Collegedunia Team Content Curator
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Osteichthyes belongs to Kingdom Animalia and consists of all bony fishes. It stands out to be the class of largest vertebrates in existence. It is so diverse and abundant that it consists of 45 orders, 435 families, and 28,000 species! The Osteichthyes have specific organs that are associated to form functional systems, each system is concerned with a specific physiological function thus they follow the pattern of organ system level of organisation.
Also Read: Difference between Osteichthyes and Chondrichthyes
Key Takeaways: Osteichthyes, Chordata, Animalia, Vertebrata, Organ system, Bilateral symmetry, Coelom, Notochord, pharynx,
What are Osteichthyes?
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Osteichthyes stands out to be the class of largest vertebrates in existence with 45 orders, 435 families, and 28,000 species. They consist of all bony fish. They have bilateral symmetry that means they have only one plane that divides their bodies into two identical halves. Their body cavity is lined by a mesoderm thus it is known as a coelom. They also possess a Notochord which is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development of the animal. Later as they grow this notochord is replaced by a cartilaginous or bony vertebral column.
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The hierarchical taxonomy of Osteichthyes is as follows.
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Division: Gnathostomata
Superclass: Pisces
Classification of Vertebrata
Class Osteichthyes is further divided into two major sub-classes. They are:
- Sarcopterygii – Lobe finned fishes
- Actinopterygii – Ray-finned fishes
Read More:
| Difference between bony fish and cartilaginous fish | Phylum Chordata |
| Biological Classification | Difference Between Chordates and Non-Chordates |
Characteristics of Osteichthyes
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Some of the main characteristics of Osteichthyes are as follows.
- The main characteristics of Osteichthyes are that they have a bony endoskeleton.
- They include marine, brackish, and freshwater fish.
- They have a streamlined body (tapering at the ends).
- They have paired pectoral and pelvic fins along with other anal, dorsal, and caudal fins.
Fins of Osteichthyes
- Their mouth forms one of the terminals while their digestive tract leads up to the anus.
- They have a covering of operculum (gill cover- it covers the aperture) on each side of the four pairs of gills.
- They have skin that is covered with ganoid/ cycloid/ ctenoid scales which are dermal (skin) in origin.
- They regulate buoyancy with the help of the air bladder (also known as the swim bladder which looks like sac outgrowth). With the help of the air bladder, they can swim in deeper or shallower waters with up and down moments.
- They have a two-chambered heart- one auricle and one ventricle: along with sinus venosus and conus arteriosus. Lungfish is an exception that has three chambers in its heart with two auricles and one ventricle.
Anatomy of Osteichthyes
- The red blood corpuscles are nucleated and oval in shape.
- They have kidneys that are mesonephric and majorly excrete ammonia as nitrogenous waste.
- They have a well-developed lateral line system (helps fishes in detecting small movements and different pressure gradients) which has hydrodynamic receptors.
- They are cold-blooded animals (ectothermic) which means that they lack the capacity to regulate their internal body temperature. Exceptions are larger marine fishes like swordfish, tuna which show some degree of endothermy.
- They have separate sexes.
- They reproduce sexually where fertilization occurs outside the body.
Reproduction in Osteichthyes
- They are oviparous while some exceptional fishes are ovoviviparous, and development is direct except in Anguilla.
- They have a well-developed renal portal system that functions to supply blood from the tail through the kidneys.
- These fishes keep balance because of their internal ears.
- They have a well-developed nictitating membrane (translucent or transparent third eyelid used for protection or moistening the eye) in their eyes.
- The brain of these fishes has a small olfactory and cerebellum.
- They possess 10 pairs of cranial nerves (nerves that connect the brain with other parts of the body)
- They do not possess Cloaca (common chamber for intestinal, urinary, genital tracts) while other anal and genital openings are present
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Sarcopterygii Characteristics
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Sarcopterygii is a subclass that consists of lobe-finned fishes. Some of the examples are West Indian Ocean Coelacanth, African Lungfish, Lepidosiren, Neoceratodus, Protopterus. The characteristics of Sarcopterygii are as follows.
- They have lobed, paired, fleshy fins which are a part of their body connected by a single bone.
- Sarcopterygians have scaloid scales which are circular in shape with ‘cut-up’ stripes.
- They are fishes that look similar to tetrapods.
- They have separate bases which have two dorsal fins attached to them.
Sarcopterygii
- All Sarcopterygians have enamel-covered teeth.
- Only a few species of Sarcopterygii are extant. Among them are the species of Lungfishes and Coelacanth
- Coelacanth Latimeria is the oldest known lineage of Sarcopterygii. It was thought to be extinct before.
