Image copyright A E Bicskos Kaszo Image caption The image is believed to be the first photograph taken of an oceanic shark giving birth

A remarkable photograph of the live birth of a thresher shark has cast light on the lives of these elusive, vulnerable fish.

The image, taken during a research dive in 2013 and now published in the journal Coral Reefs, is believed to be the first record of a birth in this species.

Lead researcher Dr Simon Oliver from the University of Chester told BBC News that this was also "the first record of any oceanic species giving birth".

He added that seeing the image was one of the most exciting moments in his career.

Sharks: The biggest, fastest and fiercest Image copyright NPL.com The world's largest shark is the whale shark (pictured), which can grow to over 13m (40ft) long. This huge filter-feeder shark sifts zooplankton from the water

The fastest shark is thought to be the shortfin mako. The species has been recorded travelling in excess of 20mph (30km/h) and can chase down some faster bony fish, such as tuna

The smallest shark is the dwarf lanternshark. This deepwater species is found in the Caribbean Sea and reaches a maximum length of just 21cm (8in)

The shark with the largest teeth relative to its size is the largetooth cookiecutter. At only 42cm in length, this species has 17-19 rows of large lower teeth and feeds by cutting circular pieces of flesh from larger marine mammals, fish and other sharks. It has even caused problems for submarines Source: Shark Trust Watch a great white shark strike in slow motion

"We were doing a standard survey - out every day, making observations, " Dr Oliver recalled.

"One of [our team] is a photographer - Attila Kaszo. He took the picture of the shark, and when he processed the image and showed it to me, I freaked out."

Dr Simon Thorrold, a senior scientist from Woods Hole Oceanographic Institution in Massachusetts, US, described the photograph, which has now been published in the journal Coral Reefs, as "amazing".

"I have never seen a comparable image for any other pelagic shark," he told BBC News. "It may well be, or at least the first time that the event has been photographed, but this is always difficult to say definitively."

He added, though, that he thought the image was of "limited importance" in terms of shark conservation.

Dr Oliver, who studies the thresher sharks at a seamount (undersea mountain) in the Philippines, says it shows how important the study site to this vulnerable species.

This particular seamount is a shark "cleaning station" - the sharks go there to have parasites nibbled away by smaller fish called cleaner wrasse.

Dr Oliver explained: "It looks like this area is not just a cleaning station, which is already massively essential, it's also serving as a pupping ground.

"We've seen lots of [pregnant] females there, so I don't think this is a one-off."

He and his colleagues want the seamount designated as a marine protected area.