A sea-dwelling species which has lived unchanged for millions of years is under threat, prompting calls to protect the ancient Nautilus.

An internationally-renowned palaeontologist has just returned from the Philippines, where he discovered the Nautilus was close to extinction at several sites.

“The Nautilus situation we found in the Philippines was mind-boggling,” said University of Adelaide’s Professor Peter Ward.

“The Philippines have been at the centre of Nautilus fishing for decades. Now it is just about extinct there. And it is not just Nautilus. In the same environments we found almost no larger fish at all where there should be large schools of many different species.”

Nautilus – known as “living fossils” because they have existed in the same form for around 500 million years − live at depths of 100-600 metres in reefs in the Indo-Pacific region, including Australia.

Desired for their beautiful shells, they are heavily fished and traded internationally – with shells readily-available on websites such as eBay.

“Nautilus has survived every single mass extinction event that's been thrown at it over half a billion years, now it's being wiped out by humans to sit on a bathroom shelf or as a pretty button on someone's shirt,” Professor Ward said.

“We know that the largest Nautilus in the world come from WA, and we are seeing them being sold on eBay even though there is supposed to be regulation in Australia.

“There is good reason for concern about WA deep reefs.

Over the past four years, Professor Ward has conducted a census of Nautilus using special underwater technology developed at University of Queensland.

“We found Nautilus at rates of 10-15 per square kilometre in the Great Barrier Reef, but in the Philippines they were 100-200 times more rare than that ̶ virtually extinct in the Bohol Strait and completely gone, as far as we could see, from three other classic fishing locations.

“Nautilus is the ‘canary in the coalmine’ of the deep reef environment,” he said.

“It tells us about the health of our deeper reefs where little ecological study is done.

“When Nautilus isn't there, we know that the other fish at those depths are also at risk from overfishing or other environmental factors. We cannot rule out high acidity and warming of these formerly cool, deep waters caused by climate change, and from rising levels of silt caused by nearby deforestation,” he said.