A fluke picture of a cast of spider crabs and a predatory octopus has clinched its owner the 2017 Australian Geographic Nature Photographer of the Year prize.
Justin Gilligan snapped the photo at Mercury Passage, between Maria Island and the Tasmanian coast.
"I wish I could say it took weeks of planning and days in the field to capture the shot but with this image I was fortunate it was a total surprise," he said.
"[It was] a completely unexpected encounter with a group of hundreds of spider crabs, down off Tasmania.
"In their midst was a giant Maori octopus, which is the largest species of octopus in the southern hemisphere.
Gilligan, a freelance photographer from New South Wales, was diving with researchers from the University of Tasmania who were experimenting with kelp on artificial reefs when he snapped the prize-winning image.
"I've been entering this competition since about 2008, so it's nine years now and I've never had the opportunity to win the top prize, so this is definitely a thrill," he said.
'Amazing' images offer insights into natural world
The awards are run by the South Australian Museum and Gilligan has won a trip to Antarctica and $10,000 for his triumph.
"The Australian Geographic Nature Photographer of the Year competition continues to grow, with 2017 seeing a record number of entries," museum director Brian Oldman said.
Some of the other winners include Jennie Stock's photo of a windblown egret, Scott Portelli's shot of green turtles devouring a giant jellyfish, and Julie Fletcher's aerial landscape.
"I'm constantly amazed at the calibre of entries we receive and Mr Gilligan's winning entry is yet another example of the role this competition plays in connecting people with the natural world in which we live."
The winners and finalists have gone on display at the South Australian Museum and at the Australian Museum in Sydney.