Requiem Sharks

The requiem shark family (Carcharhinidae) is comprised of 11 genera and at least 56 described species worldwide. This is one of the largest, most important shark families, and includes many common and wide-ranging species. These sharks are dominant (in terms of their biodiversity, abundance and biomass) in tropical continental shelf and offshore habitats, but some species are also found in subtropical and warm-temperate seas. Several members of this family are closely associated with coral reefs and oceanic islands, while other species range far into ocean basins. One pelagic requiem shark, the Blue Shark, has one of the greatest geographic ranges of any shark or marine vertebrate, from high latitude, cool-temperate waters into the tropics. A few other species are found in temperate waters and some even at great depths, but none are truly specialised deepsea sharks (compared with many species of the families Squalidae and Pentanchidae). Some requiem shark species occur in freshwater rivers and lakes. Although members of other families may enter river-mouths and ascend rivers for a short distance, the little-known Central Indo-Pacific river sharks, Glyphis species, and the broadly distributed Bull Shark appear to be the only living sharks that can live in fresh water for extended periods. The Bull Shark has a wide range in tropical and warm-temperate rivers and lakes around the world and is remarkable for the apparent ease with which it can move from saline to freshwater conditions and back again.

Although some species are relatively small (65-100cm), most are medium to large in size, with maximum lengths ranging to nearly 400cm. They have a long, arched mouth with blade-like teeth (often broader in the upper jaw), most with short labial furrows (except Rhizoprionodon), no nasoral grooves or barbels. Usually round (to horizontal) eyes with internal nictitating eyelids, usually no spiracles. Two dorsal fins, one anal fin, first dorsal fin medium to large with base well ahead of pelvic bases, second usually much smaller. Precaudal pits, caudal fin with a strong lower lobe and lateral undulations on upper margin. Mostly unpatterned (particularly in Carcharhinus). The extremely rare river sharks (Glyphis species), the only truly freshwater shark species, are very difficult to distinguish without tooth and vertebral counts.

All requiem sharks are viviparous with a yolk-sac placenta and have litters ranging in size from just one or two pups to 135 in the Blue Shark. They are strong, active swimmers, occurring singly or in small to large schools. Some species are ‘ram-ventilators’ needing to swim continually to oxygenate their gills, while others are capable of resting motionless for extended periods on the bottom. Some are solitary or socialise in small groups, some are social schooling species. There is a clear hierarchical dominance between certain species: Oceanic Whitetip Sharks are dominant over Silky Sharks of the same size, which in turn can dominate Grey Reef Sharks; Galapagos Sharks are dominant over Blacktip Sharks but subordinate to the Silvertip.

Requiem sharks are extraordinarily fast and effective hunters. Their elongated, torpedo-shaped bodies make them quick and agile swimmers, so they can easily attack any prey. Some species are continually active, while others are capable of resting motionless for extended periods on the bottom. They have a range of food sources depending on location and species, including bony fish, squid, octopus, lobster, turtles, marine mammals, seabirds, other sharks and rays; smaller species tend to select a narrow range of prey, but some very large species, especially the tiger shark (Galeocerdo cuvier), are virtually omnivorous. They are often considered the “garbage cans” of the seas because they will eat almost anything, even non-food items like trash. They are migratory hunters that follow their food source across entire oceans. They tend to be most active more active at night, or dawn and dusk, where their impressive eyesight can help them sneak up on unsuspecting prey. It is worth mentioning that the tiger shark, belongs to the Galeocerdo family. Most requiem sharks hunt alone, however some species like the whitetip reef sharks and lemon sharks are cooperative feeders and will hunt in packs through coordinated, timed attacks against their prey. Some of the species have been shown to give specialized displays when confronted by divers or other sharks, which may be indicative of aggressive or defensive threat.