The Lungfish: A Master Of Evolution

The lungfish is a master of evolution: it is a fish that can breath air, it has fins and a tail but can walk, it eats just about anything it can fit into its mouth, and it is thought to share a common ancestor with tetrapods, the first animals to crawl up onto land. A prehistoric manifestation in modern times, some species of lungfish have existed unchanged for over a hundred million years. The lungfish is Vagabondjourney.com’s animal of the week.

What the heck is a lungfish?

The Lungfish looks like the result of an orgy involving an eel, a carp, and a salamander. Having a long, tubular body, a dorsal fin, a fish-like tail, four flippers that look like a cross between fins and legs, and a large, lateral mouth, the lungfish is not pretty to look at, but it is like no other animal on the planet. Being able to grow up to six feet in length and weigh 25 pounds, this aquatic monstrosity is by no means small.

There are currently six know species of lungfish, each named after its respective geographic location in South America, Australia, and Africa (which boasts four of the species). Though it is thought that lungfish once spanned the globe, their current range has been dramatically decreased and many species have gone extinct throughout the interplay of eons.

As they are mostly air breathing fish, lungfish prefer shallow aquatic areas, generally living in lakes, streams, ponds, or swamps. This enables them to easily rise to the surface for air. As we will find later, the Australian lungfish is the only species that can still use its gills. Lungfish breath through their nostrils, and can such in air even when the mouth is closed, which is a clutch adaption when water levels in their habitat get low.

The earliest lungfish known to science lived during the Devonian period (416 to 360 mya), and it is now proposed that many of the fossilized “foot prints” that scientists once attributed to tetrapods — the first terrestrial animals — may actually have been done by ancient lungfish. A 100 million year old fossil has also been found of the Queensland lungfish: it looked then exactly as it does today. For reference, the dinosaurs went extinct 65 million years ago. The lungfish is truly an ancient animal.

The Intriguing Attributes of the Lungfish

Lungfish can breathe air

True to its name, the lungfish has lungs, and breathes air. Lungfish and tetrapods (the first animals to live on dry land)  inherited their lungs and air breathing ability from a common ancestor. Although most lungfish have gills, only the Australian species can still use them. In the South American and African species of lungfish, the gills have atrophied into virtually useless body parts. Typically, lungfish breath air through their nostrils and into their swim bladders — a lung like apparatus that some other species of fish (such as the tarpon) share — which is used for both respiration and buoyancy. As the nomenclature tells us, these swim bladders are more developed in the lungfish than in the other fish, and have multiple air sacs which maximizes oxygen intake. If, for some reason, a lungfish could not reach the surface to respire all but the Australian species would drown.

The Australian lungfish, also called the Queensland lungfish, is truly a super adaptive animal worthy of further mention. While it has only one lung (other lungfish have two) and can still breath air, but it mostly respires through its gills — only coming up to the surface to breath after periods of heightened physical exertion or when the oxygen quantity of the water is low. When it is obtaining oxygen from its gills, its circulatory system is configured like a common fish. The Queensland lungfish is among the only species on earth that can successfully extract oxygen from both air and water over an extended period of time, and it is perhaps this adaptability that has given it the competitive advantage to survive through the ages.

Lungfish can walk

“If you just looked at the skeleton of the lungfish, you would think it’s impossible for it to walk,” University of Chicago’s Heather King pointed out a key differentiation point between paleontological study and observing live animals.

King was inspired to do a deep study of lungfish locomotion after observing one that her college kept in a tank as a pet.

Though Lungfish are considered to be related to the first creatures to colonize land, their scrawny, weak looking fins have lead scientists to believe that the ability to walk must have evolved later in terrestrial quadrupeds. That is until Heather King released her findings this month in the Proceedings of the National Academy of Sciences.

Setting up video cameras pointed at her collegue’s African lungfish from the side and below, King determined that lungfish can in fact walk on its rear limbs while displacing the weight of the rest of its body in the water. Even though the forelimbs look very similar to the pelvic fins, they are not used while “walking,” as the lungfish propels itself forward in both bounding (where both limbs move together) and alternating (where one let moves before the other) motions.

Vagabondjourney.com asked Heather King why she thinks the lungfish may have evolved the ability to “walk,” and what advantages this gave them.

