape. The pectoral fins are supported by a jointed median axis with radials, providing strength and flexibility. The caudal fin is three-lobed, contributing to their unique swimming style.
  2. Cosmoid Scales: The body scales of Crossopterygii are covered by a layer of cosmine, a type of enamel-like substance. These cosmoid scales provide protection and support to their body.
  3. Absence of Cloaca: Unlike many other fish species, coelacanths lack a cloaca, which is a common opening for excretion and reproduction in aquatic animals.
  4. Reduced Swimbladder: Crossopterygii have a vestigial or reduced air sac or swimbladder. This organ, once important for buoyancy control in fish, has lost its original function in coelacanths.
  5. Presence of Certain Bones: Coelacanths possess bones such as premaxillae, maxillae, and squamosal bones, which contribute to the structure and function of their skull and jaw.
  6. Internal Nares and Spiracles: Crossopterygii may have internal nares, which are openings connecting the nasal passages to the mouth cavity. Additionally, they have spiracles, which are small openings behind the eye that aid in respiration.

Coelacanths, the most well-known representatives of the superorder Crossopterygii, exemplify a group of ancient and unique fish. Their lobed fins, cosmoid scales, absence of cloaca, reduced swimbladder, and the presence of specific bones and openings distinguish them from other fish species. Coelacanths, such as Latimeria and Coelacanthus (extinct), hold great scientific significance as “living fossils” that provide valuable insights into the evolutionary history of vertebrates.


Order-1: Coelocanthiformes

Order-1: Coelacanthiformes (Coelacanths)

Coelacanthiformes, belonging to the order Coelacanthiformes, exhibit several distinctive characteristics:

  1. Two Dorsal Fins: Coelacanthiformes have two dorsal fins located on their back. These fins provide stability and maneuverability in the water.
  2. Three-Lobed Caudal Fin: The caudal fin of Coelacanthiformes is three-lobed, meaning it has three distinct lobes. This type of caudal fin, known as dephycercal, helps in generating powerful swimming movements.
  3. Equal-Sized Teeth: Coelacanthiformes have jaws equipped with equal-sized teeth. These teeth are adapted for grasping and capturing prey, allowing coelacanths to feed on a variety of aquatic organisms.

Example: Latimaria

One prominent example of the order Coelacanthiformes is Latimaria, commonly known as the coelacanth. Latimaria is a genus of coelacanth fish that has captured the attention of scientists and the public alike due to its remarkable characteristics and evolutionary significance. Latimaria exemplifies the unique features of Coelacanthiformes, including the presence of two dorsal fins, a three-lobed caudal fin, and jaws with equal-sized teeth. As a “living fossil,” Latimaria provides valuable insights into the evolutionary history and adaptation of ancient fish lineages.

Super order-2: Dipnoi (Gr., di = double; pnoe = breathing)

The superorder Dipnoi, commonly known as lungfishes, exhibits several distinctive characteristics:

  1. Elongated and Large Body: Lungfishes have elongated and large bodies, which enable them to thrive in their aquatic habitats.
  2. Dorsal Fin: Lungfishes typically have a single dorsal fin or, in some cases, it may be absent altogether.
  3. Lung-like Organ: The air sac in lungfishes undergoes a transformation into a lung-like organ, allowing them to breathe air.
  4. Dephycercal Caudal Fin: The caudal fin of lungfishes is dephycercal, meaning it is symmetrical and lacks distinct dorsal fins. This type of tail fin aids in swimming and maneuverability.
  5. Cloaca: Lungfishes possess a cloaca, which is a common opening for excretion and reproduction.

