"In other words, the fish consciously decides to rotate the spine" to deploy its in-built weapon. A stonefish deploys the sabre by squeezing its cheeks. Although the lachrymal sabre isn't connected to venom, it'd be mighty painful if it skewered you - or, for that matter, a marine predator trying to make a meal of the stonefish. That defensive protection is likely to be the function the sabre performs, Smith says. But the sabre also glows, which may indicate it has another function.

"Beyond defensive mechanisms, it could play a role in mating, mate attraction, or territory defense," says Smith. "In that sense, it could be like the horns on a ram or the antlers of a deer." Why scientists are excited Apart from raising some interesting questions about how stonefish evolved to possess this complex "switchblade" among its other defensive mechanisms, it has also been identified as a defining feature of the species. There are 134 described species of stonefish, and Smith says the sabre is common to each of them – which means it gives scientists a new, relatively easy way to classify the fish. "So much of what biodiversity researchers do in this day and age is restricted to work with DNA, and so we often have difficulty classifying newly recognised or potentially dangerous animals or plants into groups," Smith says.

"This readily apparent feature allows new species to be immediately recognised as members of this group, which greatly simplifies the description of new species. To put it another way, when we find an animal with mammary glands and hair, we immediately know it is a mammal. That rapid classification is extremely useful to biodiversity scientists." Is the stonefish our most underrated dangerous animal? Although treading on a stonefish is up there with the most terrifyingly painful experiences you can get, the creature is often forgotten among the other dangerous and deadly animals found in Australia. An effective stonefish antivenom was first produced in 1968. The severity of stonefish injuries depend on a range of factors, including how many spines penetrate the foot (which is usually where people are injured, as they step on the fish thinking they're rocks or coral), and how deeply.

Pain, swelling, paralysis and spasms can affect the entire limb and last between days and months. Stonefish usually lie motionless, partially buried around coral, rocky reef, rubble or aquatic plants. Credit:Surf Life Saving Queensland. The excruciating pain has been known to cause delirium or unconsciousness, which can lead to death through drowning. Without treatment, respiratory distress and heart failure can also result. Neurophysical symptoms can persist for months after the initial sting. But long-term effects are less likely with immediate proper treatment, including anti-venom. Marine biologist Julian Pepperell says while they are "probably the world's most venomous fish", stings from stonefish are not terribly common, and there have been no recorded deaths from the animals in Australia – although it has certainly happened overseas.

Dr Pepperell says there are two species of dangerous stonefish in Australian waters – the estuary stonefish, Synanceia horrida and the reef stonefish, Synanceia verrucosa. "Both are mainly tropical, extending to the subtropics, right around the northern half of Australia, from around Shark Bay in WA to the Qld/NSW border, or a little beyond," he says. But for Professor Smith, who first encountered the lachrymal sabre dissecting his pet stonefish 15 years ago, they are more "under-appreciated" than underrated. "I work on stonefishes, lionfishes, and sea robins, so any time that I could get them as pets, I would buy them," Professor Smith explained. Although he took a 10 year break from keeping them when he had children, he said. People need to treat stonefish with proper care, "and they should be handled with the recognition that they are extremely dangerous," he says.