CHAGOS ISLANDS - A large, land-dwelling crustacean known as a coconut or robber crab has been filmed attacking and killing a seabird on a small island in the Indian Ocean.

It is the first time such predatory behaviour has been observed in the species, and the discovery has significant implications for how they may affect ecosystems where they dwell, the Guardian newspaper reported on Friday (Nov 10), citing the New Scientist.

The phenomenon was witnessed and captured on video by a biology professor from Dartmouth College, Mr Mark Laidre, who was studying the giant crabs in the remote Chagos Archipelago, a group of atolls in the British Indian Ocean Territory.

According to Mr Laidre, the crab climbed a tree and attacked the seabird in its nest situated on a branch close to the ground. The crab broke the bird's wing, causing it to fall out of its nest. It then attacked the bird with its claws and broke its other wing.

Once the bird was unable to move, other coconut crabs arrived and pulled the still-alive bird apart in scenes Mr Laidre described as "pretty gruesome", according to the Guardian.

Coconut crabs, which can weigh up to 4kg and grow up to one-metre wide, are common in coral atolls across the Indian and Pacific Oceans.

Mr Laidre's discovery suggests that the crabs, who were previously thought to be scavengers, may dominate their ecosystems and could discourage other animals, particularly seabirds, from inhabiting islands where they would be forced to nest on the ground.