“To answer your question,” King began, “I do have some ideas about the advantages of underwater walking, at least in lungfish. Using the limbs rather than the entire body axis (as many fishes do) for movement would require much less energy, since the muscles in the limbs are much smaller than the muscles in the body axis. Using little energy might be important for an animal like the lungfish, which has a slow metabolism and probably doesn’t move much compared to many other fishes. In addition, using the limbs instead of the entire body would result in less perturbation of the water, which might be helpful for both prey capture and predator avoidance – this ability might help the lungfish move more ‘quietly’ underwater, especially since lungfish sometimes live in swampy areas where visibility is low (sensing water movement would then be more important than using the eyes to sense the environment).”

“In terms of why walking might have originally evolved,” King continued, “hypotheses have typically centered around a need for moving on land, potentially because of rivers drying up or changing course and leaving animals stranded, or changing environmental conditions resulting in oxygen-poor water. However, our study suggests that walking may have evolved underwater first. This might not be that surprising because the habitat that tetrapods evolved in (370 million years ago) might have been similar to the habitat that the lungfish we studied lives in. It’s possible that some of the same factors influenced the evolution of walking behaviors way back in the Devonian, but this is a difficult hypothesis to test!”

What is known for sure is that lungfish are one of a few bony fish that can move their fins similar to how land animals control their limbs. While most fish are only able to raise or lower their fins, the lungfish has appendages attached to its body similar to how arms and legs are attached to that of a human.

Lungfish shed light on the colonization of land by sea animals

An African lungfish

Ancestors of the lungfish are thought to have been the animals that first bridged the gap between sea and land 370 million years ago. With King’s findings that the ability to “walk” may have first developed underwater along with University of Oregon’s Gregory J. Retallack’s theory that primitive terrestrial mobility was not a response to a disaster scenario where seas and ponds dried up, a very different picture is emerging over how animals first colonized land. It seems as if the ability to walk was first used in low water habitats, and that the first tetrapods opportunistically applied their aquatic-borne ability to breathe air and walk towards slowly entering into a terrestrial environment.

At the heart of this investigation lies the lungfish, who, like early tetrapods inhabit low water level environments and, as King recently proved, can lift up its body and walk along a sub-aquatic surface. In point, the lungfish is a window not only back upon ancient life but also upon the initial colonization of land.

Lungfish are not picky eaters

Lungfish eat just about any type of food they can fit into their mouths. This includes plants, insects, other fish, reptiles, amphibians, insects, crustaceans, worms, mollusks, and, if they get the chance, the fingers of humans. The lungfish eats by sucking prey into their mouths and crushing it between their jaws, chewing thoroughly before swallowing — a truly unique way for a fish to eat, which further links it with cud and meat chewing terrestrial animals.

Lungfish can hibernate out of water

Marbled lungfish

Another intriguing attribute of the lungfish is that they can hibernate out of water. While the Queensland lungfish can live for a few days out of water if it stays moist, the African and South American lungfish can live on standby for years at a time.

During times of drought or through dry seasons when their typically low water level habitats my dry up, the African and South American lungfish burrow into the mud and go into hibernation mode. During this time, their metabolisms drop to 1/60th its normal rate.

While the South American lungfish just builds a nest in the mud, its African cousins go even further by secreting a mucus layer over its entire body except its nostrils which will dry into a leathery cocoon, essentially sealing in essential moisture. The hibernating lungfish will thus wait in their mud burrows until the water returns, and it has been reported that they can live like this for up to two years.

Lungfish can live for a very long time

There is a Queensland lungfish in the Shedd Aquarium in Chicago that has been on exhibit since 1933 — nearly 79 years!

Lungfish conclusion

Having survived for hundreds of millions of years through mass-extinctions, climatic changes, and the breakup and global movement of continents, as well as serving as a window into the first terrestrial animals, and being an all round, super adaptive survivor, the lungfish is Vagabondjourney.com’s animal of the week.

Australian lungfish

Travel is an exercise in learning about the world we live in. The Animal of the Week series on EnviroChronicle.com shoots for this target by doing interviews with experts, sharing our own observations, and conducting research on some of the most intriguing, little known, or downright strange animals we share this planet with. EnviroChronicle.com would like to extend a gracious gesture of thanks to Heather King for sharing with us her perspective shaking insights into the world of the lungfish.