Methuselah, an Australian lungfish, has been declared the oldest living fish in every aquarium in the world. But other than “at least 85,” we didn’t really 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. Before that, his past was a mystery.
Scientists used a DNA-based technique to get a better estimate of Methuselah’s age: She is 92, give or take a few years.
Research may help to better understand the longevity (and conservation) of the species, but as Methuselah is the oldest living example, we will likely never find out its exact birthday.
Air-breathing Australian lungfish, which live in southeast Queensland, have the largest known genome of any animal. It has 43 billion base pairs: to put it in context, the human genome is about 14 times smaller, with just 3.2 billion base pairs. This makes it difficult to study your DNA.
The researchers had already collected DNA samples from 141 lungfish of known age. All lungfish had known birth dates or could be radiocarbon dated. The carbon method only works on fish born after the 1940s and 1950s because it is based on carbon levels that have changed due to nuclear bomb testing.
The researchers specifically looked for “methylation”: where a small molecule (called a “methyl group”) sticks to certain spots in the DNA. Methylation rates in these spots increase as the organism ages.
Studying the methylation pattern of these lungfish gave them a “clock” that 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 small fin clip, less than half a centimeter long, to collect this DNA.
This allowed them to establish Methuselah’s age at 92 ± 9 years. Based on the clock, there’s a 95% chance she’s between 83 and 101 years old, and probably in the middle of that range.
Dr David Roberts, a senior research scientist who studies lungfish at Seqwater, the Queensland Water Supply Authority, said Cosmos that the fact that we know that Methuselah is at least 85 years old is a good confirmation that his estimates are in the right range.
“Our lowest possible age is not far from what it could be. It would be worrying if, for example, we aged him to 86 ± 9 years.”
Because it is the oldest living sample researchers know of, it is outside the clock’s range, meaning there will always be some uncertainty surrounding its age.
“We can continue to calibrate this algorithm from samples of known-aged lungfish as they age, so that estimates of very old lungfish become much more accurate than they currently are,” says Roberts.
Roberts and colleagues previously used this technique to estimate the age of another lungfish, called Granddad, that died in 2017. They estimated the grandfather’s age at 109 ± 6 years when he died. This places lungfish in a rare category of centuries-old fish, shared only with 11 other species.
The data can help conservationists manage endangered lungfish.
“Accurately knowing the age of fish in a population, including maximum age, is vital to its management,” says Dr Ben Mayne, CSIRO researcher.
“This tells us how long a species can survive and reproduce in the wild, which is critical for modeling a species’ population viability and reproductive potential.”
The researchers are planning to publish their findings later this year.
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