Additional Characteristics:

  • Cycloid Scales: The skin of lungfishes is covered with cycloid scales, which provide protection and reduce friction in the water.
  • Paired Lobed Fins: Lungfishes have lobed pectoral and pelvic fins, which assist in propulsion and stability.
  • Symmetrical Tail Fin: The tail fin of lungfishes is symmetrical (diphycercal) without separate dorsal fins, contributing to efficient swimming.
  • Internal Nares and Absent Spiracles: Lungfishes have internal nares that open into the mouth, facilitating respiration. They lack spiracles, which are openings behind the eyes found in some fish species.
  • Lung-like Air Bladder: Lungfishes possess a single or paired air bladder that has evolved into a lung-like organ for breathing atmospheric air.
  • Cartilaginous Vertebrae and Notochord: Lungfishes have cartilaginous vertebrae and retain a notochord as an unconstricted rod, providing structural support.
  • Unique Jaw Structure: Lungfishes lack premaxillae and maxillae, and their lower jaw (palato-quadrate) is firmly fused to the neurocranium, a characteristic known as autostylic jaw suspension.
  • Digestive System: Lungfishes have no stomach, and their intestine is ciliated. They possess a spiral valve, which aids in digestion and nutrient absorption.
  • Circulatory System: Lungfishes have a partly divided auricle and a ventricle completely divided by a ridge. The ventral aorta is shortened into a spirally twisted muscular bulbus cordis with valves. They have separate pulmonary and systemic circulation.
  • Examples: The superorder Dipnoi includes three living genera of lungfishes: Neoceratodus, Protopterus, and Lepidosiren. These species represent the remarkable adaptations and survival of lungfishes in different aquatic environments.

Lungfishes, with their unique anatomical and physiological features, are remarkable examples of evolutionary adaptation and provide valuable insights into the transition of aquatic vertebrates to terrestrial life.

This super order includes two orders:

Order-1: Ceratodiformes

Ceratodiformes is an order of fish that includes the fascinating Australian lungfish (Neoceratodus) as its primary example. This order showcases unique characteristics and adaptations that distinguish it from other fish species.

  1. Paddle-shaped Fins: The pectoral and pelvic fins of Ceratodiformes are paddle-shaped, which aids in locomotion and maneuverability in their aquatic habitats.
  2. Minute Scales: The body of Ceratodiformes is covered with small, minute scales, providing protection and reducing friction in the water.
  3. Lung-type Air Sac: Ceratodiformes possess one lung-type air sac or, in some cases, a bi-lobed air sac. This specialized organ enables them to breathe atmospheric air, supplementing their respiration in oxygen-depleted aquatic environments.
  4. Example: Neoceratodus (Australian Lungfish): Neoceratodus, commonly known as the Australian lungfish, is a notable member of the Ceratodiformes order. It is found in freshwater habitats in Australia and represents the unique adaptations of lungfishes to survive in various environments.

The Ceratodiformes order, exemplified by the Australian lungfish, Neoceratodus, showcases specific anatomical and physiological traits that allow these fish to thrive in their habitats. Their paddle-shaped fins, minute scales, and lung-type air sacs are distinctive features that contribute to their survival in oxygen-limited environments.

Order-2: Lepidosireniformes

Lepidosireniformes is an order of fish that includes the African lungfish (Protopterus) and the South American lungfish (Lepidosiren) as its prominent examples. These fish exhibit unique characteristics and adaptations that set them apart from other species.

  1. Lung-like Air Sacs: Lepidosireniformes possess two lung-like air sacs or, in some cases, a bi-lobed air sac. These specialized organs allow them to breathe atmospheric air, enabling them to survive in oxygen-depleted environments.
  2. Minute Scales: The body of Lepidosireniformes is covered with small, minute scales, which provide protection and reduce friction as they navigate through their aquatic habitats.
  3. Tubular-shaped Fins: The pectoral and pelvic fins of Lepidosireniformes are tubular-shaped, which aids in their locomotion and maneuverability in water.
  4. Examples: a) Protopterus (African Lungfish): The African lungfish, belonging to the genus Protopterus, is a notable member of the Lepidosireniformes order. It is found in freshwater habitats across Africa and exhibits remarkable adaptations for surviving in harsh conditions, such as aestivation during drought periods.b) Lepidosiren (South American Lungfish): The South American lungfish, known as Lepidosiren, is another representative of the Lepidosireniformes order. It inhabits freshwater environments in South America and possesses unique physiological adaptations, including the ability to breathe atmospheric air and aestivate during dry seasons.

The Lepidosireniformes order, exemplified by the African lungfish (Protopterus) and the South American lungfish (Lepidosiren), showcases remarkable adaptations for survival in oxygen-depleted environments. Their lung-like air sacs, minute scales, and tubular-shaped fins contribute to their ability to thrive in diverse aquatic habitats.

