The question of whether sharks are fish often sparks curiosity. Despite their aquatic habitat and finned appearance, unique characteristics lead to uncertainty regarding their biological classification. Understanding the biological definition of fish and the attributes of sharks clarifies their place within the animal kingdom.
Defining What Makes a Fish
Fish are aquatic vertebrates with gills for respiration and fins for movement. They extract oxygen from water via their gills. Most are ectothermic, meaning their internal body temperature is influenced by their surrounding aquatic environment.
The superclass Pisces encompasses all true fish, which are aquatic vertebrates. This classification includes two major groups: Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). Cartilaginous fish, like sharks, have skeletons primarily composed of flexible cartilage. Bony fish have skeletons predominantly made of bone, representing the most abundant group.
The Distinctive Biology of Sharks
Sharks possess a skeletal system made entirely of cartilage, which is lighter and more flexible than bone. Their skin is covered by dermal denticles, or placoid scales. These tiny, tooth-like structures give their surface a rough, sandpaper-like texture, streamline the shark’s body, and protect against parasites.
Unlike most bony fish with a single gill cover (operculum), sharks typically feature five to seven visible gill slits on each side of their head. These openings allow water to pass over their gills for oxygen exchange. Sharks also lack a gas-filled swim bladder for buoyancy control. Instead, they rely on a large, oil-filled liver and continuous swimming to maintain their position in the water column.
Sharks exhibit diverse reproductive strategies. Some species lay eggs, while others retain eggs internally until hatching, or give birth to live young. Fertilization in sharks is internal, distinguishing them from many bony fish that reproduce through external fertilization.
Why Sharks Are Indeed Fish
Sharks are classified as fish. They meet the fundamental biological criteria: they are aquatic vertebrates, possessing a backbone, gills for respiration, and fins for movement. These characteristics place them within the biological definition of fish.
Their classification within the class Chondrichthyes, or cartilaginous fish, distinguishes them from bony fish (Osteichthyes). While their cartilaginous skeletons, multiple gill slits, and placoid scales are unique traits, these features define a specific subgroup within the fish lineage. The common misconception often arises because “fish” is frequently associated with the more numerous bony fish, overlooking the diverse forms within the biological classification. Sharks are an ancient and successful group of fish, thriving in oceans for millions of years.