Sharks Are the Ocean’s Apex Predators

Sharks play a critical role in the marine environments because they are apex predators. If too many sharks are fished commercially or killed by pollution, all other species further down the food chain are affected. Without sharks some populations swell until there is no more food for them to eat, then they dwindle. Most sharks live alone, although a few sometimes gather in large groups.

Most sharks are strong and agile swimmers, which helps them to hunt prey. Some species make long migrations, covering many hundreds of miles in search of food. Sharks do not need to feed as often as other kinds of fish and can go months without meals. Most sharks have good eyesight and a keen sense of smell. Many can detect tiny amounts of blood in the ocean. They are also sensitive to sound waves traveling through the ocean.

Sharks typically have a tough skin that is dull gray in color and is roughened by toothlike scales. They also usually have a muscular, asymmetrical, upturned tail; pointed fins; and a pointed snout extending forward and over a crescentic mouth set with sharp triangular teeth. Sharks have no swim bladder and must swim perpetually to keep from sinking to the bottom.

There are more than 400 living species of sharks, taxonomically grouped into 14–30 families. Several larger species can be dangerous to humans. Numerous sharks are fished commercially. However, overfishing has substantially reduced the populations of some shark species.

The color of sharks varies from gray to cream, brown, yellow, slate, or blue and are often patterned with spots, bands, marblings, or bulges. The oddest-looking sharks are the hammerheads, whose heads resemble double-headed hammers and have an eye on each stalk. The whale shark and the basking shark, both of which may weigh several tons, are harmless giants that subsist on plankton strained from the sea through modified gill rakers. All other sharks’ prey on smaller sharks, fish, squid, octopuses, shellfish, other invertebrates. The largest among the more predatory species is the voracious 20’ white shark, which attacks seals, dolphins, sea turtles, large fish, and occasionally people.

Normally, sharks feed on fish, often attacking in schools. Open-ocean species such as the mackerel, mako, and thresher sharks frequently feed near the surface and are much sought after with rod and reel for sport. Beautifully streamlined and powerful swimmers, those open-ocean sharks are adept at feeding on fast tuna, marlin, and the like. Bottom-feeding species of sharks are stout, blunt-headed forms that tend to have more-sluggish habits. The shellfish eaters among them have coarse, pavementlike, crushing teeth.

Fertilization in sharks is internal. The male introduces sperm into the female by using special copulatory organs (claspers) derived from the pelvic fins. The young in many species hatch from eggs within the female and are born alive. Sharks reproduce more slowly than other fish and usually do not begin until at least 6 years of age.