A dolphin is not a shark. Despite both being streamlined, top predators in the ocean, they belong to entirely different classes of the animal kingdom. This fundamental difference dictates nearly every aspect of their biology, from skeletal structure and breathing mechanisms to reproduction. Understanding these differences offers a clearer look into how life has evolved to thrive in the marine environment.
Fundamental Classification
The most significant distinction is their biological grouping. Dolphins are classified in the class Mammalia, defined by being warm-blooded, giving live birth, and nursing their young with milk. Sharks belong to the class Chondrichthyes, meaning they are cartilaginous fish. As fish, sharks are generally cold-blooded and extract oxygen from the water. Their skeletons are made primarily of cartilage, a flexible tissue, rather than dense bone.
Physiological Differences
Physiological differences involve respiration and body temperature. Dolphins must surface regularly to breathe air using their lungs through a blowhole located on the top of their heads. Their breathing is volitional, meaning they must consciously decide to take each breath. Sharks, like all fish, possess gills that efficiently extract dissolved oxygen directly from the surrounding water. They do not need to surface to breathe, and many use a method called ram ventilation, forcing water over their gills simply by swimming forward. Dolphins are endotherms, or warm-blooded, maintaining a consistent high internal body temperature. Most sharks are ectotherms, or cold-blooded, with their body temperature closely matching the ambient environment.
Structural Distinctions
The internal structure of each animal reflects its class, particularly regarding the skeleton and method of propulsion.
Skeleton and Skin
Dolphins have a bony skeleton, similar to that of land mammals, which provides a rigid framework for muscle attachment. Their skin is smooth and elastic, covering a layer of blubber that aids in insulation and buoyancy. In contrast, the shark’s skeleton is composed entirely of cartilage, which is lighter and more flexible than bone. Their skin is covered in tiny, tooth-like scales called dermal denticles, giving it a rough, sandpaper-like texture.
Propulsion
The method of swimming is also fundamentally different. Dolphins propel themselves by moving their horizontal tail flukes up and down, a movement inherited from their terrestrial mammal ancestors. Sharks swim by moving their vertical caudal fin side-to-side, a characteristic motion of fish.
Reproductive Strategies
Reproductive processes solidify the separation between mammals and fish. Dolphins exhibit live birth, delivering fully developed young underwater. The mother nurses the calf with high-fat milk produced by mammary glands and typically provides extended parental care. Shark reproduction is varied, but never involves nursing or true parental care. Some sharks are oviparous, laying eggs encased in a protective shell, while others are viviparous, giving birth to live pups. Even live-bearing species are fully independent from birth, lacking the maternal dependence seen in dolphins.