Sharks are diverse cartilaginous fishes belonging to the class Chondrichthyes, characterized by a skeleton composed entirely of cartilage instead of bone. This flexible endoskeleton is a defining feature of the group, which also includes rays and skates. With over 500 species identified globally, sharks inhabit every ocean, ranging from shallow coastal waters to the vast, cold depths of the open sea. Their body plans exhibit a wide array of adaptations, but nearly all share multiple gill slits, placoid scales (dermal denticles), and a lack of a swim bladder. Instead, they rely on a large, oil-rich liver for buoyancy. This ancient lineage ranges in size from the tiny dwarf lanternshark (17 centimeters) to the colossal whale shark (over 12 meters).
Ground Sharks and Mackerel Sharks
The Ground Sharks (Order Carcharhiniformes) and Mackerel Sharks (Order Lamniformes) are the two most recognized groups of active, free-swimming predators, exhibiting distinct anatomical differences. Carcharhiniformes is the largest order, including over 270 species. Members are characterized by a nictitating membrane, a translucent third eyelid that sweeps across the eye for protection, especially during a strike. This group, often called requiem sharks, features familiar species such as the Tiger Shark, the Bull Shark, and the unique Hammerhead Sharks. The Hammerhead’s flattened, laterally extended head (cephalofoil) is thought to enhance sensory perception and maneuverability. Ground sharks typically possess five gill slits and two dorsal fins, and they are common in tropical and temperate coastal waters worldwide.
Mackerel Sharks (Order Lamniformes) are built for sustained speed and power, lacking the nictitating membrane. Instead, species like the Great White Shark roll their eyes back into their sockets just before impact with prey. A significant adaptation in the Lamnidae family, which includes the Great White and the agile Mako Sharks, is the rete mirabile (Latin for “wonderful net”). This complex network of blood vessels uses a countercurrent exchange system to retain metabolic heat generated by their muscles. This makes them partially warm-blooded (endothermic), enabling them to hunt effectively in colder waters and sustain high-speed pursuits.
Carpet Sharks and Angel Sharks
The Carpet Sharks (Order Orectolobiformes) and Angel Sharks (Order Squatiniformes) have evolved specialized body forms that prioritize camouflage and ambush over open-water speed. Carpet sharks are predominantly sluggish bottom-dwellers, often found in the Indo-Pacific. Their skin patterns resemble a rug or carpet, aiding in concealment on the seafloor or coral reefs. Characteristic features include sensory barbels near the nostrils, which help locate hidden prey, and prominent grooves connecting the nostrils to the mouth. The Wobbegong sharks possess fleshy dermal lobes around their head, effectively breaking up their outline against the substrate.
This order also contains the Nurse Shark, a nocturnal species that uses powerful suction to extract invertebrates from crevices, and the massive Whale Shark, the world’s largest fish. Despite its classification as a carpet shark, the Whale Shark is pelagic, traveling vast distances to filter-feed on plankton and small schooling fish using its enormous terminal mouth and specialized gill rakers. Angel Sharks (Order Squatiniformes) take the bottom-dwelling adaptation to an extreme, with their bodies dorsoventrally flattened to closely resemble rays. Unlike true rays, their enlarged pectoral fins are not fused to the head, and their five gill slits are located laterally. These sharks are masterful ambush predators, often burying themselves in sand or mud for days, waiting to capture unsuspecting fish or crustaceans.
Primitive and Deep-Water Sharks
The Hexanchiformes and Squaliformes orders encompass sharks representing ancient lineages and those adapted to extreme, deep-sea environments. Hexanchiformes, known as cow and frilled sharks, are considered the most primitive living sharks, retaining archaic traits. Their defining characteristic is the presence of six or seven pairs of gill slits, contrasting with the five slits found in nearly all other sharks. The Frilled Shark possesses a slender, eel-like body and six frilly gill slits. The Sixgill and Sevengill Sharks are robust, deep-water predators with a single, posteriorly positioned dorsal fin.
Squaliformes, or Dogfish Sharks, are a large, diverse group of cold-water and deep-sea species that lack an anal fin and a nictitating membrane. Many dogfish species, such as the Spiny Dogfish, have sharp spines preceding both dorsal fins. This order includes the Greenland Shark, a massive, sluggish predator that holds the record for the longest lifespan of any known vertebrate, potentially living for over 500 years in the frigid Arctic waters. Another notable member is the small Cookiecutter Shark. This ectoparasite uses specialized lips and razor-sharp lower teeth to attach to and carve out perfectly circular plugs of flesh from much larger marine animals, including whales and Great White Sharks.