This image taken at Fakarava Atoll in the Pacific has earned Laurent Ballesta the prestigious title of Wildlife Photographer of the Year (WPY).
Jury chair Roz Kidman Cox said it was a technical tour de force.
"It's partly the setting, taken during a full Moon, but also the timing of it, knowing when to take the picture."
The annual spawning of camouflage groupers occurs in July. It's been known to draw up to 20,000 fish, along with many reef sharks looking for a meal. Overfishing threatens the groupers, but this picture was captured in a reserve that gives them some protection.
"We spent five years in this place, 3,000 hours of diving, to get this particular moment," Laurent said.
"I'm attached to this picture because of the shape of the cloud of eggs: it looks like an upside-down question mark. It's a question of the future of these eggs because only one in one million will (survive to) become an adult, but it's maybe more symbolic of the future of nature. It's a very important question about the future of nature."
As well as celebrating WPY's Grand Prize, the French photographer also wins the competition's Underwater category.
Ten-year old Vidyun R Hebbar from India is the Junior Wildlife Photographer of the Year for this picture of a tent spider in its web. The image is called Dome Home.
The blurred green and yellow colours in the background belong to one of those three-wheeled tuk-tuk taxis.
"Its focusing is pin sharp," Roz Kidman Cox told BBC News. "You can actually see the little fangs if you blow up the picture. I love the way it's been framed and the way you can see all the texture of the web, its lattice structure."
Vidyun recalled: "It was challenging to focus the tent spider because the web shook every time a vehicle passed by."
Started in 1964, WPY is organised by London's Natural History Museum.
The competition attracts tens of thousands of entries each year. Scroll down to see some of the individual category winners.