Sharks: Predatory Cartilaginous Fishes
Sharks are cartilaginous fishes of the order Selachii (class Chondrichthyes), predatory in habit. Along with rays and skates, they form the subclass Elasmobranchii. Unlike other elasmobranchs, sharks have a fusiform body shape, gill slits on the sides of the head, hard skin roughened by toothlike scales, pointed fins, and a crescent-shaped asymmetrical tail. They lack swim bladders and must swim continuously to avoid sinking.
Species and Diversity
Over 400 living shark species exist, classified into 14–30 families. Many large species can be dangerous to humans. Sharks vary in color from gray, cream, brown, yellow, slate, or blue, and may display spots, bands, or marbling. Unique species include hammerheads (Sphyrna) with their distinctive wide heads and wobbegongs (family Orectolobidae) camouflaged to match the seafloor. Common names also highlight color, like the blue (Prionace glauca), great white (Carcharodon carcharias), and lemon (Negaprion brevirostris) sharks.
Size and Feeding
The whale shark (Rhincodon typus) and basking shark (Cetorhinus maximus) are massive plankton feeders, growing up to 18 meters (59 feet) and 14 meters (46 feet), respectively. Most other sharks are carnivorous, preying on smaller sharks, fish, squid, octopuses, shellfish, and occasionally humans. The white shark, reaching 6 meters (20 feet), feeds on seals, dolphins, turtles, and large fish, while the Greenland shark (Somniosus microcephalus) scavenges whales and hunts in deep cold waters.
Behavior and Hunting
Sharks often hunt in schools. Open-ocean species like mackerel, mako, and thresher sharks feed near the surface on fast prey such as tuna and marlin and are popular in sport fishing. Bottom-feeding species are slower, with blunt heads and specialized crushing teeth for shellfish. Sharks’ diverse adaptations make them effective predators across a wide range of marine environments.
