In 1836, European scientists discovered a peculiar animal from the River Amazon that they struggled to identify. Its eel-like body was a few feet long and its air-filled lungs persuaded anatomists it must be a reptile.
A year later, another specimen was found in Africa, and based on the structure of its heart, it was declared an amphibian. Three decades of debate later, scientific consensus settled on the idea that these are fish – only instead of breathing water through gills, they have lungs. Enter the lungfish.
If they don’t have access to the water’s surface to suck in air, West African lungfish (Protopterus annectens) will drown. Their withered gills are too small to provide enough oxygen. But lungfish have another unusual ability to help them survive in their natural environment of swamps and rivers, which seasonally dry up.
Lungfish chew a burrow in the mud and build a chamber filled with mucus in which they curl up in a state of dormancy, not eating or moving until the rains return. In the wild, this can last seven or eight months. Some lungfish have been known to slumber for four years.
Lungfish first evolved more than 400 million years ago. But despite the species’ age, there’s still much that’s mysterious about the creature, including the link between its lungs and another fishy feature: the swim bladder, which most fish use to help them float underwater, as well as to hear and make noises.
Several varieties of fish have lungs, including bichirs and bowfins, but none has both lungs and a swim bladder. One organ could be a version of the other, but which came first? This is proving a difficult question to answer.
In fish embryos, both the swim bladder and lungs develop from a pocket in the gut. Scans showing the way lungs and swim bladders are connected to the blood system in various fish support the theory that lungs are a more ancient organ. They also hint that swim bladders came along later and are, in fact, modified lungs.
Another question that has long puzzled scientists is whether lungfish are the closest living relatives to all the land-dwelling, four-legged animals – that is, all the mammals, birds, reptiles and amphibians.
It’s either lungfish or another group of mysterious fish, the coelacanths, and recent studies are edging towards it being lungfish. They’re certainly more distantly related to bony fish, such as tuna and herring, than they are to terrestrial vertebrates – including us humans.
There are six known species of lungfish alive today, including four from Africa. A female Australian lungfish, known as Methuselah, holds the title of the oldest fish alive in captivity.
She arrived at an aquarium in San Francisco in 1938 and, from studies of her DNA, scientists think she’s at least 93 years old. If she carries on, she may outlive another Australian lungfish, called Grandad, who died in an aquarium in Chicago at the age of 109, give or take a couple of years.
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