The whale shark, often considered the largest fish in the world, frequently causes confusion due to its immense size and common name. Despite its “whale” designation, it is not a marine mammal but a fish, specifically a shark. This article clarifies why the whale shark is classified as a shark, examining its biological characteristics and contrasting them with those of whales.
Defining a Shark
Sharks are a diverse group of cartilaginous fishes, distinguished by several biological features. Their skeletons are composed of cartilage, a flexible tissue, rather than bone. Sharks typically possess five to seven gill slits on each side of their head, used for extracting oxygen from water.
Their bodies are streamlined, facilitating efficient movement through water. Sharks have various fins, including dorsal fins on their back, pectoral fins on their sides, and a caudal (tail) fin, which aids in propulsion. Most sharks are cold-blooded, meaning their internal body temperature is regulated by the surrounding water temperature. These anatomical and physiological traits define what constitutes a shark.
Whale Shark Characteristics
The whale shark exhibits all the defining characteristics of a shark. Like other sharks, its skeletal structure is entirely cartilaginous, providing flexibility and reducing weight. This contrasts with the bony skeletons found in most other fish and all mammals. Whale sharks possess five large gill slits on each side of their head, which they use to breathe underwater, a method distinct from the lung-based respiration of mammals.
They also display shark fins, including two dorsal fins, pectoral fins, and an anal fin. Despite their enormous size, whale sharks are cold-blooded, like other fish. Their filter-feeding mechanism, where they strain plankton and small fish from the water, is a specialized adaptation. This does not alter their classification as a shark.
Why Not a Whale
Whale sharks are distinct from whales, which are marine mammals, due to several biological differences. Whales are mammals, characterized by being warm-blooded, breathing air with lungs, and nourishing their young with milk. In contrast, whale sharks are fish, which are cold-blooded, breathe using gills, and do not produce milk for their offspring.
A skeletal difference lies in their composition: whales possess bony skeletons, whereas whale sharks have cartilaginous skeletons. Reproduction also differs; whales give birth to live young that are nursed. Whale sharks are ovoviviparous, meaning they produce eggs that hatch internally, with live young emerging, but without parental nursing. Additionally, whales propel themselves by moving their tails up and down, while whale sharks, like other fish, move their tails from side to side. These distinctions highlight why, despite some superficial similarities, whale sharks are not whales.
The “Whale” in the Name
The inclusion of “whale” in the whale shark’s common name refers to its massive size and specific feeding behavior, rather than its biological classification. Whale sharks are the largest known fish species, capable of reaching lengths comparable to some whale species. This immense scale prompted the comparison to whales, the largest animals on Earth.
The “whale” aspect also refers to its filter-feeding method. Like some baleen whales, whale sharks consume vast quantities of plankton and small organisms by filtering water through their mouths. This shared feeding strategy, along with their imposing size, led to the descriptive common name “whale shark.” This name is purely descriptive and does not reflect a taxonomic relationship with marine mammals.