The grey reef shark (Carcharhinus amblyrhynchos) is a requiem shark belonging to the family Carcharhinidae. It is one of the most common reef sharks in the Indo-Pacific region, often found in shallow waters near coral reefs. Known for its inquisitive nature and potential for aggression when threatened, understanding its diet and behavior is crucial for both conservation efforts and safe interaction.
Identification and Physical Characteristics
The grey reef shark has a classic "reef shark" shape, with a broad, round snout and large eyes. It's dark grey on the dorsal surface and paler, almost white on the ventral side. A key identifying feature is the dark margin on the entire trailing edge of the caudal fin. Some populations in the Western Indian Ocean and Red Sea have a white-edged first dorsal fin, sometimes classified as a distinct species (Carcharhinus wheeleri). They lack an interdorsal ridge running between the first and second dorsal fins.
The teeth of the grey reef shark are triangular and serrated, with 13-14 teeth in each jaw half. The upper teeth are narrow and serrated, semi-erect to oblique in shape with high cusps, while the crown feet have coarser serrations.
Males grow up to about 185 cm in length, and females to about 190 cm. Maturity occurs at about 120-140 cm for males and about 125 cm for females, around 7 years old for both sexes. Size at birth is between 45-75 cm. Maximum age is believed to be about 25 years.
Distribution and Habitat
The grey reef shark is limited to the Pacific and Indian Oceans. It's found in the Indo-Pacific, from Northern South Africa, Madagascar, the Mauritius-Seychelles region, and up into the Red Sea. In the western Pacific Ocean, its range extends from southern China to northern Australia and the Tuamoto Archipelago. It prefers shallow tropical and subtropical waters near coral atolls and lagoons adjacent to reef habitats, with a depth range of 0-164 feet (0-50 m), although they have been known to dive to 1,000 m (3,300 ft). Tagging studies show that sharks living near ocean reefs are nomadic and travel along the reef habitat each day.
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Diet and Feeding Behavior
Reef fishes provide the majority of the grey reef sharks’ prey, along with smaller quantities of cephalopods (squid and octopus), and crustaceans (shrimp and lobster). Bony fish including cowfish, surgeonfish, and butterflyfish are also part of their diet. Most feeding activity occurs at night, when the shark is most active. Grey reef sharks are voracious carnivores and nocturnal predators that prey on almost anything that it can take.
They hunt alone or in groups and have been seen trapping schools of fish against reef walls. Groups of up to 700 grey reef sharks have been observed hunting together. They are skilled at catching fish swimming in open water, with an exceptional sense of smell capable of detecting minute amounts of tuna in vast quantities of seawater.
Social Behavior and Aggression
Grey reef sharks are social, gathering in groups during the day and hunting alone at night. During the day, these sharks often form groups of five to 20 individuals near coral reef drop-offs, splitting up in the evening as the sharks begin to hunt. Adult females also form groups in very shallow water, where the higher water temperature may accelerate their growth or that of their unborn young. They are inquisitive and commonly approach divers. They are also believed to be territorial and have been documented to arch their backs and push their pectoral fins downward and swim in an exaggerated pattern as part of a territorial display.
The grey reef shark is considered one of the more aggressive sharks but will typically only show aggression towards a person when it feels threatened. If cornered or threatened, the grey reef shark displays explicit threat behavior including raising its snout, depressing the pectoral fins, and arching its back while swimming with an exaggerated sway. If the threat continues, the shark may immediately flee or may deliver a quick bite prior to retreating.
Reproduction
The grey reef shark is a viviparous species. The embryos are nourished via a yolksac placenta during gestation inside the mother. The gestation period lasts approximately 12 months, followed by live birth of a litter of 1-6 pups. During mating, the male grey reef shark bites at the female's body or fins to hold onto her for copulation.
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Conservation Status
The grey reef shark exhibits strong site fidelity to coral reefs, a habitat that is becoming increasingly more restricted, making them easier to catch and more susceptible to overfishing. In addition, their life history characteristics (small litter size and relatively late age at maturity) make them particularly susceptible to population decline. The grey reef shark is currently listed by the World Conservation Union (IUCN) as “Near Threatened”. More fisheries data is required for future assessment.
The grey reef shark is fished commercially, mainly for their fins which are used in shark fin soup. The meat is also consumed and used as fishmeal.
Interactions with Humans
Grey reef sharks often show curiosity and approach divers. However, they can become dangerous in the presence of food, and tend to be more aggressive if encountered in open water rather than on the reef. There have been several known attacks on spearfishers, possibly by mistake, when the shark struck at the speared fish close to the diver. This species will also attack if pursued or cornered, and divers should immediately retreat (slowly and always facing the shark) if it begins to perform a threat display.
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