How old is Methuselah, the oldest living aquarium fish?

Methuselah, an Australian lungfish, has been declared the oldest living fish in all the world’s aquariums. But beyond ‘at least 85’, we didn’t actually know how old she was – until now.

The Australian lungfish (Neoceratodus forsteri) came to the California Academy of Sciences’ Steinhart Aquarium in San Francisco in November 1938. Prior to that, her past is a mystery.

Scientists have used a DNA-based technique to get a better estimate on Methuselah’s age: she’s 92, give or take a few years.

The research can help better understand the longevity (and conservation) of the species, but since Methuselah is the oldest living example, it’s likely we’ll never figure out her exact birthday.

Air-breathing Australian lungfish, which live in south-east Queensland, have the largest known genome of any animal. It’s 43 billion base pairs long: for context, the human genome is around 14 times smaller, with just 3.2 billion base pairs. This makes it difficult to study their DNA.

The researchers had previously sampled the DNA of 141 lungfish of known age. The lungfish all either had known birth dates, or they could be dated using radiocarbon dating. The carbon method only works on fish born after the 1940s and 50s, because it relies on carbon levels that changed because of nuclear bomb tests.

The researchers looked specifically for “methylation”: where a small molecule (called a “methyl group”) sticks on to certain points in DNA. Methylation rates in these spots increase as the organism ages.

Studying the methylation pattern of these lungfish gave them a “clock”, which they could use to estimate the age of other lungfish.

In this study, they examined the DNA of Methuselah and 32 other lungfish from aquariums in the US and Australia. The researchers only needed a tiny clip of a fin, less than half a centimetre, to collect this DNA.

This allowed them to establish Methuselah’s age as 92 ± 9 years. Based on their clock, there’s a 95% chance she’s between 83 and 101 years old, and most likely in the middle of that range.

Dr David Roberts, a senior research scientist who studies lungfish at Seqwater, the Queensland Water Supply Authority, tells Cosmos that the fact we know that Methuselah is at least 85 is a good confirmation that their estimates are in the right range.

“Our lowest possible age is not far off what it could be. It would be concerning if, for example, we aged it at 86 ± 9 years.”

Because she’s the oldest living sample researchers know about, she’s out of the range of their clock, meaning there will always be some uncertainty around her age.

“We can keep calibrating that algorithm from samples of known age lungfish as they get older and older, so that estimates of the very old lungfish become much more accurate than they currently are,” says Roberts.

Roberts and colleagues have previously used this technique to estimate the age of another lungfish, called Granddad, which died in 2017. They estimated Granddad’s age at 109 ± 6 years when he died. That places lungfish in a rare category of centenarian fish, only shared with 11 other species.

The data can help conservationists to manage threatened lungfish.

“Accurately knowing the ages of fish in a population, including the maximum age, is vital for their management,” says Dr Ben Mayne, a researcher at the CSIRO.

“This tells us just how long a species can survive and reproduce in the wild, which is critical for modelling population viability and reproductive potential for a species.”

The researchers are planning to publish their findings later this year.