Two weeks ago I wrote about flying frogs, which I noted are two words that don’t often go together. Well, today’s animal is another one of those, because really, who hears ‘fish’ and thinks ‘lungs’? But that’s one of the things that makes lungfish so interesting!

The Australian, or Queensland, lungfish which in many ways is quite different from the other five extant species of lungfish. CC BY-SA 2.5, via Wikimedia Commons

There are six species of lungfish, four of which are found in Africa, one in South America, and one in Australia. All species of lungfish are freshwater fish, preferring slow-moving or still waters, such as swamps, wetlands, or river deltas. Lungfish are demersal, meaning they live at the muddy bottoms of their preferred habitats. 

The largest of the six species of lungfish is the marbled lungfish, at lengths of up to 200cm, which is nothing to sneeze at. The honour of being the smallest lungfish goes to the gilled lungfish, which reaches a paltry 44cm in length. Lungfishes look a bit like eels, with elongated bodies and a long caudal, or tail fin.

Most lungfish aren’t particularly flashy, though the marbled lungfish does look pretty cool with its namesake marbled patterns. Other species of lungfish are various shades of brown, grey or olive colour. Such colouration makes sense when you consider the muddy waters that lungfish prefer. 

Marbled lungfish look quite pretty… until you see their faces. Image by OpenCage, CC BY 2.5, via Wikimedia Commons

Of course, the most interesting thing about lungfish are their lungs. Other air-breathing fish accomplish this impressive feat by using modified gas bladders, which are fairly simple. But lungfish take this process a step further, with lungs that have small air sacs, creating a larger surface area for the exchange of oxygen. All but one species of lungfish have two lungs (the Australian lungfish being the exception, because all Australian wildlife needs to be different in some way). 

Almost all species of lungfish are obligate air breathers, meaning they have to breathe air to survive. The Australian lungfish is again the exception here, as they can breathe both using their lungs and using their gills. There’s a whole slew of complicated things that have to happen in the Australian lungfish’s circulatory system to allow the fish to switch between breathing air and breathing water, which I won’t go into because it’s quite technical. Who knew having two methods of breathing would be so complex?

You might be wondering why lungfish bother breathing air at all. After all, most fish are quite content with their watery respiration. But waterways are not static, and droughts or dry seasons can almost completely dry up some rivers or swamps. Lungfishes’ ability to breathe air means they are well adapted to changing water levels: they can survive for up to a year outside of water! 

A closeup of a marbled lungfish’s face. Not the most attractive fish. Image by George Berninger Jr., CC BY-SA 4.0, via Wikimedia Commons

Breathing air isn’t the only strategy lungfish have for surviving dry seasons. When water levels are low, they wiggle their way into the mud, and enter a state called estivation. This is basically a hibernation-like state, where the fish slow their metabolic rate up to one sixtieth of its normal rate. They also stop excreting protein waste as ammonia, which is toxic when it can’t be secreted directly into water, and start converting it to urea (which is how humans do it). Of course, I should mention that the black sheep of the lungfish order, the Australian lungfish, can only survive a few days out of water. 

One final interesting point about the marbled lungfish: this odd and inconspicuous looking fish has one of the largest genomes in the world. There are only two known organisms that have larger genomes than the marbled lungfish, an ameboid and a species of plant. For reference, the human genome has 3 billion base pairs. The genome of the marbled lungfish has 133 billion base pairs. So it’s just a wee bit bigger. 

Though lungfish don’t appear to be particularly exciting fish, they’ve got a lot to be proud of! I figured fish with ‘lung’ in their name would be pretty cool, but lungfish are even more interesting than I thought. 

Cover image by Mathae, CC BY-SA 3.0, via Wikimedia Commons, cropped to fit