A rarely seen oarfish washed up on the Aramoana Spit salt marsh near Dunedin.

It's rarely seen, can grow up to eight metres long and swims vertically, but does it really bite off its own tail?

That is the question marine scientists are grappling with after an oarfish washed up on a salt marsh on the Aramoana Spitat the entrance of Otago Harbour on Thursday.

The University of Otago's New Zealand Marine Studies Centre posted photos of the bizzare sea creature on its Facebook page after it was discovered by a local man on a morning walk.

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According New Zealand Marine Studies Centre the fish was three metres long but the species was known to reach lengths more than 8m.

Based on information from the Smithsonian Tropical Research Institute(STRI), the New Zealand Marine Studies Centre said on its Facebook page the fish were known to reduce their length by "biting off their tail".

STRI research associate Tyson Roberts penned a book on the oarfish in 2012, which said all specimens larger than 1.5m have shortened their bodies, often several times, by autotomy, the process of "self-amputation".

Although he never mentioned oarfish actually eat their own tail.

University of Canterbury's Chris Glover, from the school of biological sciences, believed there may have been confusion around the term autotomy - "a process where animals can cleave off parts of their bodies" without using their mouths.

"So like lizards, oarfish are thought to be able to autotomise sections of tail. Exactly why they do this, nobody seems to know, but it's not thought to be due to predation," he said.

According to Roberts' 2012 publication on "one of the most bizarre and rare fishes in the sea", oarfish are long-distance ocean drifters usually found hanging in vertical position in the up to 300m deep.

Healthy oarfish are known to wash up on beaches and are sometimes found near the water surface.

Department of Conservation (DOC) services manager David Agnew was one of the first to see the fish in Aramoana.

He said he had never come across such a sea creature.

"It was really fresh, it had just washed up on the night tide and looking at it, it was a pretty weird looking creature.

"It didn't have scales like other fish, it had smooth skin, like tinfoil, and if you rubbed it the silver would come off."

Members of the Otago Museum had taken tissue and organ samples of the fish, but what has been done with its body since remains a mystery.