Requiem shark

Description

Requiem sharks have the usual features of their order, including round eyes and a nictitating membrane—a third eyelid that sweeps across the eye to protect it during a strike.^[2][1] Requiem sharks have long, streamlined bodies that look like torpedoes, which helps them swim fast and turn quickly.^[3] Scientists think the size and shape of requiem sharks' pectoral fins are just right for cutting down the energy they use while swimming.^[2]

Biology and ecology

Their food sources vary by location and species, including bony fish, squid, octopus, lobster, turtles, marine mammals, seabirds, other sharks and rays.^[3] Smaller species tend to select a narrow range of prey, but some very large species are virtually omnivorous.^[3] Requiem sharks are most active at night, when their excellent vision helps them creep up on prey that doesn't see them coming.^[4] Some requiem shark species stay active all the time, while others can lie still on the ocean floor for long stretches.^[3]

Most requiem sharks hunt by themselves, but a few species, like whitetip reef sharks and lemon sharks, work together and hunt in groups with carefully timed attacks.^[5] When divers or other sharks get close, some requiem shark species put on special displays that might signal they feel threatened or are getting ready to attack.^[4] Every requiem shark species gives birth to live young that are already fully developed when they're born.^[2] Requiem sharks typically mate from around spring through autumn.^[6]

Distribution and habitat

Most species are coastal, working reef edges, drop-offs, surf zones and estuaries.^[1] A smaller number, including the blue shark, live out in open water and cross entire ocean basins.^[1] Requiem sharks include the river sharks of the genus Glyphis, which live in muddy tropical rivers and estuaries.^[1] The bull shark moves between salt and fresh water at will, and has been recorded far up the Mississippi, the Amazon and the Zambezi.^[1]

Requiem sharks live in marine, brackish, and freshwater environments.^[7]

Human interactions

According to the ISAF, requiem sharks are among the top five species involved in shark attacks on humans.^[8] Even trained experts have trouble telling many requiem shark species apart.^[9] Late maturity and a handful of pups per litter leave requiem sharks with no capacity to absorb fishing pressure.^[1] They are heavily represented in the fin trade and in tuna and swordfish bycatch.^[1]

Several species are assessed as Endangered or Critically Endangered, including the dusky, the daggernose and the river sharks.^[1] White, tiger, and bull sharks are the "Big Three" for attacks—they're large, swim where people do, and have teeth that shear flesh.^[9] Identifying which shark attacked is very hard because victims rarely get a good look during the encounter.^[9]

Taxonomy and phylogeny

The tiger shark was treated as a requiem shark for most of the twentieth century, but genetic work has since separated it into its own family, Galeocerdonidae.^[1][5] In 1896, two scientists named D.S. Jordan and B.W. Evermann first proposed the scientific name Carcharhinidae as a subfamily of Galeidae.^[10][7] Only a few fossils of this group have been found from before the Cenozoic era began.^[11]