Sharks are ancient ocean predators that have thrived for hundreds of millions of years. Their success is rooted in specialized adaptations. Understanding these animals requires learning a specific vocabulary that describes their physical makeup and complex behaviors. This overview defines terminology related to shark anatomy, senses, reproduction, and actions.
Unique Anatomical Terminology
Sharks belong to the class Chondrichthyes, a group of jawed fishes distinguished by skeletons composed primarily of cartilage rather than bone. Cartilage is lighter and more flexible, contributing to the shark’s streamlined efficiency and buoyancy, though they still lack the swim bladder found in bony fish. The tough, sandpaper-like texture of a shark’s skin is due to thousands of tiny, tooth-like scales called dermal denticles, also known as placoid scales. These structures are composed of dentine and enamel, similar to human teeth, and provide protection.
Dermal denticles also play a significant role in hydrodynamics by reducing drag and turbulence as the shark moves through the water. The backward-facing tips of the denticles create micro-vortices that help the water flow smoothly over the body, enabling efficient swimming. Sharks breathe by drawing water over their gills, which exit the body through a series of gill slits located on the side of the head. These typically number between five and seven pairs, and water must constantly flow over the gill filaments for oxygen exchange.
The shark’s powerful thrust for propulsion comes from its heterocercal tail, or caudal fin. This tail is asymmetrical, featuring a larger upper lobe and a smaller lower lobe. This design pushes water downward and backward, generating an upward force. This upward force helps compensate for the lack of a gas-filled swim bladder, providing the lift necessary to maintain position during forward movement.
Specialized Sensory Functions
Sharks possess sensory capabilities that allow them to perceive their environment effectively. The lateral line system is a network of fluid-filled canals beneath the skin that runs along the head and flanks. This system uses specialized sensory cells called neuromasts to detect minute changes in water pressure, vibration, and movement. The lateral line provides a sense of “distant touch,” allowing the shark to map its surroundings, navigate in darkness, and sense the struggling movements of prey from a distance.
A unique sensory adaptation is electroreception, facilitated by the Ampullae of Lorenzini. This is a network of tiny pores concentrated around the snout and head. These pores open into jelly-filled canals that detect weak electrical fields generated by the muscle contractions and heartbeats of other organisms. This ability allows a shark to locate prey buried beneath the sand and navigate vast distances by sensing Earth’s magnetic field.
Sharks also rely heavily on chemoreception, their sense of smell, which is highly refined for detecting dissolved compounds in the water. Water flows through the paired nostrils, passing over specialized sensory membranes called olfactory lamellae. This allows them to detect extremely low concentrations of chemicals, such as amino acids released by potential food sources. The ability to detect differences in concentration between the two nostrils helps the shark track a scent trail with high precision.
Reproductive and Life Cycle Glossary
All sharks reproduce using internal fertilization, unlike most bony fish. Male sharks are identified by the presence of claspers, which are paired, grooved, cartilaginous extensions of the pelvic fins. During mating, the male inserts one clasper into the female’s cloaca to transfer sperm.
Shark species exhibit three distinct reproductive strategies for developing their young.
Reproductive Strategies
- Oviparous species, such as the Port Jackson shark, lay eggs encased in a leathery, protective shell often called a “mermaid’s purse.” The embryo develops outside the mother’s body, relying entirely on the yolk sac for nourishment until it hatches.
- The most common strategy is ovoviviparous reproduction, where the eggs hatch internally within the mother’s oviduct. The pups are sustained by their yolk sac, but they do not form a placental connection to the mother. The young are then born alive and fully functional.
- Viviparous species, including bull sharks and hammerhead sharks, give birth to live young nourished internally via a placental connection. This provides the developing embryo with oxygen and nutrients directly from the mother’s bloodstream, similar to mammalian gestation. These young are typically born at a relatively large size.
Essential Behavioral Vocabulary
Certain shark behaviors require unique terminology to describe their function. Ram ventilation is the process where a shark swims with its mouth slightly open, forcing water over the gills for gas exchange. Species that must swim continuously to breathe, such as the Great White and Mako shark, are known as obligate ram ventilators. Sharks that can rest on the seafloor often use buccal pumping, actively drawing water in through their mouths and spiracles using muscular contractions.
Another distinctive behavior is tonic immobility, a temporary, trance-like state that some sharks enter when they are gently inverted or their snout is stimulated. In this state, the shark’s muscles relax, breathing becomes deep, and sensory responsiveness decreases. Researchers often use this phenomenon to safely handle and study sharks, and it is also believed to play a role in the mating process.
Many shark species display countershading, a form of camouflage where the top (dorsal) surface is darker than the bottom (ventral) surface. This coloration allows the shark to blend in with the dark ocean depths when viewed from above and disappear against the bright surface light when viewed from below.
Sharks engage in migration, which is the seasonal movement over long distances. These movements are typically driven by factors such as fluctuating water temperature, the pursuit of migratory prey, or the need to reach specific mating and pupping grounds.