Most fish have skeletons made of true bone (Osteichthyes). However, an older, less numerous lineage of jawed fish never develops this hard, calcium-rich tissue. This unique group retains a skeletal structure composed almost entirely of a more flexible material. Their distinct anatomy sets them apart from the bony fish that dominate aquatic ecosystems today.
What Defines Cartilaginous Fish
The class of cartilaginous fish, formally known as the Chondrichthyes, is defined by an endoskeleton made entirely of cartilage. This flexible tissue provides structure without the high density of true bone, a characteristic that has persisted for millions of years. These fish also possess unique anatomical features that distinguish them from their bony relatives.
Cartilaginous fish typically lack the bony plate called an operculum that covers the gills in most other fish. Instead, they feature five to seven exposed gill slits that open directly to the outside environment. Their rough skin has a texture similar to sandpaper, due to thousands of tiny, tooth-like structures known as placoid scales or dermal denticles.
They do not possess a gas-filled swim bladder for buoyancy control. To remain afloat, they rely on a large, oil-filled liver that can comprise up to 30 percent of their body mass. This low-density organ helps compensate for the lack of a gas bladder, though many species must still swim continuously to avoid sinking. Many cartilaginous fish also possess a mouth located on the underside of the head.
Structural Differences Between Cartilage and Bone
The difference between cartilage and bone is a fundamental biological and chemical distinction. Bone is a highly mineralized connective tissue composed primarily of calcium phosphate deposited around a collagen framework. This process, called ossification, creates a dense, rigid tissue that provides maximum structural support and protection.
Cartilage is a more pliable connective tissue made of cells called chondrocytes embedded in a matrix of collagen and elastin fibers. This composition makes cartilage significantly lighter and more flexible than bone, allowing for greater maneuverability and reduced body weight. Although cartilaginous fish lack true bone, their cartilage is often hardened through calcification.
Calcification involves depositing calcium salts into the cartilage matrix, giving it greater stiffness and strength, especially in areas like the jaws and vertebrae. This process does not involve the complete cellular and matrix replacement that defines true ossification. In many species, the cartilage surface is covered in tiny, mineralized tiles called tesserae, which provide external reinforcement. The resulting lighter skeleton is an advantage for active marine predators, reducing the energy cost of swimming.
Key Groups and Examples
The cartilaginous fish are divided into two main subclasses. The Elasmobranchii is the larger and more familiar group, containing the sharks, rays, and skates. Sharks exhibit a streamlined, torpedo-shaped body plan highly adapted for speed and predatory hunting in the open ocean.
Rays and skates, also part of the Elasmobranchii, have evolved a dramatically different form. They are characterized by flattened bodies and enormous pectoral fins fused to the side of the head. This body shape is ideal for life on the seabed, allowing them to camouflage themselves and feed on bottom-dwelling prey.
The second subclass, the Holocephali, is a smaller group commonly known as chimaeras or ghost sharks. These deep-sea inhabitants are distinct from elasmobranchs because they have a single external gill opening on each side of the head, covered by a flap of tissue. Chimaeras possess specialized tooth plates for crushing hard-shelled invertebrates, unlike the continuously replaced teeth found in most sharks and rays.