Nuclear weapons solve the Lungfish riddle

The lungfish is a particularly weird fish.

As well as the usual gills, as its name suggests it also has a fully functioning lung that allows it to survive out of water in drying river beds or lakes for extended periods.

That lung has led some to believe it essentially represents a surviving link in the evolutionary chain between fishes and amphibians.

However, similarities to humans don’t stop at the lungs – a new study has found it also has a similar life span to us, potentially up to 80 years old.

That’s the conclusion made by Stewart Fallon from the Australian National University, in findings published in the journal PLoS ONE. And befitting such a weird fish, the unexpected key to solving the question was nuclear weapons testing.

A long-term question about longevity

Despite intriguing scientists since the 19^th century, exactly how old the Australian lungfish (Neoceratodus forsteri) live had never satisfactorily been answered.

“A lot of fish have what’s called an Otolith – basically a solid stone in their inner ear. As the fish grows, the stone grows as well and there’s usually little annual marker bands on there, so we can count them and know how old the fish is – but the lungfish doesn’t have that stone,” comments Fallon.

With lungfish also being a threatened species, any approaches which resulted in the death of the fish were also not an option.

Nuclear testing provides the key

Instead, Fallon and his team developed a different approach, instead measuring the amount of Carbon 14 in the fish’s scales.

The amount of Carbon 14, also sometimes referred to as radiocarbon or bomb carbon, began increasing in the atmosphere in the mid-1950s thanks to the testing of nuclear weapons. By the 1960s amounts had almost doubled, hitting their peak in the southern hemisphere in 1965. After the Nuclear Test Ban Treaty came into effect level of Carbon 14 started decreasing again, and by the 1990s levels were around 20% higher than in 1950.

“That carbon’s been basically mixing in with all the carbon in the Earth since then,” explains Fallon.

That Carbon 14 in the atmosphere is absorbed up by plants, which are then eaten by animals. Those animals are then eaten by other animals, and the Carbon-14 ends up in the food chain.

“So we have this distinct curve (of Carbon 14 levels over time), and when we tested the fish we were able to reproduce that curve, and tell when the fish was born.”

The team ended up making around 1200 measurements over several years, finding fish between three years old and 78.

While one fish in captivity has previously lived to around 80, earning the name “Granddad”, it had previously been assumed they survived to around 20-25 years old.

Finding the missing youngsters

Being able to find youngsters is an important discovery – for years scientists have struggled to find any juvenile fish.

For spawnings to produce juvenile fish, conditions need to be ideal for both egg-laying and the growth of eggs and young. The specific mix of conditions is only thought to occur every 5 or so years, but other factors can then come into play.

“I actually don’t know how they’ve survived in Australia for so long. They like to lay their eggs in the shallow parts of the river where there are plants for the eggs to cling onto. Whenever we have big floods it just wipes everything away, so in these time periods we may find there were big floods just beforehand, and then it takes several years for the plants to grow back,” say Fallon.

That raised concerns that the entire population of fish was ageing, and could eventually completely die off. However, the long-life of the Lungfish suggests they may have a better chance of surviving as a species than previously thought.

They did, however, find that there were long periods when no fish were born at all.

“For example in the Mary River in the 1970s and 80s we didn’t see many fish born,” comments Fallon.

With the lungfish a threatened species, understanding the lifecycle and longevity could prove valuable information to understanding the population and their survival, say the researchers.

This article was first published on Australia’s Science Channel, the original news platform of The Royal Institution of Australia.