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Species:

Habitat: tropical waters in the Indo-Pacific – no deeper than 800 metres or their shells implode

Take a night-time dive over a coral reef in the Philippines and you may well see a nautilus or two. Looking like a cross between an octopus and a snail, it fixes you with its pale white eyes before , wobbling gently with the currents.

Its movement is ungainly and slow, but it has survived virtually unchanged for at least 450 million years, so it must be doing something right. Its relatives the ammonoids dominated the oceans for millions of years before going extinct along with the dinosaurs 65 million years ago – but the nautilus came through that disaster and is still with us today.

For now, anyway. Recent findings confirm something long suspected: nautiluses reproduce very slowly, so they are particularly vulnerable to overfishing. In the Philippines, .

Shiny shell

Nautiluses do not make good eating – you have to – but their are highly prized.

When the animals first gained those shells , some 550 million years ago, it was a nifty new trick. The early nautiloids used it to trap layers of air, which allowed them to stay at one depth without expending any effort. Nautiluses also have muscles attached to the shell, which squeeze water out in a jet that propels the animal along.

Such jet power is a cumbersome way of getting around the seas, and most modern cephalopods have largely abandoned it. Despite its primitive way of getting around, however, the nautilus is no mental slouch.

They have a simple form of memory, despite having . In a 2008 study, nautiluses were able to learn that a blue light meant food was about to be delivered. Since then it has emerged that nautiluses can and remember them for at least two weeks, as well as to help them find their way.

Nautiluses mostly scavenge for dead crustaceans, worms and starfish, often digging for them in mud and biting into them with their . , tracking odours from up to . Their eyes do not have lenses, and instead work just like pinhole cameras. As a result, . It may well be of little use to them, as they hide in the depths during the day and hunt by night.

Slow to grow

Secretive they may be, but fishermen still catch them in droves. To find out how this might be affecting their populations, of the University of Queensland in Brisbane, Australia, and colleagues have been observing nautiluses living on Osprey reef, off the north-east coast of Australia, since 1998. This isolated population is not exploited by humans, so it offers a good baseline for conservationists to work out how many nautiluses can be safely caught.

The answer is, not many. There are around 3000 nautiluses living on and around Osprey reef, an area of roughly 195 square kilometres. Four out of five are male, and less than 10 per cent are juveniles.

By capturing nautiluses, marking them and recapturing them later, Marshall’s team found that it takes the cephalopods over 15 years to grow to maturity. Females lay no more than 20 eggs per year, and there are not many females. As a result, the population would be slow to recover from even a small fishing catch.

Nautiluses have not yet been assessed for the , the database of threatened and not-so-threatened species maintained by the . As a result we don’t really know how they are doing. It might be time to find out.

Journal references: and

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