Subclass-3: Actinopterygii (Gr., actis – ray; pteryx = fin)

Actinopterygii is a diverse subclass of fish, commonly known as ray-finned fishes. It encompasses a vast array of species exhibiting unique characteristics and adaptations. Here are some key features of the Actinopterygii subclass:

  1. Absence of Lobe Fins: Actinopterygii lacks lobed fins, which are found in other subclasses of fish. Instead, their fins are supported by fin rays, providing flexibility and control during swimming.
  2. Bony Skeleton: The skeleton of Actinopterygii species is primarily composed of bone, providing structural support and protection.
  3. Caudal Fin Structure: Unlike some other fish subclasses, Actinopterygii species have a caudal fin that does not contain an epicordal part. The caudal fin may be homocercal (symmetrical) in most species, while some may exhibit heterocereal or semi-heterocercal structures.
  4. Diverse Scale Types: The body of Actinopterygii is covered by various types of scales, including cycloid, ctenoid, or ganoid scales. These scales provide physical protection and contribute to the overall appearance of the fish.
  5. Fin and Radial Structure: Actinopterygii species have thin, broad paired fins without fleshy basal lobes. The fins are supported by dermal fin-rays, which aid in maneuverability and propulsion. The radials of paired fins are not arranged bi-radially.
  6. Single Dorsal Fin: Actinopterygii typically possess a single dorsal fin, which may be divided into distinct sections.
  7. Gill Slits and Spiracles: The gill-slits of Actinopterygii species are covered by an operculum, which helps protect the gills. Unlike some other fish, they lack spiracles, which are additional respiratory openings.
  8. Olfactory Sacs and Internal Nares: Actinopterygii species have olfactory sacs that are not connected to the mouth cavity. Additionally, they lack internal nares, which are nasal passages found in certain fish species.
  9. Absence of Squamosal Bone: The squamosal bone, which is present in some other fish subclasses, is absent in Actinopterygii.

The Actinopterygii subclass, consisting of ray-finned fishes, showcases remarkable diversity and adaptations. With their bony skeletons, unique fin structures, and various scale types, these fish have successfully inhabited a wide range of aquatic environments.

It is divide into 3 super orders;

Super order-1: Chondrostei

Chondrostei is a superorder of fish that encompasses a group of primitive ray-finned fish characterized by certain distinct features. Here are some key characteristics of the Chondrostei superorder:

  1. Partially Cartilaginous Skeleton: The skeleton of Chondrostei species is partially cartilaginous, meaning it contains both cartilaginous and bony elements. This feature distinguishes them from fish with fully bony skeletons.
  2. Ganoid Scales: Chondrostei species possess ganoid scales, which are thick, hard scales with a diamond-shaped or rhomboid pattern. These scales provide effective protection for the fish.
  3. Heterocercal Caudal Fin: Chondrostei species have a heterocercal caudal fin, which means the upper lobe of the tail fin is larger than the lower lobe. This type of tail structure provides stability and improved swimming efficiency.
  4. Large and Wide Mouth: Chondrostei species typically have a large and wide mouth opening. This adaptation allows them to capture and consume a variety of prey items.

Chondrostei represents a group of primitive ray-finned fish with unique characteristics. Their partially cartilaginous skeletons, ganoid scales, heterocercal tail fins, and large mouths contribute to their overall morphology and ecological adaptations. While Chondrostei species are relatively less diverse compared to other superorders, they have played an important role in the evolutionary history of fish.

Order-1: Polypteriformes

Polypteriformes is an order of fish that includes a single family, Polypteridae, commonly known as bichirs or ropefish. These unique fish possess several distinct characteristics. Here are some key features of the Polypteriformes order:

  1. Bony Skeleton: The skeleton of Polypteriformes species is primarily composed of bones, distinguishing them from cartilaginous fish. This bony structure provides support and protection for the fish’s internal organs.
  2. Ganoid Scales: Polypteriformes have ganoid scales, which are thick, hard scales with a shiny, enamel-like surface. These scales offer excellent defense against predators and environmental hazards.
  3. Dorsal Fin with Sail-Like Finlets: One of the notable features of Polypteriformes is the presence of a dorsal fin that contains a series of elongated finlets. These finlets give the appearance of a sail and are a characteristic trait of bichirs.
  4. Example: Polypterus: The genus Polypterus comprises various species of bichirs. They are known for their elongated bodies, distinctive snouts, and the ability to breathe air using a modified swim bladder. Polypterus species are often found in African freshwater habitats.

