A shark is indeed a fish, though this fact often causes confusion due to the animal’s powerful reputation and distinct appearance. Sharks meet all the general biological criteria for classification as fish, yet they belong to a separate, ancient lineage. Most people picture bony fish, such as salmon or goldfish, which creates a misconception that anything different must be something else entirely. Sharks possess unique adaptations and internal structures that differentiate them from these common, bone-structured species.
The Broad Classification of Fish
Fish are defined as aquatic, gill-bearing vertebrates that possess fins but lack limbs with digits, which immediately includes sharks in this category. This definition encompasses a massive and diverse group of animals, constituting more than half of all vertebrate species on Earth. All members of this group are ectothermic, meaning they are cold-blooded and their body temperature fluctuates with the surrounding water temperature.
These creatures utilize specialized organs called gills to extract dissolved oxygen from the water. They also rely on fins for all aspects of movement, including steering, balance, and propulsion through the water column. Sharks, like all other fish, possess a backbone, placing them within the subphylum Vertebrata. This baseline classification confirms that sharks satisfy the fundamental requirements to be scientifically considered fish.
The Defining Feature: Cartilage
The primary distinction separating sharks from the majority of fish lies in their skeletal composition. Sharks, rays, and skates belong to the class Chondrichthyes, derived from the Greek words for “cartilage” and “fish,” signifying their entirely cartilaginous skeleton. Cartilage is a flexible and lighter material compared to bone.
In contrast, the vast majority of commonly known fish, such as tuna and cod, belong to the class Osteichthyes, or bony fish, which have a hard skeleton made of calcium-rich bone tissue. The cartilaginous structure provides sharks with a lighter body mass, aiding in buoyancy and flexibility. They lack the true bone tissue found in Osteichthyes, placing them on a separate evolutionary branch that diverged hundreds of millions of years ago.
Specialized Shark Anatomy
Beyond the skeleton, sharks possess several unique anatomical features that further differentiate them from bony fish. One prominent difference is their respiratory system and the structure of their gills. Bony fish typically have a single, protective bony flap called an operculum that covers and moves water over their gills. Sharks lack this cover and instead have five to seven visible, external gill slits on the side of their head through which water exits.
Their skin is also highly specialized, covered not with flat, overlapping scales like most fish, but with tiny, tooth-like structures called dermal denticles, or placoid scales. These denticles are structurally similar to teeth and help reduce drag and turbulence as the shark swims, improving hydrodynamic efficiency.
Another significant difference is how sharks maintain their position in the water column without sinking. Bony fish use a gas-filled organ called a swim bladder to regulate buoyancy, allowing them to remain suspended with minimal effort. Sharks do not possess a swim bladder and instead rely on a massive, oil-filled liver, which can constitute up to 30% of their body weight, to provide lift. They must also continually swim to generate dynamic lift using their pectoral fins and asymmetrical tails, a method known as ram ventilation, to keep water flowing over their gills and maintain their depth.