Sharks

Sharks are cartilaginous fish — their skeletons are made entirely of cartilage rather than bone, making them lighter and more flexible than bony fish. Their skin is covered in tiny tooth-like scales called dermal denticles, which reduce drag and allow them to swim efficiently. Sharks have multiple rows of teeth that are continuously replaced throughout their lives — some species produce tens of thousands of teeth over a lifetime. Their sensory systems are extraordinary: electroreceptors called ampullae of Lorenzini detect the electrical fields produced by all living animals, while a lateral line system senses vibrations and pressure changes in the water.

The diversity of sharks is remarkable. The whale shark (Rhincodon typus) is the world's largest fish, reaching up to 12 metres in length, yet feeds entirely on plankton and small fish. The great white shark (Carcharodon carcharias) is the ocean's most iconic predator, capable of breaching completely out of the water when hunting. The hammerhead shark's distinctive cephalofoil head enhances its sensory capabilities, while the wobbegong lies camouflaged on the seafloor. Deep-sea species like the goblin shark and frilled shark represent some of the most bizarre and ancient lineages of vertebrate life.

Sharks are keystone species — their presence or absence has disproportionate effects on the entire ecosystem. As apex predators, they regulate the populations of species below them in the food chain, preventing any single species from dominating and maintaining the balance of marine ecosystems. When shark populations decline, the effects cascade through the entire food web in a process known as a trophic cascade. Healthy shark populations are therefore an indicator of healthy ocean ecosystems.

Despite their evolutionary success, sharks face unprecedented threats from human activity. Overfishing — both targeted and as bycatch — kills an estimated 100 million sharks every year. The practice of shark finning, where fins are removed and the animal discarded at sea, has decimated populations of many species. Climate change is altering ocean temperatures and chemistry, affecting shark distribution and prey availability. Many shark species are slow to mature and produce few offspring, making population recovery extremely slow. Over a third of all shark species are now classified as threatened with extinction by the IUCN.