Skeletal System of Fish (original) (raw)
Last Updated : 23 Jul, 2025
The **skeletal system of fish is the bones and the cartilage that support the fish's body. Even the basic hagfishes have a skeletal system. The skeletal system of sharks and rays is complicated and mainly consists of cartilage rather than bones. Based on the type of skeleton the fishes are classified into bony fishes and cartilaginous fishes. The main function of the skeletal system is to provide protection, support, and aid in locomotion. In this article, we will look into the **meaning, composition, and function of the skeletal system of fish.
Table of Content
- Skeletal Structure of Fish
- Fish Skeletal System Labeled Diagram
- Composition of the Fish Skeleton System
- Fish Bones
- Classification of Fish Based on the Habitat and Skeletal System
- Function of the Fish Skeletal System
Skeletal Structure of Fish
Fishes are vertebrates that belong to the superclass **Pisces and are aquatic creatures with a strong skeletal system. The skeletal system of a fish forms the basic foundation of the body and is mainly made up of bones, cartilage, and connective tissues. The system's primary function is protection and movement and offers shape and support to the body.
Fish Skeletal System Labeled Diagram
The fish skeletal system labelled diagram is given below:
Composition of the Fish Skeleton System
The fish skeletal system comprises about 150 bones that feature the vertebral column, jaw, ribs, cranium, and intramuscular bones. The skeletal system is divided into two parts:
- The **Endoskeleton: Comprises the skull, vertebrae, and ribs.
- The **Exoskeleton: Consists of pectoral and pelvic girdles and the bones of the appendages.
The Endoskeleton
The endoskeleton of the fish is covered with scales and it is comprised of:
- **The Skull: Itis a double structure that consists of two boxes of bone, one is enclosed by the other. The outer skeleton is an armour of the dermal bone seen in primitive extinct bony fishes. In modern fishes, the outer skeleton is the remains of the armour. The elements of the inner skull form the cranium and consist of many small bones. The skull is made up of about 130 small loosely connected bones, it has three different parts:
- **Chondrocranium: It is a cartilaginous structure that forms the skull in cartilaginous fishes like sharks and rays. In bony fishes, the cartilaginous chondrocranium is replaced by an endochondral bone called neurocranium.
- **Splanchnocranium: It is formed by the branchial arches or gill arches and this cranium is also known as visceral skeleton. The main function of this part is to support the oral cavity including the teeth and jaws.
- **Dermatocranium: It is made of dermal bones and forms the major part of the skull and protects the brain. The dermatocranium is made up of roofing bones, primary palatal bones, upper and lower jaw bones, and opercular bones
- **Vertebral Column: A series of endochondral bones called vertebrae form the vertebral column. The vertebral column has a large spool-shaped structure in the center called the centrum and extending through the middle of each centrum is a canal for the notochord to pass through. On the top of the center, an arch of bone is present that protects the spinal cord. A dorsal projection called the neural spine extends from the vertebral column. The vertebral column is divided into two parts:
- **The Trunk: A neck is absent in fish, the first two trunk vertebrae are modified and lack ribs while the rest of the vertebrae have **ribs.
- **The Tail (caudal): The vertebrae of this region possess a ventral portion that forms a hemal arch that surrounds the blood vessels.
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The Exoskeleton
The exoskeleton mainly supports the fins of the fish and it is divided into three parts namely:
- **Median Fins: The dorsal fins have fin rays for support whereas the anterior dorsal fin has ossified fin rays. These are spiny and provide stiff support, while the fins of the posterior dorsal fin are soft as they are not ossified and flexible. The first two fins of the anal fin are ossified and the caudal fins are completely made of soft and unossified fin rays.
- **Pectoral Girdle and Fins: These fins are attached to a bony girdle called the pectoral girdle. It is composed of several fused elements and is also fused to the skull. The fins are supported by soft fin rays.
- **Pelvic Girdle and Fins: The pelvic fins are attached to the pelvic girdle and are composed of two bony pelvic plates that are fused along the midline. The pelvic girdle is not attached to the vertebral column or the pectoral girdle but is embedded only in the muscle.
Fish Bones
The skeletal system of the fish is made up of many small bones, they are dense and less flexible than cartilage. The different bones in fish are:
- **Vomer: The vomer midline bone is present on the roof of the mouth and mainly supports the teeth.
- **Articular: It is a movable joint that is formed by a pair of angular-shaped bones and it forms the posterior end of the lower jaw.
- **Maxillary: These are a pair of bones that are fused with palatal fissure to form the upper jaw.
- **Retro Articular: Paired bones that are located at the lower posterior corner of the articular.
- **Opercle: It is a paired, flat bone that is connected to the hyomandibula bones to form the majority of the gill cover.
- **Centrum: The centrum is the central cylindrical component of each vertebra, it obstructed the embryonic notochord during the ossification process.
- **Dentary: These are large paired dental bones that are united medially nd create the front of the lower jaw
- **Scapula: The scapula is a pair of flat bones that articulate with the coracoids anteriorly and cleithra posteriorly. Small bones called radials connect the fin rays that articulate anteriorly with the scapula and coracoid bones.
Classification of Fish Based on the Habitat and Skeletal System
Fishes are classified into two types based on the type of skeletal system. They are:
- **Cartilaginous fishes (Chondrichthyes): The skeleton of these fishes is mainly made of cartilage that is lightweight and flexible and helps in the swift and efficient movement of the fish in the water. It is highly durable and protects the internal organs. The bone marrow is absent and they rely on different mechanisms to maintain buoyancy. These fishes have a dense oil-filled liver to maintain buoyancy and do not have swim bladder like bony fishes. They are found in saltwater such as oceans.
- **Bony Fish (Osteichthyes): These fishes are made up of bones that are hard and calcified and their main function is to provide support and protection. These bones also offer a strong framework for the muscles to attach and enable powerful swimming motions. The fish have a swim bladder that is filled with gas to regulate buoyancy in the water. They also exhibit various adaptations based on the habitat they live in.
**Also Read: Skeletal System: Anatomy and Function, Diagram
Function of the Fish Skeletal System
There are various functions of the skeletal system in fishes, some of which are:
- **Support and Structure: Like any other vertebrate, the skeletal system of fishes also offers physical support where it gives shape and structural integrity.
- **Protection of Vital Organs: The skeletal system forms a protective covering around all the vital organs that are protected from external threats and pressure of underwater environments. For example, the skull encases the brain and the sensory organs while the ribs and vertebrae form a protective cage that protects the heart, lungs, swim bladder, and other important internal organs.
- **Locomotion: this is one of the important functions of the skeletal system, the fins that are supported by the skeletal structures are the propellers of the fish which allows stability, steering, and **propulsion. The muscles and the bones of the fish enable efficient swimming and conserve energy while navigating through water and escaping from predators. The framework enables the attachment of muscles that facilitate movement and swimming. The bones can either be rigid or flexible cartilage based on the type of fish and modes of locomotion.
- **Buoyancy Control: Buoyancy is the ability to maintain a desired depth without using much energy, this is partially regulated by the skeletal system. In bony fishes, the swim bladder helps in controlling buoyancy and it also helps in maintaining **equilibrium and facilitates effortless vertical movement in the water.
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Conclusion - Fish Skeletal System
The fish skeletal system is complex and very diverse as it reflects a vast array of evolutionary adaptations enabling the fish to survive in any aquatic environment. Based on the composition of the skeletal system there are two types of fishes; cartilaginous fishes and bony fishes. The main function of the skeletal system is to provide mechanical support, and protection and help in locomotion.
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