Polypteriformes, represented by the family Polypteridae, showcases unique adaptations and characteristics that set them apart from other fish species. Their bony skeleton, ganoid scales, dorsal fin with sail-like finlets, and the presence of species such as Polypterus make them fascinating members of the fish kingdom.

Order-2: Acipenseriformes

Acipenseriformes is an order of fish that includes two families, Acipenseridae (sturgeons) and Polyodontidae (paddlefish). These unique fish exhibit several distinctive characteristics. Here are some key features of the Acipenseriformes order:

  1. Cartilaginous Skeleton: Acipenseriformes species have a skeleton primarily composed of cartilage rather than true bones. This cartilaginous structure provides flexibility while maintaining the structural integrity of the fish’s body.
  2. Long Rostrum: One of the notable features of Acipenseriformes is the presence of a long rostrum, which is an elongated snout that extends in front of the head. This rostrum is a defining characteristic of sturgeons and paddlefish, giving them a unique appearance.
  3. Body Covering: The body of Acipenseriformes species can be covered with either bony plates or be naked, depending on the family. Sturgeons typically have rows of bony plates called scutes along their body, while paddlefish have a naked skin.
  4. Subterminal Mouth: Acipenseriformes have a subterminal mouth, meaning the mouth is positioned on the ventral side of the head, closer to the body rather than the snout. This mouth placement is a distinguishing feature of these fish.
  5. Examples: Acipenser, Polyodon: The genus Acipenser includes various species of sturgeons, known for their elongated bodies, bony scutes, and the ability to thrive in both freshwater and saltwater environments. Polyodon represents paddlefish, characterized by their elongated rostrum and unique filter-feeding apparatus.

Acipenseriformes, comprising the families Acipenseridae and Polyodontidae, exhibit remarkable adaptations and characteristics that set them apart from other fish species. Their cartilaginous skeleton, long rostrum, body covering of bony plates or naked skin, and species such as Acipenser and Polyodon contribute to the diversity and fascination of this order.

Super order-2: Holostei

Holostei is a superorder of fish that includes two distinct orders: Amiiformes (bowfins) and Lepisosteiformes (gars). These ancient fish exhibit several unique characteristics that distinguish them from other groups. Here are some key features of the Holostei superorder:

  1. Bony Skeleton: Holostei species have a skeleton primarily composed of bones, which provides them with a more rigid and sturdy framework compared to cartilaginous fish.
  2. Body Covering: The body of Holostei fish can be covered with either cycloid or ganoid scales. Cycloid scales are round and smooth, while ganoid scales are thick, enamel-like scales with a diamond-shaped or rhombic appearance.
  3. Mouth Size: Compared to some other fish species, Holostei fish have a relatively small mouth. While the size of the mouth can vary between species and orders, it is generally not as large as seen in other groups like sturgeons or pike.
  4. Rudimentary Caudal Fin: The caudal fin of Holostei fish is often rudimentary and heterocercal, meaning that the upper lobe of the tail is larger than the lower lobe. This characteristic gives their tails a distinct asymmetrical appearance.

Order-1: Semlonotiformes

Semionotiformes is an order of fish that includes several species, with Lepisosteus (Gar pike) being a prominent example. These fish exhibit distinctive characteristics that set them apart from other orders within the class Actinopterygii. Here are some key features of the Semionotiformes order:

  1. Elongated Body: Semionotiformes fish have an elongated body shape, often with a pointed snout. This streamlined design allows for efficient swimming and maneuverability in their aquatic habitats.
  2. Ganoid Scales: The body of Semionotiformes is covered in ganoid scales. These scales are thick, hard, and often have a diamond-shaped or rhombic appearance. In Semionotiformes, the ganoid scales are obliquely arranged, providing an added layer of protection for the fish.
  3. Dorsal Fin Placement: Semionotiformes typically possess a small dorsal fin located near the base of the caudal fin. This dorsal fin aids in stability and balance during swimming, contributing to their overall agility in the water.
  4. Stout and Cylindrical Body: The body shape of Semionotiformes fish is often stout and cylindrical, giving them a robust appearance. This body structure, combined with their elongated form, allows for efficient movement and helps them navigate their environment effectively.

