Despite common misunderstandings, sharks are indeed fish. They share fundamental characteristics with all fish while also possessing specialized traits that set them apart.
Common Misconceptions About Sharks
Confusion often arises because sharks do not resemble the typical bony fish many people imagine, such as a goldfish or tuna. Their large size and predatory nature contribute to the misconception that they might be more akin to marine mammals like dolphins or whales. These animals, however, are warm-blooded, breathe air with lungs, and give birth to live young, fundamentally differing from sharks. Popular media portrayals also frequently depict sharks as aggressive “man-eaters,” which further separates them in public perception from other, seemingly less formidable, fish species.
Defining the Characteristics of Fish
To understand why sharks are classified as fish, it helps to review the general characteristics that define this group. Fish are aquatic, gill-bearing vertebrates that live exclusively in water. They obtain oxygen by drawing water over their gills. Most fish are cold-blooded, or ectothermic, meaning their body temperature fluctuates with the surrounding water. Additionally, fish typically have fins for movement and balance, and their bodies are often covered in scales.
Unique Traits of Sharks as Fish
While sharks meet the criteria for being fish, they possess several unique adaptations that distinguish them from the more common bony fish. A primary difference is their skeleton, which is composed entirely of cartilage rather than bone. This flexible and lighter material helps sharks move efficiently through water and contributes to their buoyancy. Unlike most bony fish that have a single gill opening protected by a flap called an operculum, sharks feature multiple, distinct gill slits, typically five to seven pairs.
Sharks also have a distinctive skin covering made of placoid scales, also known as dermal denticles. These tiny, tooth-like structures are embedded in the skin and point backward, giving the shark’s skin a rough, sandpaper-like texture. Placoid scales help reduce drag as the shark moves through water, enhancing swimming efficiency.
Furthermore, sharks do not possess a swim bladder, an air-filled organ that bony fish use for buoyancy control. Instead, sharks rely on a large, oil-filled liver, which can constitute a significant portion of their body mass, along with the lower density of their cartilaginous skeleton, to aid in buoyancy. Many shark species also generate dynamic lift through continuous swimming, using their pectoral fins to prevent sinking.