Read More: Difference between Aestivation and Hibernation
Actinopterygii Characteristics
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Actinopterygii is a subclass that consists of fish with thin and elongated fins. Some of the examples are Labeo, Lates, Clarius, Hilsa, etc. The characteristics of Actinopterygii are as follows.
- They do not have any scales in the head region while their body is covered with cycloid/ ganoid/ ctenoid scales. Some exceptional actinopterygian lack scales.
- The long caudal and dorsal fins are not connected to the spine. In some species, the fins are not supported by fin rays.
- They have a comparatively large mouth aperture.
- They lack accessory respiratory organs.
Actinopterygii
- They are commonly present in freshwater or marine habitats.
- They lack conus arteriosus in the heart.
- They have an external nare on the top of their head.
- Usually, Actinopterygii does not have a spiracle.
- Their Caudal fins are homocercal or heterocercal in nature.
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Things to Remember
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- Osteichthyes belongs to Kingdom Animalia and consists of all bony fishes.
- Very few species of Sarcopterygii are not extinct. There are 8 living species of Coelacanth and 6 species of lungfish.
- There are a total of 45 orders under the Class Osteichthyes
- There are a greater number of Sarcopterygian species than Actinopterygian
- The main difference between the two sub-classes Actinopterygii and Sarcopterygii is their distinct fin structure.
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Sample Questions
Ques. How important is the presence of air bladder in Pisces? (2 marks)
Ans. The air bladder is the respiratory or hydrostatic organ of the fish. It is an outgrowth of the alimentary canal. The air bladder in Pisces helps in buoyancy. The air bladder is filled with air which helps in ascending or descending in water current. It controls the movement of the fish. It prevents the fish from sinking. It helps in saving the energy of the fish in swimming.
Ques.‘All vertebrates are chordates, but all chordates are not vertebrates’ Justify this statement. (2 marks)
Ans. All Chordates have a notochord. Chordates include Cephalochordates, Urochordates, and vertebrates. For Vertebrates, the notochord is present in the embryonic stage, but it is replaced by a bony vertebral column as they grow while the Cephalochordates, Urochordates do not possess the vertebral column. Thus all vertebrates are chordates, but all chordates are not vertebrates.
Ques. What is the difference between homocercal or heterocercal Caudal fins? (2 marks)
Ans. The Homocercal and the Heterocercal are tail fins. The homocercal tail fin has two lobes that are equal to each other. The heterocercal tail fins have different-sized lobes. They are asymmetrical. We can see such fins in sharks. The upper part of the fin is made of the larger lobe while the lower part of the fin has the smaller lobe.
Ques. Why are Sarcopterygian (lobe-finned fishes) endangered? (2 marks)
Ans. The increase in demand for fish can be a reason but another majorly affecting reason is water pollution. Some extreme fishing techniques like heavy trawls tend to squander the ecosystem and biological cycle on which the fish is dependent. It takes considerably long time for the ocean to recover from such losses.
Ques. Why can Osteichthyes fishes breathe without swimming? (2 marks)
Ans. Osteichthyes fishes can breathe without swimming because they have an air bladder that stores air for them.
Ques. What are the major differences between class Chondrichthyes and class Osteichthyes? (2 marks)
Ans.
| Class Chondrichthyes | Class Osteichthyes |
|---|---|
| The mouth is positioned ventrally, and the body is covered with placoid scales. | The mouth is positioned apically, and the body is covered with cycloid or ctenoid, or ganoid scales |
| Gill slits, usually five pairs, open separately and directly to the exterior. | Gill slits, usually four pairs, open in a gill chamber. |
| The endoskeleton is cartilaginous. | The endoskeleton is bony with amphicoelous vertebrae. |
| The operculum is always absent. | Operculum with branchiostegal membrane always present. |
Ques. What are prominent glands present in Osteichthyes species? (2 marks)
Ans. They have mucus-secreting glands, poison glands (in some species), Photophores (light-emitting) glands as in some Osteichthyes species.
Ques. What makes cartilaginous fish ureotelic? (2 marks)
Ans. Ammonia is more poisonous than urea, yet it requires less water. The uricotelic organism excretes uric acid or uric acid ions. Because they don't have a normal urinary system, urea is concentrated in their blood and excreted via the skin. Sharks, like the majority of cartilaginous fish, are hence ureotelic.
Ques. Why are sharks classified as cartilaginous fish? (2 marks)
Ans. Sharks, skates, and rays all have cartilaginous skeletons because cartilage has a low density, which allows sharks to swim quicker in the water while using less energy and keeps them afloat. Because cartilage has a lower density than bones, it has more flexibility and can bend more easily than bony fish.
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