An example of a fish belonging to the order Semionotiformes is Lepisosteus, commonly known as the Gar pike. Gar pikes are known for their long, slender bodies, pointed snouts, and their characteristic ganoid scales. They have a fierce predatory nature and inhabit freshwater environments such as lakes, rivers, and swamps.

The Semionotiformes order, with its elongated body, obliquely arranged ganoid scales, small dorsal fin, and species like the Gar pike, showcases a unique group of fish within the broader class Actinopterygii. These adaptations have allowed them to thrive in various aquatic ecosystems and establish their ecological niche as skilled hunters.

Order-2: Amiformes

Amiiformes is an order of fish that includes the species Amia, commonly known as the bowfin. These fish exhibit distinct characteristics that differentiate them within the class Actinopterygii. Here are some key features of the Amiiformes order:

  1. Rounded Snout: Fish belonging to the Amiiformes order have a rounded snout, giving their head a more blunt and rounded appearance compared to other fish species.
  2. Cycloid Scales: The body of Amiiformes fish is covered with cycloid scales. These scales are thin, smooth, and round-edged, providing some protection while maintaining flexibility and maneuverability.
  3. Elongated Dorsal Fin: Amiiformes typically possess an elongated dorsal fin, extending along the back of the fish. This fin contributes to stability and propulsion, assisting the fish in maintaining balance and precise movements.
  4. Rounded Caudal Fin: The caudal fin of Amiiformes is rounded and gephycercal, meaning it has a well-developed upper lobe and a smaller, ventrally located lower lobe. This type of caudal fin aids in generating thrust and maneuvering in the water.

An example of a fish belonging to the order Amiiformes is Amia calva, commonly known as the bowfin. Bowfins are freshwater fish found in North America and are known for their elongated bodies, rounded snouts, and distinctive cycloid scales. They have a unique combination of primitive and derived traits that have allowed them to adapt to various aquatic environments.

The Amiiformes order, with its rounded snout, cycloid scales, elongated dorsal fin, and species like the bowfin, showcases a unique group of fish within the larger class Actinopterygii. These adaptations have enabled them to thrive in freshwater habitats and demonstrate their ecological significance as predatory and resilient species.

Super order-3: Teleostei

Teleostei is a diverse and highly successful superorder of fish, comprising the largest group of living vertebrates. These fish exhibit a wide range of adaptations and characteristics that set them apart within the class Actinopterygii. Here are some key features of the superorder Teleostei:

  1. Bony Skeleton: Teleostei fish have skeletons primarily composed of bone, which provides structural support and protection. Their bones are relatively lightweight, allowing for increased maneuverability and agility in the water.
  2. Variable Scales: The scales of Teleostei species can be cycloid, ganoid, or absent altogether. Cycloid scales are thin, flexible, and overlap each other, providing a smooth body surface. Ganoid scales, on the other hand, are thicker and more rigid, often with a diamond-shaped or rhombic structure. Some Teleostei fish have evolved scales that are greatly reduced or absent, giving them a naked or smooth skin appearance.
  3. Homocercal Caudal Fin: Teleostei fish possess a homocercal caudal fin, meaning that the upper and lower lobes of the tail are of equal size. This tail fin design allows for efficient propulsion and precise control of movement, contributing to their swimming abilities.
  4. Swimbladder or Air Sac: Teleostei species may have a swimbladder, which is an internal gas-filled organ that helps control buoyancy and assists with respiration. However, not all Teleostei fish possess a swimbladder, as it can be reduced or absent in certain species.
  5. Movable Premaxilla and Maxilla: The premaxilla and maxilla, which are bones in the upper jaw, are movable in Teleostei fish. This flexibility enables them to have a wide range of mouth movements, facilitating various feeding strategies and enhancing their ability to capture prey.

