RESEARCH: Reef Sharks have friendship groups

Marine research scientists have found out that Grey Reef Sharks hang out with the same friends in the same spot for years.

Researchers at the Florida International University in America have studied Grey Reef Sharks (Carcharhinus amblyrhynchos) for four years in the remote Palmyra Atoll in the Pacific Ocean.

To study the Grey Reef Sharks, the researchers tagged about 40 individual sharks with acoustic transmitters that emit a unique high-frequency sound. A network of 65 receivers recorded the identiy of any tagged shark that came within 300 metres of any of the receivers. The batteries on the transmitters last for four years.

The marine research scientists found that sharks feed at night, and during the day they return to a particular spot on the reef, forming groups of about 20 friends.

“We don’t think of sharks as social animals, but they do have social groups,” said Yannis Papastamatiou of Florida International University.

The social behaviour study found that the social groups of the Grey Reef Sharks are very stable, with the same individuals associating together in the same groups year after year. The situation of individuals moving from one group to join another group was extremely rare – the sharks prefer to stay in the same group.

Papastamatiou and colleagues think that the sharks can recognize other sharks individually.

The sharks are not friends like humans or some other animals are friends. They are more like social individuals or social associates. Papastamatiou said, “They are not friends in the same sense of having any emotional bond with each other.” 

Many shark species, such as Great White Sharks and Hammerhead Sharks are solitary, but the Grey Reef Sharks are unusually social.

The scientists are unsure whether the same individuas hunt together when they leave the home area at night, or not. At Palmyra, the Grey Reef Sharks hunt and catch their prey mostly in open waters at night, too far from the reef to be detected by the receiver network equipment.

The Grey Reef Shark is a marine (saltwater) requiem shark in the Carcharhinidae family. It is an elasmobranch cartilaginous fish—a fish that does not have a bony skeleton. 

The Grey Reef Shark is slim with a grey body. It has a wide, rounded snout (nose) and large, oval-shaped eyes. 

It grows to about 160 centimetres (63 inches) in length. 

It is a very common shark. It is native to the Pacific Ocean. It prefers to live in the shallow waters of tropical coral reefs near the shore. It eats bony fish, crustaceans (shrimp, crabs, and lobsters), squid, and octopus. 

The Grey Reef Shark is a social shark, living in groups called aggregations. 

It is viviparous. The female is pregnant for 7-11 months, before giving birth to 2-5 live young called pups. 

Journal reference: Proceedings of the Royal Society B

DOI: 10.1098/rspb.2020.1063

Location of photographs: Aquarium in Almaty, Kazakhstan

Photographer: Martina Nicolls


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