Sharks are indeed a type of fish, classified within the vast group of aquatic vertebrates. Although their appearance and certain biological features might lead some to question this classification, sharks meet all fundamental criteria that define a fish.
What Defines a Fish?
A fish is broadly defined as an aquatic vertebrate animal possessing gills for breathing and fins for locomotion. Most fish are cold-blooded, meaning their body temperature fluctuates with their surrounding water, though some active species can maintain a higher core temperature. They typically have a streamlined body shape and are covered in scales, providing protection and reducing drag in water.
Gills are specialized organs that allow fish to extract oxygen dissolved in water and release carbon dioxide, performing a function similar to lungs in terrestrial animals. Fins, which vary in type and structure, enable movement, steering, and stability within their aquatic environment.
Sharks: Unique Fish Characteristics
Sharks belong to a distinct group of fish known as cartilaginous fish, characterized by skeletons made primarily of cartilage rather than bone. This flexible yet strong connective tissue provides a lighter and more agile framework compared to the dense bone of most other fish.
Their respiratory system features multiple gill slits, typically five to seven pairs, which are exposed on the sides of their heads. In contrast, most bony fish have a single gill opening covered by a protective operculum. Sharks extract oxygen from water as it passes over these gill filaments, either through continuous swimming (ram ventilation) or by actively pumping water when at rest.
Shark skin is covered in placoid scales, also known as dermal denticles, which are structurally similar to tiny teeth. These scales are tough and provide a protective layer, differing significantly from the flatter, overlapping scales found on bony fish. The unique arrangement of these denticles also helps to reduce drag, allowing for efficient movement through water.
Sharks possess several types of fins, including pectoral, pelvic, dorsal, and caudal fins, which aid in balance, steering, and propulsion. Unlike the flexible fins of many bony fish, shark fins are generally rigid and lack fin rays, contributing to their powerful and controlled swimming.
Buoyancy in sharks is primarily maintained by a large, oil-filled liver, which can constitute up to 30% of their body mass, rather than a gas-filled swim bladder common in bony fish. The lower density of the oil-rich liver and their cartilaginous skeleton helps offset their tendency to sink, often requiring continuous swimming for depth maintenance.
Why the Confusion About Sharks Exists
Confusion regarding whether sharks are fish often stems from their distinct appearance and biological differences when compared to the more commonly envisioned bony fish. Their sleek, often intimidating bodies and predatory nature set them apart from familiar fish species like cod or salmon, which typically have a different body plan and visible scales. The absence of a bony skeleton, a defining feature for the vast majority of fish species, also contributes to this misunderstanding.
Most people associate “fish” with bony fish, overlooking the cartilaginous fish group to which sharks belong. While both are aquatic vertebrates with gills and fins, the internal skeletal composition is a major divergence.
Occasionally, people mistakenly associate sharks with marine mammals, such as dolphins or whales, due to their large size, streamlined shapes, and sometimes predatory roles. However, crucial biological distinctions separate sharks from mammals, including their method of breathing via gills versus lungs, being cold-blooded (with some exceptions) rather than warm-blooded, and their dermal denticles instead of hair. While some sharks give birth to live young, a trait shared with mammals, this reproductive strategy does not override their fundamental fish characteristics.