Teleostei encompasses an extensive range of fish species found in both freshwater and marine environments. They exhibit remarkable diversity in their size, shape, coloration, and ecological adaptations. From colorful reef fish to swift predators and peaceful herbivores, Teleostei showcases the remarkable success and adaptability of this superorder.

The superorder Teleostei, with its bony skeleton, variable scales, homocercal caudal fin, presence or absence of swimbladder, and movable premaxilla and maxilla, represents a vast array of fish species that have conquered aquatic habitats worldwide. Their evolutionary innovations and versatility have made them dominant players in aquatic ecosystems and a subject of great fascination for researchers and fish enthusiasts alike.

Superorder-3: Teleostei divided into 31 Orders

The superorder Teleostei, consisting of a wide variety of fish species, is further classified into 31 distinct orders. Each order possesses unique characteristics and adaptations. Let’s explore these orders:

  • Order-1: Elopiformes: Elopiformes are medium to large-sized fish with a deep body. They have the pelvic fins situated at the abdominal region. The lobe of their caudal fin is deeply forked. Examples of Elopiformes include Elopes and Megalops.
  • Order-2: Anguiliformes:Anguiliformes are characterized by their elongated and slender bodies. They lack pectoral and pelvic fins, while their dorsal and anal fins are attached to the caudal fin. They may have minute scales or be scaleless. The European eel (Anguila) is an example of this order.
  • Order-3: Notacanthiformes: Notacanthiformes have slender and eel-like bodies. They possess a pectoral fin situated at the abdominal region and lack a caudal fin. These fish typically inhabit deep-sea regions. Notacanthus is an example of the Notacanthiformes order.
  • Order-4: Clupeiformes: Clupeiformes have scaleless heads and opercula. Their bodies are covered with cycloid scales, and their fins contain soft fin rays. Fish in this order include species like Tenualosa, Gadusia, and Setipina.
  • Order-5: Osteoglossiformes: Osteoglossiformes have flexible fin rays and a bony tongue. They possess a small dorsal fin and a large anal fin. Examples of Osteoglossiformes include Osteoglossum and Notopterus.
  • Order-6: Salmoniformes: Salmoniformes have flexible fin rays and pelvic fins located at the abdominal region. Some species in this order have an adipose fin, which is a fatty fin. Salmon and Esox are examples of Salmoniformes.
  • Order-7: Myctophiformes: Myctophiformes have a fatty or adipose fin and may lack a swim bladder or have a swim bladder filled with fat. These fish possess fluorescent organs in their bodies. Myctophum is an example of Myctophiformes.
  • Order-8: Cypriniformes: Cypriniformes have a swim bladder that is 2-3 chambered and connected to the pharynx. They also have a Weberian ossicle. Some species have barbels around their mouths. Labeo and Catla are examples of Cypriniformes.
  • Order-9: Siluriformes: Siluriformes are characterized by the presence of barbels around their mouths. They lack scales on their bodies but have teeth in their jaws. Some species have an adipose fin. Clarias, Wallago, and Mystus are examples of Siluriformes.
  • Order-10: Beloniformes: Beloniformes have cycloid scales and a dorsal fin located just above the anal fin. Their jaws are elongated into a rostrum-like shape. Xenontodon and Exocoetus are examples of Beloniformes.
  • Order-11: Cyprinidontiformes: Cyprinidontiformes have their dorsal and pelvic fins located towards the posterior region of the body. They have movable lips and are covered with cycloid scales. Aplocheilus is an example of this order.
  • Order-12: Echeneiformes: Echeneiformes have a dorsal fin that is modified into a sucker organ. They lack a swim bladder but retain their other fins. Echeneis is an example of this order.
  • Order-13: Lophiformes: Lophiformes have a thick pectoral fin and a dorso-ventrally flattened body. They possess fluorescent organs in their bodies. Lophius is an example of Lophiformes.
  • Order-14: Gadiformes: Gadiformes, in some species, have three dorsal fins and two anal fins. Their fins contain soft fin rays, and their pelvic fins are situated at the front of the body. Gadus is an example of Gadiformes.
