UNIVERSITY of Washington biologist Peter Ward has recorded a formal sighting of a rare species of Nautilus, 29 years after it was last spotted.

The Allonautilus scrobiculatus differs from other nautilus species in their gills, jaws, shell shape and male reproductive structures, as well as having a thick, hairy, slimy covering on its shell, Dr Ward said.

It was previously discovered off Ndrova Island in Papua New Guinea in 1984 by Bruce Saunders from Bryn Mawr College, with Dr Ward collecting samples of the animal a few weeks later.

After a second recorded sighting by Dr Saunders in 1986, the Allonautilus disappeared until July 2015, when Dr Ward returned to Papua New Guinea to survey nautilus populations.

Suspending fish and chicken meat on a pole between 500 and 1,300 feet below the sea surface off Ndrova Island, Dr Ward and his team would film activity around the bait for 12 hours.

One night, the crew was lucky enough to film an Allonautilus approaching the bait, soon followed by another nautilus, with the two fighting for access to the bait until a sunfish arrived on the scene.

“For the next two hours, the sunfish just kept whacking them with its tail,” Dr Ward said.

The team also used baited traps to capture several nautiluses, including Allonautilus, at a depth of about 600 feet, collecting tissue and shell samples before releasing them back into the wild.

Through these studies, researchers have learned that most nautilus populations are isolated from one another because they can only inhabit a narrow range of ocean depth.

“They swim just above the bottom of wherever they are,” Dr Ward said.

“Just like submarines, they have ‘fail depths’ where they’ll die if they go too deep, and surface waters are so warm that they usually can’t go up there. Water about 2,600 feet deep is going to isolate them.”

These restrictions on where nautiluses can go mean that populations near one island or coral reef can differ genetically or ecologically from those at another.

The findings also pose a challenge for conservationists, as once the animals are gone from an area, they may be gone for good.

“It’s only near this tiny island,” Dr Ward said.

“This could be the rarest animal in the world. We need to know if Allonautilus is anywhere else, and we won’t know until we go out there and look.”