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: It is 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.

Also Read: Blood – Components, Functions, Groups and Disorders

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.

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.