This Octogenarian Is the Oldest Fish in Captivity. And She Likes Belly Rubs.

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Methuselah is no ordinary fish, and this month, she's swimming into her 80th year as a resident of the California Academy of Sciences' Steinhart Aquarium in San Francisco.

The mature, 4-foot-long (1.2 meters) fish came to the aquarium from Australia in 1938 when she was already adult size, so experts estimate she's between 85 and 90 years old. That likely makes her the world's oldest fish in captivity.

Appropriately named after the longest-living biblical figure (in the bible, Methuselah lived to be 969 years old), it's not just Methuselah's age that makes her stand out. As an Australian lungfish (Neoceratodus fosteri), she represents one of only six living species of air-breathing lungfish, which belong to the class Sarcopterygii. [Photos: The Freakiest Looking Fish]

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This ancient group of freshwater fish has been around for 400 million years and is characterized by a unique swim bladder that not only controls buoyancy but also functions as a primitive lung that allows the fish to breathe air.

Besides Australian lungfish, there are also African lungfish and South American lungfish. The two latter groups are known to burrow in the mud and breathe air for months at a time when their ponds disappear in the dry season. And, they sometimes use their four fins to "walk" to another pond.

So, what's the secret to Methuselah's longevity?

"I want to say it's my care, but it's not — it's genetics," said Allan Jan, senior biologist and Methuselah's primary caretaker at the Steinhart Aquarium.

Lungfish are long-lived species. The previous record for the oldest captive fish was held by Granddad, a lungfish at Chicago's Shedd Aquarium, who died in 2017 at the age of 84.

Although she's genetically programmed for a long life, Methuselah also has a few traits that may contribute to her endurance.

For example, she has always had a healthy appetite, Jan said. "She eats everything that I offer her, and she eats the most." And although she's not picky, she's refined her palate over the years so that her favorite foods are prawns and figs, Jan said.

Similar to many of us, Methuselah also has a preference for certain people. Jan said he's had many volunteers who want to feed Methuselah over the years, but despite her healthy appetite, there are a few people she simply refuses to take food from.

Jan has also learned that the octogenarian fish prefers the calm tranquility of her small tank as opposed to the larger tank where the Steinhart Aquarium's other two lungfish reside — both are about half Methuselah's age.

When Methuselah was placed in the larger tank, with more space and increased water flow, she responded by resting upside down, on her back. Jan and his colleagues never figured out why Methuselah insisted on her belly-up position, but once they moved her back to her smaller tank, she flipped right-side up and "was absolutely fine," Jan said. "It wasn't a [health] scare, but we didn't want to concern the public."

During her eight decades at the aquarium, Methuselah has seen millions of visitors, who have been delighted by her large size and unusual appearance.

Although these living fossils look like formidable armored fighters, the lungfish are really just big softies, Jan said. Methuselah and the other lungfish enjoy frequent belly rubs and head scratches, just like "underwater puppies," he said.  

Australian lungfish are considered a protected species, according to the Australian Museum. The species can be captured only with a special permit and export is strictly regulated. 

Methuselah serves as an important ambassador for her wild counterparts, whose populations are struggling from human-caused habitat loss, Jan said, and he bets she'll continue do so for years to come.

Originally published on Live Science.