by Matt Walker Editor, BBC Earth News

The once secret life of a huge, recently discovered species of manta ray has been unveiled.

Biologist Dr Andrea Marshall has discovered that the giant fish, which she first described as new to science last year, undertakes huge journeys.

As well as making the longest migration known across the Indian Ocean, the fish gathers in large numbers to feed and survives attacks by sharks.

The behavior is revealed in the BBC documentary series Natural World.

Growing up to 7m wide, manta rays are the largest living ray in the ocean and one of the largest of all fish.

Previously, it was thought there was just one species of manta ray, known by the scientific name Manta birostris.

But the more Dr Marshall swam with them, the more she noticed how different manta rays that frequent reefs and the open ocean are, both in their behavior and markings.

Mantas evolved from sting rays, and it was thought they had lost their sting. But Dr Marshall has found that the larger ocean-going mantas have retained a vestigial sting on their tails, proving that the two are separate species.

In July last year, she announced the discovery that there are not one but two species of manta ray at the American Elasmobranch Society’s annual conference in Montreal, Canada.

The larger giant mantas retain the name Manta birostris, while the reef-going mantas have been newly named Manta alfredi, in tribute to Alfred Whitley who first scientifically described manta rays in the 1930s.

However, Dr Marshall’s studies have uniquely revealed a host of manta ray behaviors.

The rays sometimes swim across the ocean floor, skimming the reefs with their mouths, a behavior filmed by the camera crew for the first time.

Usually the fish feed on plankton, but what they feed on when skimming is unclear.

Dr Marshall studies manta rays off the coast of Tofo in southern Mozambique.

It was always thought that manta rays stayed in shallow waters.

Manta ray mating behavior

But Dr Marshall has discovered that the larger giant species of manta ray dives deeply, and migrates 700 miles (1,100km) in just 60 days to the Maldives, the longest migration known for a fish living in the Indian Ocean.

In the Maldives, researchers have also uncovered a unique gathering place of reef mantas, where hundreds of the fish gather.

Around 80% of the fish are female, with many pregnant, suggesting the area is a critical breeding site for the species.

Studies by Dr Marshall and colleague Dr Simon Pierce of the Manta Ray and Whale Shark Research Centre based at Tofo Beach in southern Mozambique have also revealed that manta rays living off Tofo bear huge scars inflicted by large sharks.

Manta rays appear capable of surviving such attacks and will queue up to have their wounds tended by cleaner fish, which nibble at the wound to remove dead tissue and prevent infection.

Dr Marshall’s studies have also revealed new aspects to the fishes’ reproduction.

Many male mantas follow a single female, mirroring her behavior in a bid to attract her as a mate.

Females, which give birth to a single pup after a 12-month gestation, rarely give birth in consecutive years, Dr Marshall has found.

That extremely slow reproduction could place the fish in danger from overfishing, both for subsistence and for export to be used in traditional Chinese medicines.

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