November 29, 2023

She’s super-gentle, and doesn’t get overly excited. She enjoys eating earthworms, fruits and vegetables, and slowly moving around her tank. Her favorite food – at least for what is in season now – is a fig.

If Methuselah sounds like a grand old dame, it’s because she is: she is the oldest living fish in captivity, aged somewhere upwards of 92 and potentially as high as 101 years. She arrived on a steamship from Australia along with 230 other fish to the Steinhart aquarium in San Francisco in 1938 as a young, small fish. And Methuselah’s story unfolded in a typical way, for a fish in an aquarium: she grew. Humans came to look at her. She peered back through glass at humans.

But 1938 was a different time: bread cost nine cents a loaf. A racehorse named Seabiscuit was winning races. Germany was persecuting Jews, foretelling a coming conflict in Europe.

Then there is Methuselah, who is no ordinary fish. She’s the only fish still living from the steamship. And most important, she’s a lungfish – a species more closely related to humans or cows than to ray-finned fish like salmon or cod – which can breathe air using a single lung when streams become stagnant, or when water quality changes. Lungfish are also believed to be an example of the original creatures that crawled out of water and moved to land in evolutionary history. The species was discovered in 1870 – and the scientist who first described the fish originally thought it was an amphibian.

Lungfish like Methuselah have long-held secrets, but scientists have only recently attempted to understand their evolution and life history. For one thing, the fish’s genome is the largest of any animal, containing 43bn base pairs – roughly 14 times the number in the human genome. The previous record holder, the Mexican axolotl, has a genome made up of 32bn base pairs.

“Genetics is really quite straightforward for normal fish – but for lungfish they’re so unique and so different that all of those techniques didn’t or don’t work,” said David T Roberts, a senior scientist with Seqwater, the statutory authority of the government of Queensland in Australia, where the fish still live in a handful of rivers in the wild. “It’s always pushed the envelope on uncovering some of its secrets to be able to manage and conserve it – and age is a really important one.”

Methuselah lives at the Steinhart aquarium at the California Academy of Sciences in Golden Gate park in San Francisco, California.
Methuselah lives at the Steinhart aquarium at the California Academy of Sciences in Golden Gate Park in San Francisco, California. Photograph: John G Mabanglo/EPA

A fish’s age is critical to know because it tells scientists information like growth rates, maturity, longevity and how long they breed – which is vital fundamental knowledge to manage a protected species.

Lungfish – a vulnerable species – have proved especially challenging to date because they grow a lot at the beginning of their lives, but then grow extremely slowly (yet continuously) for the remainder of their lives. Ear bones that are harvested after most fish’s death can be counted like tree rings, but lungfish, always the outlier, don’t have the same composition to their ear bones.

So scientists started to use radiocarbon to date the fish – relying on a technique that basically imprints living things with a carbon signature resulting from the atomic bomb tests back in the 1950s. But that doesn’t work well in animals born before 1950, when the carbon signature changed.

Now, scientists are using DNA tools that look at methylation – the way that DNA is turned on or off – to date the fish. For younger fish, it can offer an exact number, but for older fish it gives a range of dates.

It wasn’t the first time this technique had been used. Last year, scientists estimated a lungfish named Granddad that lived at the Shedd aquarium in Chicago to be 109 years old (give or take six years) at the time of its death, confirming that lungfish can live well over 100 years. The analysis also revealed that Granddad started its life in the Burnett River in Queensland, Australia, the location of the species’ original discovery in 1870.

In the study on Methuselah, aquarium workers took samples the size of a peppercorn from the lungfish in captivity and extracted the DNA from that in order to estimate their age for the first time ever. They found Methuselah to be at least 92 years old. The scientists plan to release their findings of 30 other lungfish later this year, as part of a library of living lungfish across the world.

“Knowing how long they potentially live and understanding more about how long they could reproduce could drive how we’re caring for habitat to help keep that species afloat in the wild,” says Brenda Melton, director of animal care and welfare at the Steinhart aquarium. “It just really opens the doors for a lot of other conversations and questions that might be able to be asked about how we can better care for them in the wild and preserve habitat.”

Roberts is inspired to continue to conserve the fish – after all, lungfish were around before dinosaurs became extinct, and their cousins possibly split off into animals with legs and then crawled on to land and then became humans, he says. “They’re a cousin to all land animals, basically.”

Methuselah’s age is now known, but she still holds other mysteries – even her biological sex. The handlers use she/her pronouns, but they actually don’t know if Methuselah is a male or female. Some fish have gender differences in size or shape – but not lungfish. And behaviorally, they suspect she’s a female, but they will not be able to find out for sure until after she dies.

Another question is if the fish is feeling old – and how do fish change when they’re geriatric? Melton says it varies widely. Most fish live only a few years – so it’s rare to see really old fish in the wild. But there are some hints: some spinal changes, like a curved back, or weight loss, cloudy eyes or looking a little gray in the scales.

Two of the other fish in the new study were estimated to be 50 and 54 years old – and Melton says they look a little more similar in coloration, while Methuselah has gotten a little lighter in color over the years. “We don’t know that that’s actually tied to her age, but it’s the only thing that we have seen physically that looks different for this fish.”

Melton says that just the existence of something that has lived for so long leaves her in awe. She wonders what Methuselah thinks of all her companions and living situations over the many years she’s spent at the aquarium – as the fish has the longest institutional memory of anything in the building.

“It’s incredible to me that after all of these years of having her in our care,” she says, “we’re still learning and we still have the ability to learn from animals in ways that we can’t even conceive yet.”

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