An Australian lungfish believed to be the longest-living fish in captivity has died at a Chicago aquarium.

Granddad, who was housed at the Shedd Aquarium, weighed 11 kilograms and was euthanased after showing signs of organ failure.

It is believed he was aged in his late 90s, but Queensland researchers are hoping genetic testing will confirm his actual age.

Granddad had been at the aquarium for more than 80 years. ( Supplied: Chicago Shedd Aquarium )

Granddad was one of Australia's oldest exports, originally collected from Queensland's Mary or Burnett Rivers in 1933 by Taronga Zoo staff.

Over the course of his time at Shedd Aquarium, Granddad received more than 104 million visitors, and according to aquarium staff, will be sorely missed.

"For a fish who spent much of his time imitating a fallen log, he sparked curiosity, excitement and wonder among guests of all ages who would hear his story and learn about the incredible biology that makes his species a living fossil," Shedd president Bridget Coughlin said.

Unlocking DNA secrets

Lungfish can live to be more than 100 years old and are a protected species in Australia.

They have a single primitive lung and are among a small number of fish that can breathe air.

They have existed for nearly 400 million years and fossils show they have remained unchanged for more than 100 million years.

Tom Espinoza says Queensland researchers are using carbon dating technology to demystify the fish's secret life cycle. ( Supplied: Tom Espinoza )

Queensland researcher Tom Espinoza has been using technology to demystify the secret life cycle of the ancient and unique lungfish.

He is hoping that researchers will finally be able to put the debate to rest over how old Granddad really was.

"The scale would be the primary sample we would want, but just a little bit of his body tissue for genetic analysis would be good as well," he said.

"The difficult thing with this species is that they're not aged using traditional methods — normally you would use ear bones and cross-section it and count rings like a tree.

"This animal you can't do it to, so the primary objective of our overall research has been to figure out how to age the species."

Granddad has called the aquarium home since 1933. ( Supplied: Shedd Aquarium )

Researchers have been examining lungfish scales using carbon dating to calculate their age.

Mr Espinoza is hoping the process will teach researchers both here and at the Shedd Aquarium a little bit more about the animal.

"In terms of ageing the scale, we can give them a better estimate of Granddad's age," he said.

"In the genetic sense, it would be good to know if an animal that old would have some unique genetic signatures that we could use in our study and vice-versa, we could inform them where their animal actually came from."

Lungfish facts Lungfish grow up to 1.5 metres in length and have bone-like structures in their fins

Lungfish grow up to 1.5 metres in length and have bone-like structures in their fins They are thought to have remained unchanged for 140 million years, and can live for up to 100 years

They are thought to have remained unchanged for 140 million years, and can live for up to 100 years The species are native to the Burnett and Mary Rivers in Queensland, with a small population relocated to the Brisbane River in 1895

The species are native to the Burnett and Mary Rivers in Queensland, with a small population relocated to the Brisbane River in 1895 A lungfish was first presented to Dr Gerard Krefft in 1870 by the Honourable William Forster, the NSW Minister for Lands, as the "Burnett salmon"

A lungfish was first presented to Dr Gerard Krefft in 1870 by the Honourable William Forster, the NSW Minister for Lands, as the "Burnett salmon" On Granddad's 80th birthday, the Australian consul-general, the honourable Roger Price, presented the Shedd Aquarium with a proclamation recognising "the tremendous relationship between the John G Shedd Aquarium and the Government of Australia"

Tale of lungfish love

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Following the announcement of Granddad's death on social media, hundreds of people commented about how much they loved to visit the Queensland native at Shedd Aquarium.

Nathan de Rover of Chicago commented that he and his wife were married in front of Granddad's tank so the lungfish could be a witness.

"We didn't ask permission because we would be afraid we would be told no," Mr de Rover told ABC Local Radio.

"We just kind of went there as guests and in front of Granddad we cleared everybody out and said we're going to get married now.

"Our friend Michael, who was the officiant, started marrying us and said, 'Ladies and gentlemen and fish'.

"We're still members [of the aquarium] and we still go several times a year, they always have new things to look at.

"It's beautiful, it really is one of the gems of Chicago."