Those who are familiar with the Bible may remember a man named Methuselah, the grandfather of Noah, who lived longer than any other Biblical person – 969 years.
A fish of the same name has just achieved global recognition as the world’s oldest living aquarium fish. This particular Australian lungfish (Neoceratodus forsteri) was transported from Australia to San Francisco’s Steinhart Aquarium in 1938. Simple math indicates that Methuselah the lungfish (who, based on behavior, is likely a female) would be turning at least 85 years old this year. However, scientists recently conducted a study and found that she is likely around 93 years of age or older (up to 102 years)! So, how did they do it?
Otoliths can tell age
Traditional means of estimating the age of a fish rely on the use of calcified (hardened) structures. Simply put, scientists examine the structures and count the annual growth rings. These rings form as a result of differing growth rates by season.
Otoliths, which means “ear-stones,” are today’s most popular structure for aging fishes. Every fish has a pair of otoliths, one in each inner ear. Actually, all vertebrates – including humans – have otoliths. The primary function of otoliths in living organisms is to assist with hearing and balance. Their size and shape vary by species. Unfortunately, extracting otoliths always requires sacrificing the fish, and therefore is discouraged for individuals of Threatened or Endangered species.
Australian lungfish are listed as Endangered on the International Union for Conservation of Nature (commonly known as the “IUCN”) Red List – and, more importantly, Methuselah is a beloved resident of the Steinhart Aquarium! – so otoliths could not be used to estimate Methuselah’s age.
Other structures can tell age, too
There are other calcified structures that can sometimes be used to age fishes, and these include scales, fin rays and/or fin spines. Although these structures can typically be extracted from individuals without the need for sacrifice, they possess their own shortcomings. Primarily, the early growth rings in scales, fin rays and fin spines can shrink and/or vanish, which can cause scientists to underestimate the age of a fish. This issue is particularly prevalent in older individuals, so these structures would not have worked well for Methuselah!
DNA methylation tells Methuselah’s age
Now that we have discussed traditional aging methods and the reasons why those methods could not be used to estimate Methuselah’s age, you are probably wondering how the scientists accomplished this feat. The answer lies in cutting-edge DNA techniques. First, Drs. Ben Mayne and David Roberts obtained a fin clip, or a piece of Methuselah’s fin. From that fin clip, they extracted a tiny tissue sample and used a process called DNA methylation to estimate Methuselah’s age. They also estimated the ages of several other Australian lungfish and created a “DNA age clock” that will allow scientists to better predict the longevity of the species, as well as its population viability and reproductive potential. The clock will be revised over time as scientists are able to estimate the ages of more individuals of this species. DNA-based techniques like this one are vital for learning more about Threatened and Endangered species without harming their populations.
Lungfishes are particularly fascinating due to their evolutionary history: they are some of the oldest living (most “primitive”) fishes and have remained the same over the past 100 million years. In response to environmental conditions of extreme drought, wherein their home pools become anoxic (deprived of oxygen) or nearly dry up, lungfishes evolved to possess a single lung that enables them to breathe air.
Learn more about Methuselah, including her favorite foods, in this Creature Closeup and this video feature.
Meet the author
Amanda Jargowsky
Marine Fisheries Specialist
Amanda Jargowsky is a marine fisheries specialist with the MASGC-supported Marine Fisheries Ecology Program at the Mississippi State University Coastal Research and Extension Center. She conducted... Read more
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