It is a common misconception that sharks might be mammals, perhaps due to their large size or their presence in marine environments alongside true marine mammals. However, sharks are not mammals; they are classified as fish. Understanding the biological characteristics that define these two distinct animal classes clarifies why sharks belong to the fish category.
Defining Mammals
Mammals are a diverse group of vertebrate animals characterized by several key biological traits. A fundamental characteristic is the presence of mammary glands, which produce milk to nourish their young. This distinguishes them from all other animal classes.
Mammals are also warm-blooded, maintaining a constant internal body temperature regardless of their external environment. They typically possess hair or fur at some point in their lives, which provides insulation. Most mammals give live birth and breathe air using lungs. Mammals have a four-chambered heart and a neocortex in their brain.
Defining Fish
Fish are aquatic vertebrates that possess gills for extracting oxygen from water. Their bodies are streamlined, aiding movement through water, and they use fins for propulsion and stability. Most fish are cold-blooded, with their body temperature fluctuating with the surrounding water.
While many fish have scales, sharks possess a unique skin covering called dermal denticles, which are tooth-like structures made of dentine and enameloid. These denticles are structurally similar to teeth and help reduce drag, allowing for efficient swimming. Fish lack limbs with digits and have a skeleton that can be cartilaginous or bony.
Classifying Sharks
Sharks are classified as fish, specifically belonging to the class Chondrichthyes, which includes cartilaginous fish like rays and skates. Their skeleton is composed entirely of cartilage, a flexible and durable tissue, rather than bone. This contrasts with bony fish (Osteichthyes) that have calcified skeletons.
Sharks breathe by extracting oxygen from water as it passes over their gills. They do not have lungs and cannot survive out of water for extended periods. While most sharks are cold-blooded, some species, like the great white and mako sharks, exhibit regional endothermy, allowing them to maintain a warmer core body temperature. However, this is not true warm-bloodedness like in mammals, where the entire body maintains a stable, high temperature.
Regarding reproduction, sharks display diverse strategies. Some species lay eggs encased in leathery “mermaid’s purses” (oviparous), while many give birth to live young (viviparous or ovoviviparous). Despite live birth, sharks do not possess mammary glands and therefore do not produce milk to feed their offspring. Young sharks are independent at birth, lacking the parental care seen in mammals.
Why the Confusion About Sharks
The common confusion about whether sharks are mammals often stems from superficial similarities and specific biological traits that can be misleading. One primary reason is that many shark species give birth to live young, a characteristic often associated exclusively with mammals. This reproductive method, however, does not negate their classification as fish, as they lack other defining mammalian traits like mammary glands or hair.
Another factor contributing to the misconception is their large size and streamlined body shape, which can resemble marine mammals such as dolphins or whales. These resemblances are a result of convergent evolution, where unrelated species develop similar traits due to adapting to similar environments, rather than indicating a close evolutionary relationship or shared classification. Sharks also lack complex cognitive processes and social behaviors found in marine mammals.