Lungfish

How to make it through Corona: inspiration comes from the lungfishiest of places

If there was ever a year when I would have wished I was a lungfish, it must have been 2020. The year of the Coronavirus pandemic. Now hold on a minute, you will say, why would you want to be in the one group of fishes that is actually at risk of contracting a respiratory disease while there is a pandemic going on? Well, let’s dive in and find out why this makes sense. We’ll start with the basics – it is true that lungfish do have lungs. The cue is in the name. And indeed, the lungs of lungfish are yet another incarnation of an organ that was already present in the ancestor of all modern fish and tetrapods (four-limbed animals, including birds, mammals, etc.). This ‘proto-lung’ became the lung of lungfish, the swim bladder of ray-finned fishes, and the lung of tetrapods; evolutionary biologists would say these three types of organs are ‘homologous’. That being said, lungfish lungs are not quite as elaborate as, say, human lungs. But they are more elaborate than simple swim bladders in the sense that they have multiple chambers and allow for sufficient oxygen exchange. The reason for this somewhat intermediate form between lungs and swim bladders is in the fish’s somewhat intermediate lifestyle: they are truly aquatic, generally needing to be in water for survival and reproduction, but they are less dependent on water than other fish. As such, they are very good at dealing with changing environmental conditions. If you are the usual run-of-the-evolutionary-mill type of fish, you are doomed the moment your pond or river dries out. But if you are a lungfish, you shrug your (non-existent) shoulders, dig yourself a nice little burrow and just wait until water returns. This lifestyle is so successful that lungfish have been around since way before dinosaurs roamed this planet. Indeed, they are often referred to as ‘living fossils’ because their morphology hasn’t drastically changed for hundreds of millions of years. Within an individual lungfish’s lifespan, this flexibility with regard to the environment means that they are unlikely to be killed by a random extrinsic event like a draught – and have thus evolved fairly long lives. It is estimated that they can reach ages well above 80 years, partly thanks to their incredible regeneration powers; they are pretty good in re-growing limbs and tissues. Quite the feat for a fish that does not grow much larger than 1.5m (about 5 foot). It probably helps that at least some of them can switch between breathing with their lungs or their gills, as needed. Another interesting fact about them is that lungfish have among the largest genomes (i.e. the most DNA per cell) of all vertebrates. But where this fits into all of their other cool biology is not quite clear yet. Currently, there live six species of lungfish. One each in Australia and South America and a total of four in Africa. This geographic distribution also roughly corresponds to their evolutionary relatedness: the Australian species is least closely related to the other five, and the four African species are more closely related to one another than to the non-African lungfish.


Now in all of these factoids, where is there anything that seems desirable during a global pandemic? The ability to not use lungs and simply breathe with gills? Sure, that helps with not having to wear a face mask. But technically any fish could do that. And most lungfish cannot sustain themselves for prolonged periods by gill respiration alone. No, it is that burrowing behaviour lungfish show. I find that quite enviable right now. Here is what the lungfish do (with the exception of the Australian species that appears not to have evolved this behaviour as they live in fairly permanently wet habitats): as their pond or stream dries out, they start digging into the mud. I wouldn’t say that they look particularly dignified or skilled during that act. But then again, who am I to criticise a behaviour that is orders of magnitude older than our own species? The lungfish basically use their whole body in that act; luckily they are pretty eel- or snake-like in their appearance, greatly helping their style of burrowing. They wriggle and twirl until they are at their desired depth of approximately 50cm (20 inches) underground. Here, they perform some more wriggling and twirling until they have made a nice, comfortable cave for themselves. And then the fun starts! They secrete mucus. Upon mucus. Upon mucus. Until their whole hole is laced with it. And then that mucus dries out and becomes a fish-made layer of moisture insulation for the lungfish’s new home. This way, evaporation from the burrow is minimised and the fish stays nice and wet as its surroundings dry out. That is what I would call efficient wallpaper hanging. Interior design, the lungfish way. Even cheaper than Ikea. And less harmful to the environment. Or society. With its mucus-walled cave in fine order, the lungfish begins to… do nothing, but with great gusto. It reduces its metabolism, it changes how it secretes organic waste, it starts digesting its own muscle (quite literally eating itself), and it just sits. And waits. For rain. So if I could mimic this, just waiting this bloody pandemic out until things are back to ‘normal’, that would be great. But any time I start collecting mucus to lace our walls, I am threatened with homelessness. Oh how cruel to be a human and not a lungfish in 2020/21!

Further reading

1) Check out this short article about the most famous of Lungfishes, Granddad from Chicago: https://www.southernfriedscience.com/conserving-chicagos-lungfish-legacy/

2) A nice little review about lungfish, their role in evolution, and whether they are ‘living fossils’ or not: Lee J, Alrubaian J & Dores RM: Are lungfish living fossils? Observation on the evolution of the opioid/orphanin gene family. General and Comparative Endocrinology, Volume 148, Issue 3, Pages 306-314

3) Watch an overly dramatic short documentary on African Lungfish here: https://www.youtube.com/watch?v=vGj3XJjPIuU and watch people digging out lungfish here: https://www.youtube.com/watch?v=KuEs5zYNHec&