The question of whether a shark is a mammal or a fish is common, stemming from observations of their size and their ability to give live birth. The definitive scientific answer is that sharks are fish, specifically belonging to the class Chondrichthyes, or cartilaginous fish. This classification is rooted in fundamental biological and anatomical differences that separate them entirely from the mammalian class.
Defining Characteristics of Fish
Sharks are classified as fish because they possess the defining traits of aquatic vertebrates, starting with their skeletal structure. Their internal framework is composed almost entirely of cartilage, a strong but flexible connective tissue, rather than true bone. This cartilaginous skeleton is lighter than bone, assisting with buoyancy and allowing for highly agile movement through the water.
Gas exchange is accomplished through a series of five to seven open gill slits located on the sides of the head. Sharks draw oxygen dissolved in the water across these gills, and they entirely lack the lungs required for air breathing, which all mammals possess.
Their skin is covered not in traditional scales, but in tiny, tooth-like structures called dermal denticles, or placoid scales. These structures are made of the same material as teeth and work to protect the shark while also minimizing drag, which allows for faster, more efficient swimming.
Most sharks are ectothermic, meaning their body temperature generally matches the surrounding water. Certain active predators, like the Great White Shark and Mako Shark, exhibit regional endothermy. They use specialized vascular networks, known as the rete mirabile, to retain metabolic heat and warm specific areas like muscles and the brain. This adaptation is not the constant body temperature regulation characteristic of mammals.
Why Sharks Are Not Mammals
Sharks fail to meet the biological criteria that define the class Mammalia, which are centered on a suite of specific anatomical features. A primary difference is the lack of mammary glands, specialized secretory glands used by female mammals to produce milk for nursing their young. Sharks do not nurse their pups, and their lack of hair or fur is another distinction, as all mammals possess hair at some stage of life.
The circulatory system provides a clear separation between the two classes. Mammals maintain a high metabolic rate and constant body temperature using a four-chambered heart that separates oxygenated and deoxygenated blood. Sharks, like other fish, have a two-chambered heart that pumps blood directly to the gills for oxygenation before it circulates back to the body. This simpler, single-circuit system cannot support the energy demands of constant endothermy.
Furthermore, a mammal’s lower jaw consists of a single bone, the dentary, which articulates directly with the skull. Sharks possess a more complex jaw structure that is not fused to their cartilaginous cranium.
Unique Reproductive Strategies
The greatest source of confusion regarding shark classification is their varied reproductive methods, which include giving birth to live young. Sharks reproduce using internal fertilization, where the male uses specialized organs called claspers to transfer sperm to the female. This contrasts with the external fertilization common in most bony fish.
Sharks employ three main reproductive strategies:
- Oviparous species, such as the Horn Shark, lay eggs encased in tough, protective egg cases often called “mermaid’s purses.”
- Ovoviviparous sharks retain the eggs inside the mother, where the pups hatch and are nourished by the yolk sac until they are born live.
- Viviparous sharks give birth to live young that develop inside the mother, sometimes forming a placental connection similar to mammals, as seen in the Lemon Shark.
Even in these live-bearing species, the pups are born fully independent and do not receive post-birth care or milk, confirming their separation from the mammalian lineage.