Are sharks fish? This common question arises, often due to their unique appearance and predatory nature. The answer is yes, sharks are fish. While they share fundamental characteristics with other aquatic vertebrates, they also possess distinct features that set them apart, leading to much of the public’s confusion regarding their classification. This article clarifies why sharks are categorized as fish, despite their differences.
Defining a Fish
A fish is defined as an aquatic vertebrate that breathes with gills and possesses fins for movement. Most fish are cold-blooded, meaning their body temperature fluctuates with the surrounding water. They have a streamlined body shape optimized for efficient swimming.
Fish also possess a backbone, placing them within the subphylum Vertebrata. This broad definition encompasses an incredible diversity of species found in various aquatic environments. Sharks meet these core criteria, living exclusively in water, possessing gills to extract oxygen, and utilizing fins for propulsion and steering.
Unique Characteristics of Sharks
Sharks belong to a group of fish called cartilaginous fish (Chondrichthyes), which includes rays and skates. Their distinguishing feature is a skeleton made entirely of cartilage, a flexible tissue, rather than bone. This contrasts with the bony skeletons of most other fish.
Unlike most bony fish that have a single, protective gill cover (operculum), sharks possess multiple, exposed gill slits, typically five to seven, on each side of their head. Their skin is covered by tiny, tooth-like scales called placoid scales. These give shark skin a rough, sandpaper-like texture and help reduce drag during swimming.
Sharks also lack a swim bladder, a gas-filled organ many bony fish use to control buoyancy. Instead, sharks rely on a large, oil-filled liver (up to 25% of their body mass) for lift. They also use dynamic lift, generated by their pectoral fins and constant swimming, to maintain position in the water. Unlike most bony fish that release eggs and sperm for external fertilization, sharks practice internal fertilization, with males using claspers to deliver sperm to the female.
Why the Confusion?
Confusion about whether sharks are fish often stems from their distinct characteristics and portrayal in popular culture. Their cartilaginous skeletons and unique gill structures make them visibly different from familiar bony fish, leading some to mistakenly believe they belong to a separate category.
As apex predators with an intimidating appearance, sharks are sometimes compared to marine mammals like whales or dolphins. However, mammals breathe air with lungs and give birth to live young nourished with milk, none of which apply to sharks. Despite their unique adaptations, sharks remain classified as fish.