  • Order-15: Syngnathiformes: Syngnathiformes have slender and elongated bodies. Their dorsal and pectoral fins are small, and their mouths are located at the terminal end of the rostrum. Examples of Syngnathiformes include Hippocampus and Syngnathus.
  • Order-16: Pleuronectiformes: Pleuronectiformes have flattened and asymmetrical bodies. Their eyes are movable and located on one side of the head. Both the dorsal and anal fins are large. Cynoglossus is an example of Pleuronectiformes.
  • Order-17: Channiformes: Channiformes, also known as snakehead fish, have scales on their heads. They possess an accessory respiratory organ and exhibit a unique respiratory adaptation. Channa is an example of Channiformes.
  • Order-18: Perciformes: Perciformes have the pectoral fin attached to the head. Their dorsal and anal fins contain both hard and soft fin rays. They possess a swim bladder and an accessory respiratory organ. Anabas, Colisa, and Nandus are examples of Perciformes.
  • Order-19: Symbranchiformes: Symbranchiformes have eel-like bodies. They lack fin rays in both the dorsal and anal fins, and they do not have pectoral fins. Ophisteron is an example of Symbranchiformes.
  • Order-20: Tetraodontiformes: Tetraodontiformes have rounded bodies and scale-like spines. They have sharp teeth in their jaws, and their pelvic fins are absent. Tetradon is an example of Tetraodontiformes.
  • Order-21: Mugiliformes: Mugiliformes have elongated bodies with broad and flattened heads. They possess a lateral line sense organ, and their jaws may contain small teeth or be toothless. Mugil is an example of Mugiliformes.
  • Order-22: Polynemiformes: Polynemiformes have elongated bodies with two dorsal fins. They possess a lateral line sense organ, and thread-like structures are present on the lower side of their pectoral fins. Polynemus is an example of Polynemiformes.
  • Order-23: Mastacembeliformes: Mastacembeliformes have slender and elongated bodies. Some species have spines in front of their dorsal fins. Nostrils are present at the end of their snouts. Mastacembalus is an example of Mastacembeliformes.
  • Order-24: Scorpaeniformes: Scorpaeniformes have elongated bodies, dorso-ventrally flattened heads, and cycloid or ctenoid scales, or lack scales altogether. Their pectoral and caudal fins are rounded, and many species have numerous spines on their heads. Scorpaena and Pterois are examples of Scorpaeniformes.
  • Order-25: Percopsiformes: Percopsiformes have large heads, and some species lack pelvic fins. Their dorsal fin may be of the adipose type. Percopsis is an example of Percopsiformes.
  • Order-26: Gonorhynchiformes: Gonorhynchiformes have a more or less pointed head region and a small mouth without teeth. Their pelvic fins are located ventrally in the middle region. Chanos is an example of Gonorhynchiformes.
  • Order-27: Characiformes: Characiformes have ovoid-shaped bodies with a dorsal adipose fin. They possess sharp and well-developed teeth in their jaws, and their anal fins are large. Weberian ossicles are present in this order. Pygocentrus and Piaractus are examples of Characiformes.
  • Order-28: Scopeliformes/Aulopiformes: Scopeliformes/Aulopiformes have elongated bodies, with the posterior region being flattened. They have large mouths with teeth and a small adipose dorsal fin. Harpodon and Saurida are examples of this order.
  • Order-29: Pegasiformes: Pegasiformes have two dorsal fins with spines. Their dorsal and anal fins are small, and the head region is elongated and pointed. Pegasus is an example of Pegasiformes.
  • Order-30: Beryciformes: Beryciformes are deep-sea fish with broad heads. They possess two dorsal fins, and their pelvic fins have long spines. The caudal fin contains branched fin rays. Holocentrus is an example of Beryciformes.
  • Order-31: Batrachoidiformes: Batrachoidiformes have elongated bodies with a dorso-ventrally flattened head region. They may have scales or lack them entirely. A spiny operculum is present, and they possess two dorsal fins. Batrachomoeus trispinosus (three-spined frogfish) is an example of Batrachoidiformes.

Each of these 31 orders within the superorder Teleostei represents a diverse group of fish with their own unique adaptations and characteristics, contributing to the remarkable diversity of fish species found in aquatic ecosystems around the world.

