Sharks are among the most recognized ocean inhabitants, yet they are often misunderstood as simply being a type of fish. These animals are far more complex, representing a distinct and highly specialized branch of the vertebrate family tree. Sharks have navigated the world’s oceans for hundreds of millions of years, evolving unique anatomical features and sensory abilities that enable them to persist as apex predators in nearly every marine environment.
The Cartilaginous Fish Classification
Sharks are vertebrates belonging to the Class Chondrichthyes, commonly referred to as cartilaginous fishes. This classification distinguishes them from bony fishes (Class Osteichthyes). The defining feature of a shark is its skeleton, which is composed almost entirely of cartilage rather than true bone tissue. Cartilage is a dense, flexible connective tissue that is lighter than bone, providing agility and flexibility for movement through water.
The skeletal structure is not entirely soft; areas like the jaws and vertebrae are often reinforced with calcium salts. These calcified areas provide strength and rigidity, particularly in the biting structures, but they lack the complete mineralization found in bony fish skeletons. This lightweight framework assists with buoyancy control, compensating for the absence of the gas-filled swim bladder found in most bony fish. The Chondrichthyes class also includes rays, skates, and chimaeras.
Unique Anatomical Features
The skin of a shark is covered not with typical fish scales, but with thousands of tiny, tooth-like structures called dermal denticles (placoid scales). Each denticle is composed of a pulp cavity, dentine, and enamel, similar to a true tooth. These structures are angled backward, disrupting the flow of water over the shark’s body, which reduces drag and increases swimming efficiency. This hydrodynamic surface has inspired engineers in the design of high-performance materials.
Sharks breathe differently than bony fish, utilizing five to seven open gill slits on each side of the head. Unlike bony fish, they lack a protective, movable gill cover (operculum). This means many species must maintain constant forward motion to force water over their gills, a process known as ram ventilation.
Another defining anatomical feature is the enormous, oil-filled liver, which can constitute up to 25% of the shark’s total body weight. This large organ stores low-density oils that contribute significantly to buoyancy, preventing the shark from sinking in the absence of a swim bladder.
The teeth of a shark are not rooted into the jawbone but are embedded in the soft tissue of the mouth. Sharks are polyphyodonts, meaning they have multiple rows of teeth that are continuously replaced throughout their lives. As a tooth in the front row is lost or damaged, a replacement tooth from the row behind moves forward. This conveyor-belt system ensures the shark always maintains sharp, functional teeth for seizing prey.
Specialized Sensory Systems
Sharks possess a highly refined suite of senses, including electroreception, which allows them to detect minute electrical fields in the water. This ability is facilitated by the Ampullae of Lorenzini, a network of jelly-filled pores concentrated around the head. These organs are sensitive, capable of sensing electrical currents as weak as five billionths of a volt per centimeter. The Ampullae of Lorenzini detect the faint bioelectric fields generated by the muscle contractions and nervous systems of living prey, even if the prey is buried beneath the sand.
Sharks also possess a lateral line system, a series of fluid-filled canals that run along the sides of their bodies. This system detects subtle changes in water pressure and vibration, allowing the shark to sense the movement of other organisms from a distance. The lateral line is effective in low-visibility environments, providing a continuous picture of the surrounding environment.
Chemoreception, or the sense of smell, is profoundly developed in sharks, which can detect extremely low concentrations of substances, such as blood, over great distances. Their large, paired olfactory bulbs allow them to track chemical gradients in the water with precision, guiding them toward distant food sources.
Ancient Lineage and Modern Diversity
Sharks represent one of the oldest lineages of jawed vertebrates, with fossil evidence indicating their presence in the oceans for approximately 400 million years. This ancient history highlights a successful body plan that has undergone little fundamental change over geological time, leading to their description as “living fossils.” The earliest recognizable sharks appeared during the Devonian Period, surviving multiple mass extinction events.
The modern diversity of sharks is organized into two main superorders within the subclass Elasmobranchii.
Galeomorphii
The Galeomorphii, often called Galean sharks, includes many familiar species, such as ground sharks (hammerheads and requiem sharks) and mackerel sharks (great white and mako sharks). These sharks are characterized by having an anal fin and two dorsal fins.
Squalomorphii
The Squalomorphii, or Squalid sharks, typically lack an anal fin. This diverse group includes the dogfish, angel sharks, and sleeper sharks. This group often occupies colder, deeper waters. The continued evolutionary success of both superorders demonstrates the adaptability of the shark body plan to a vast array of ecological niches.
Reproductive Strategies
Shark reproduction is highly varied, generally involving a long gestation period. This strategy, known as K-selection, focuses on producing a small number of well-developed offspring, contrasting with the R-selection strategy of many bony fish that produce millions of small eggs. Sharks are characterized by internal fertilization.
The three primary methods of embryonic development are oviparity, viviparity, and ovoviviparity.
Oviparity
Oviparous species, such as the horn shark, lay eggs encased in a tough, leathery protective case often called a “mermaid’s purse.” The embryos develop externally.
Viviparity
Viviparous sharks, including species like the hammerheads, give birth to live young that develop inside the mother with a placental connection for nourishment, similar to mammals.
Ovoviviparity
The most common strategy is ovoviviparity, where the eggs hatch internally. The developing young are nourished by the yolk sac within the mother’s oviduct until live birth. This method, seen in species like the great white shark, offers the young protection during development without a direct placental link. The reliance on producing few, larger, and more capable young makes shark populations vulnerable to threats like overfishing.