Sharks are cartilaginous fish, meaning their skeletons are primarily composed of cartilage rather than bone. This fundamental difference sets them apart from most other fish species.
Defining Cartilaginous Fish
Cartilaginous fish, scientifically known as Chondrichthyes, are a class of jawed fish distinguished by several unique characteristics. Their most defining feature is a skeleton made entirely of cartilage, which is a connective tissue less dense and more flexible than bone. Cartilaginous fish also lack a swim bladder, an organ typically found in bony fish for buoyancy control. Instead, they often rely on a large, oil-filled liver to help regulate their position in the water column.
Another distinguishing characteristic of these fish is their specialized scales, called placoid scales or dermal denticles. These scales are embedded in the skin and give it a rough, sandpaper-like texture. Additionally, cartilaginous fish possess multiple gill slits that open directly to the outside.
Sharks: A Prime Example
Sharks perfectly exemplify the traits of cartilaginous fish. Their skeletons, being cartilaginous, offer both flexibility for agile swimming and reduced weight, which aids in buoyancy. This lighter skeleton, combined with their large, oily livers, helps them maintain their position in the water.
The skin of sharks is covered in placoid scales, which are oriented backward, reducing drag and increasing their hydrodynamic efficiency. Sharks exhibit the characteristic multiple gill slits. Other well-known cartilaginous fish include rays and skates, sharing these fundamental anatomical features with sharks.
Beyond Bone: The Difference Makers
The primary distinction between cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes) lies in their skeletal composition. While cartilaginous fish possess skeletons made of cartilage, bony fish have skeletons predominantly composed of bone tissue. This skeletal difference impacts other physiological aspects. Bony fish typically utilize a swim bladder, a gas-filled sac, to control their buoyancy, allowing them to remain suspended in the water without continuous movement. In contrast, cartilaginous fish, lacking a swim bladder, rely on their large, oily livers and the lift generated by their pectoral fins and swimming motion to maintain buoyancy.
Their skin coverings also differ significantly. Bony fish possess various types of dermal scales, such as cycloid or ctenoid scales, which lie beneath the epidermis. Cartilaginous fish, however, are covered in placoid scales, which are tooth-like structures embedded in the skin, giving it a unique texture and function.