shark | Attacks, Types, & Facts
The whale shark (Rhincodon typus) and the basking shark (Cetorhinus maximus), both of which may weigh several tons, are harmless giants that subsist on plankton strained from the sea through modified gill rakers. Whale sharks may grow up to 18 metres (59 feet) in length, whereas basking sharks may reach 14 metres (46 feet) fully grown. All other sharks prey on smaller sharks, fish, squid, octopuses, shellfish, other invertebrates, and, in some species, trash. The largest among the more predatory species is the voracious 6metre (20foot) white shark, which attacks seals, dolphins, sea turtles, large fish, and occasionally people. The more sluggish Greenland shark (Somniosus microcephalus) of cold deep waters feeds on seals, large fish, and even swimming reindeer; they may also scavenge whale carcasses.

Normally, sharks feed on fish, often attacking in schools. Openocean species such as the mackerel (Lamna), mako (Isurus), and thresher (Alopias) sharks frequently feed near the surface and are much sought after with rod and reel for sport. Beautifully streamlined and powerful swimmers, those openocean sharks are adept at feeding on fast tuna, marlin, and the like. Bottomfeeding species of sharks are stout, bluntheaded forms that tend to have moresluggish 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. Other species may lay eggs or nurture their young in the uterus with a placental attachment to the mother, like humans. Some species may even consume their siblings before they are born.

The origin of sharks is obscure, but their geologic record goes back at least to the Devonian Period (419.2 million to 358.9 million years ago). Fossil sharklike fish appeared in the Middle Devonian Epoch and became the dominant vertebrates of the Carboniferous Period (358.9 million to 298.9 million years ago). Modern sharks appeared in the Early Jurassic Epoch (201.3 million to 174.1 million years ago) and by the Cretaceous Period (145 million to 66 million years ago) had expanded into the presentday families. Overall, evolution has modified shark morphology very little except to improve their feeding and swimming mechanisms. Shark teeth are highly diagnostic of species, both fossil and modern.

Sharks’ geographic ranges are not well known. Their extensive movements are related to reproductive or feeding activities or to seasonal environmental changes. Tagging returns from large sharks on the east coast of the United States indicate regular movements between New Jersey and Florida, and blue sharks have been recovered after crossing the southern Atlantic Ocean. A tagged spiny dogfish (Squalus acanthias) was recovered after traveling about 1,600 km (1,000 miles) in 129 days. Advances in tracking technologies include the use of satellite tags that continuously beam a signal to orbiting satellites whenever the shark surfaces and in that manner reveal northsouth movements, transoceanic movements, and roaming and migratory patterns that are beginning to be understood for many species.

Some members of the Carcharhinus genus—most notably the bull shark (C. leucas)—enter fresh waters. Riverine sharks are small to mediumsized and are exceptionally voracious and bold.