SHADOWS IN THE SEA

Covering my face with lavender-drenched rags and smearing Vicks vapor rub under my nose fail to eliminate the vile odor. The seagulls and petrels love it, though, eagerly plunging into the greasy water and snagging small pieces of skin and blubber with shrieks of delight. The lifeless carcass of a humpback whale, likely hit by a cargo ship or oil tanker, is ushered by wind and currents out of the busy shipping lanes toward the South African coast. Large predators found the carcass days before, their attendance revealed by the missing chunks of blubber cut with surgical precision. Some of them still linger, and I make out dark shadows swimming just below the surface. Finally, one dorsal fin after the other breaks the surface. Within minutes I count five white sharks. Like a human fingerprint, each has a unique pattern of notches and tears along the trailing edge of its dorsal fin. The half-mile-long slick of oil seeping from the whale has lured the sharks with the promise of an easy meal. Despite their reputation as one of the world's most formidable predators, there is not a hint of frenzy—sharks are keen to conserve energy whenever possible. Like airplanes in a holding pattern, they line up and swim in an orderly fashion around the carcass. Smaller sharks give way to larger ones, but in the end everyone gets their turn to tear away 30-pound (14-kg) chunks of blubber. The teeth of a shark's lower jaw are narrow, designed to impale and grip a slippery carcass, while the rows of 26 perfectly serrated, triangular teeth of the upper jaw slice meat like butter. By late afternoon a quarter of the whale has been consumed. I abandon the scene, leaving the sharks to wrestle with their prize as wind and waves push the whale into the surf zone.

A little over a mile away in the lee of a headland, I jump into the sea with a mask and snorkel. I descend through a canopy of kelp and come to rest on the seabed. As the stench of whale oil and blubber leaves my body, I spot another shark, not cruising in the open water like the whites but inches away from me. A perfectly camouflaged and thumb-size puff adder shyshark rests among the pebbles. Most people associate sharks with the classic stereotype: Dorsal fin slicing through the water and a steely gray, torpedo-shape body beneath. In fact, more than half of all sharks defy this popular image, including the tiny puffadder shyshark that now, with some coaxing, rests on my hand. Only when the urge to breathe overtakes me do I place it back on the seabed. As I ascend, its outline disappears into the maze of pebbles.

Sharks have roamed the oceans for more than 400 million years. They're the planet's oldest vertebrate predator, but this ancient pedigree doesn't mean they're primitive creatures. A steadfast adaptability to an ever-changing world has been key to their survival. Sharks began their reign as small coastal predators but gradually evolved larger bodies and the knack for colonizing deeper, offshore waters. The Carboniferous period 360 million years ago marked the golden age of sharks, when species proliferated and occupied almost every imaginable niche in the oceans.

A shark's skeleton is unique in that it is made entirely out of cartilage. Because cartilage lacks the preservation quality of bone, shark fossils are rare. So the exact morphology and natural history of early sharks have been pieced together from a mix of scales, teeth and fossilized impressions of muscles and organs. Prehistoric sharks had bizarre features, including L-shape spines, spiral jaws, and hour glass-shape dorsal fins. After an estimated 95 percent of all marine life was wiped out during the Permian–Triassic extinction event 250 million years ago, modern sharks evolved about 50 million years later in the heyday of the dinosaurs. The sharks that shared the planet with T-Rex and brontosaurs would have been very familiar to us. They were streamlined, with two dorsal fins, a powerful vertical tail, and a flexible jaw that could thrust forward to feed.

The most famous and largest of all prehistoric sharks, Carcharodon megalodon, is now extinct. Megalodons were estimated to grow to 49 to 65 feet (15–20 m) long. Their dorsal fins towered 6.5 feet (2 m) out of the water, and their jaw measured 6.5 feet in diameter. It's fairly easy to imagine them swallowing a modern-day great white shark with no problem. Impressions found on fossilized bones of whales match the dimensions of fossilized megalodon teeth (roughly the size of a human hand), indicating that early whales may have been a primary food source. Theories of megalodon extinction are contentious. Some scientists speculate that two million years ago changes in oceanographic conditions triggered whale migrations to colder waters to feed; megalodons, whose physiologies were possibly not well adapted to polar conditions, were unable to follow.

An estimated 500 species of shark (some still unnamed) survived the perils of prehistory and still roam almost every seascape in the world, from the icy seas of the North Pole to the warm tropical waters of the South Pacific. Indonesian waters harbor the highest diversity of sharks—78 species—while the northern polar seas are the most depauperate, with just eight species. The range of environmental conditions that sharks tolerate is astounding. At one end of the spectrum, the aptly named Greenland shark thrives in sub-freezing waters under the Arctic ice sheet, while the epaulette carpet shark survives in waters over 86 °F (30 °C) on sun-baked tropical reef flats. Sharks also inhabit 90 percent of the world's marine ecosystems, including kelp forests, seagrass meadows, mangroves, coral reefs, and rocky shores.

Sharks roam a vast depth range, from the reef shark of tropical latitudes, which hunts in water just centimeters deep, to the Portuguese dogfish, which has been caught at depths of 12,000 feet (3,670 m) off the coast of Ireland. To date, no sharks have been caught at deeper depths, making the abyssal and Antarctic waters the only assumed shark-free regions. Sharks also thrive across a wide salinity gradient; the milk shark of the Arabian Gulf can tolerate salinities in excess of 40 ppt (parts per thousand) during the summer months. On the other hand, bull and Ganges sharks dwell for long periods of time in fresh water and, in some cases, have been found 620 miles (1,000 km) upriver.

Sharks come in a variety of sizes. On the small end is the dwarf lantern shark, which at seven inches (19 cm) long as an adult is no larger than a megalodon tooth. Growing to 50 feet (16 m) long, the largest shark (and fish for that matter) is the whale shark. Around half of all shark species adhere to a very familiar blueprint: metallic gray skin, a prominent dorsal fin, and a jaw equipped with rows of razor-sharp teeth. Defying the stereotype, the other 50 percent of sharks are an outlandish and motley crew. There are sharks with...