Examples of Osteichthyes

Osteichthyes, or bony fishes, encompass a wide range of species. Here are some notable examples:

  1. Ray-finned fishes: This group includes numerous marine and freshwater bony fishes known for their fins supported by long, flexible rays. Examples include:
  • Hippocampus (Sea horse): These unique fish have a distinctive upright posture, a prehensile tail, and a unique mode of reproduction where males carry eggs in a specialized pouch.
  • Exocetus (Flying fish): Known for their long pectoral fins, these fishes can glide above the water’s surface to escape predators or travel long distances.
  • Lophius (Angler fish): Angler fishes have a characteristic “fishing lure” appendage on their heads to attract prey. They are known for their ambush predation strategy.
  1. Freshwater bony fishes: This category comprises various bony fish species found in freshwater habitats. Examples include:
  • Labeo rohita (Rohu): Rohu is a popular freshwater fish species in South Asia, highly valued for its taste and commercial importance.
  • Labeo catla (Katla): Katla is another important freshwater fish species found in South Asia, known for its large size and high economic value.
  • Clarias (Walking catfish or Magur): These catfish species have the ability to use their pectoral fins to move across land for short distances.
  • Mystus (Catfish): Catfish species belonging to the Mystus genus are widely distributed in freshwater ecosystems and are popular aquarium pets.
  1. Aquarium bony fishes: This group includes bony fishes commonly kept in aquariums due to their vibrant colors or unique characteristics. Examples include:
  • Betta splendens (Fighting fish): Also known as Siamese fighting fish, Betta splendens is admired for its vibrant colors and elaborate fin displays.
  • Pterophyllum (Angelfish): Angelfish are known for their graceful appearance, triangular shape, and long, flowing fins. They are popular in the aquarium trade.
  1. Lobe-finned fishes: This group includes bony fishes with fleshy lobed fins. While most species in this group are extinct, some living examples exist, such as:
  • Lungfishes: Lungfishes are air-breathing fishes found in stagnant waters of Africa, South America, and Australia. Notable species include Lepidosiren, Neoceratodus, and Protopterus.
  • Coelacanth (Latimeria): The coelacanth is a living fossil and represents the oldest known lineage of Sarcopterygii. They were once thought to be extinct until a living specimen was discovered in 1938.

These examples illustrate the diversity and uniqueness of bony fishes, showcasing their adaptability to various habitats and their importance in both natural ecosystems and human interactions.


What are Osteichthyes?

Osteichthyes are a class of fish commonly known as bony fish. They are characterized by having skeletons made of bone rather than cartilage.

How many species of Osteichthyes are there?

There are over 30,000 known species of Osteichthyes, making them the largest class of vertebrates.

What are the main features of Osteichthyes?

Osteichthyes have bony skeletons, gills for respiration, and swim bladders or lungs to control buoyancy. They also possess scales and fins for locomotion.

Do Osteichthyes include both freshwater and marine fish?

Yes, Osteichthyes include fish that inhabit both freshwater and marine environments. They are found in various aquatic habitats worldwide.

What is the diet of Osteichthyes?

The diet of Osteichthyes can vary depending on the species. Some are herbivorous, feeding on plants and algae, while others are carnivorous, consuming smaller fish, invertebrates, or even plankton.

Can Osteichthyes breathe air?

Some species of Osteichthyes have adapted to breathe air. They possess a lung-like organ called a swim bladder, which allows them to extract oxygen from the air when in oxygen-deprived water.

Are all Osteichthyes edible?

While many Osteichthyes species are considered edible and form an important food source for humans, not all of them are suitable for consumption. Some may be toxic or not palatable.

How do Osteichthyes reproduce?

Osteichthyes have various reproductive strategies. Most species are oviparous, meaning they lay eggs, while others are viviparous, giving birth to live young. Some species also exhibit parental care.

Can Osteichthyes change their color?

Yes, certain species of Osteichthyes have the ability to change their coloration to blend with their surroundings, communicate with other fish, or display courtship behaviors.

Are Osteichthyes important for ecosystems and humans?

Yes, Osteichthyes play a crucial role in aquatic ecosystems. They contribute to nutrient cycling, serve as prey for other organisms, and are economically important as a food source and in recreational fishing.



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