An Australian shark already renowned for its peculiar “balloon defence” behaviour has been captured displaying another extraordinary phenomenon – glowing green in the dark.

Biofluorescence occurs when animals absorb light from the environment and re-emit it as a vivid colour. It was discovered in local swell sharks by filmmakers involved in a project documenting the waters of Western Port Bay in southern Victoria.

Western Port Down Under founder Sheree Marris – a marine biologist and underwater photographer – described the discovery as “the kind of moment you dream about”.

“We knew biofluorescence had been documented in some overseas shark species, so it was definitely something we were curious about exploring, but honestly, seeing it appear through the lens was one of those jaw-on-the-floor moments,” she told The New Daily.

The bright green light display isn’t visible to the naked eye, so the team used specialised underwater lighting, filters and camera equipment to capture the phenomenon at Western Port, which is about an hour’s drive from Melbourne.

“Seeing that glow appear through the lens was surreal… moments like this show just how much is hiding in plain sight,” said cinematographer Jack Breedon.

Biofluorescence is different from bioluminescence, where marine creatures such as the anglerfish can generate their own light through a chemical reaction in their body – perhaps to attract prey or a mate, or to distract predators.

Animals that display biofluorescence instead absorb blue wavelengths of light and re-emit it at as a glowing green, red or orange.

The phenomenon is surprisingly widespread in the ocean, where it has been recorded in other reef fishes and sharks, as well as corals, jellyfish, sea turtles, seahorses and pipefish. Scientists believe it can help animals see each other, especially in deep waters.

“It’s one of the reasons the ocean is so endlessly fascinating,” said Marris, who has a deep passion for the sea and has written several books including Octopuses – Underwater Wonders and Melbourne Down Under.

“There’s this entire hidden neon world beneath the surface that humans are largely blind to.”

American researchers documented biofluorescence in two species of shark in 2019: The Pacific swell shark (Cephaloscyllium ventriosum) and the chain catshark (Scyliorhinus retifer). However, Marris said this is the first time it has been observed or recorded in the Australian swell shark (Cephaloscyllium laticeps), which is endemic to southern Australian waters.

“What’s really exciting is that this isn’t just visually spectacular, it opens up fascinating scientific questions about communication, camouflage and even skin chemistry,” she said.

“Scientists think the same compounds helping the shark glow may also have antimicrobial properties, almost like a built-in glowing superhero shield.”

A spotted swell shark is well camouflaged against the ocean floor. Photo: Jack Breedon / Western Port Down Under

Swell sharks can grow to around one metre long and are usually found around reefs and rocky habitats.

Marris, who said they were “completely harmless” to humans, believes they are one of the most under-rated sharks in Australian waters.

“They’re famous for one very weird behaviour,” she said.

“When threatened, they can rapidly inflate their bodies, making them look more intimidating and much harder for predators to pull out or swallow. Basically, [it’s] nature’s underwater balloon defence system.”

They also lay spiral egg cases, often called “mermaid’s purses”, in which the baby shark grows before hatching fully formed and ready to survive on its own.

The footage showing the Australian swell shark displaying biofluorescence has been verified by La Trobe University biologist Travis Dutka, who said it was reminder of how little was known about southern marine ecosystems.

“To reveal biofluorescence in a species where it hasn’t previously been documented is genuinely exciting, and highlights that even familiar animals may be hiding extraordinary adaptations,” Dutka said.

For Marris, it also encapsulates the message behind the Western Port Down Under project, which seeks to shed fresh light on this coastal environment and the diversity of its marine life.

At first glance, she said, the bay appeared to be just an expanse of mangroves, saltmarsh and mudflats, leading most people to dismiss it as unremarkable. But hiding beneath the surface are “globally unique” reefs, sponge gardens and rocky reefs, as well as marine residents such as weedy seadragons and ghost sharks.

“To see a species we thought we knew suddenly reveal a hidden side completely changes how we see our own backyard,” she said of the swell shark.

“You don’t need to travel across the world to see something extraordinary – it’s right here.”