By Piotr Naskrecki - About 400 million years ago, in the Devonian, in what was likely a shallow, freshwater pond in some tropical part of the world, a fish made a sound that started a dramatic chain of events, one that culminated in you and me being born. The sound was that of air being sucked in, as the fish lifted its mouth above the surface of the water, desperate to replenish falling oxygen levels in its bloodstream. Soon, gulping for fresh air became a necessity, as the gills failed to supply enough oxygen from the warm, muddy waters to sustain the animal’s activity. Its bladder, which up to that point helped maintain buoyancy for swimming, started to function as a gas exchange organ, an early version of lungs. At the same time, the fish’s pelvic and pectoral fins grew sturdier, and their connection to the rest of the skeleton more capable of lifting the body above the substrate. With these two steps the stage for the vertebrate conquest of land was set. We probably will never know when and where exactly this momentous transition took place, but we are pretty sure of what that fish looked like. And I am staring at it right now, as it looks back at me from the bottom of my beer cooler.
The sound of an early ancestor of this lungfish (Protopterus annectens) taking it first gulp of air signified a pivotal moment in the history of life on Earth. The emergence of this behavior, along with the development of four limbs, set the stage for the conquest of terrestrial habitats by vertebrates, and the evolution of all tetrapods.
Ever since I first set foot in Gorongosa National Park in Mozambique I have been obsessed with meeting my oldest living vertebrate cousin, the lungfish (Protopterus annectens). Despite its unassuming physique, reminiscent of a large eel, the lungfish holds a special place in the history of life on Earth. Biologists have always suspected that the lungfish gave us insight into the origin of terrestrial vertebrates, but only last year a massive molecular phylogenetic analysis , based on 251 (!) genes, proved that the lungfish is a sister taxon to all tetrapods – amphibians, reptiles, birds, and you, and the rest of mammals. Previously that place was reserved for the coelacanth, a marine fish of equally ancient provenance, but now it appears that the coelacanth is an earlier offshoot of fishes that went its own, equally interesting, if less pivotal, way.
The Southern African lungfish (Protopterus annectens)
My first encounter with the lungfish took place a couple of months ago, when I ran across a fisherman who had caught a couple of these animals. Alas, by the time I met him they had been killed and gutted, leaving me heartbroken and even more obsessed. This time I made it abundantly clear to anybody who would listen that I wanted a live lungfish, and last week a fisherman from the village of Vinho finally delivered one into my hands.
In Gorongosa National Park lungfish are common, if rarely seen, inhabitants of seasonal water pans. During the dry season, when the pans evaporate, the lungfish burry themselves in the mud and estivate for several months. During this period their metabolic rates drop by about 60% and gas exchange is done entirely through their lungs.
It is difficult to describe the nearly religious reverence I felt when I saw my first live lungfish. Here was an animal that, I am pretty sure, looked like something I would see in the ponds of the early Paleozoic, long before first amphibians, even longer before dinosaurs. As I watched the lungfish slowly moving in my cooler, it suddenly lifted its head above the water and loudly inhaled a big gulp of air. I don’t think I will ever be able to forget the sound of it.
The lungfish does not have fins like other fishes. Rather, it has two pairs of whip-like appendages that act as weak, but very much functional legs. In 2011 an interesting experimental study demonstrated that the pelvic fins are used by the lungfish in a fashion very similar to that of the land animals’ hind legs, for both walking and bounding. This in turn casts a new light on some early Devonian fossil tracks that were thought to have been left by primitive amphibians – in fact, they probably are those of semi-terrestrial lungfish ancestors. The development of articulated limbs with fingers (digited limbs) no longer seems to be the prerequisite to the conquest of land.
If you think that this looks like walking that’s because it is. Lungfish use their pelvic fins in a way very similar to that of a tetrapod’s legs – the distal part of the fin becomes a “foot” and the fins produce both walking and bounding motions.
The more we study the lungfish the more fascinating it becomes. It is now clear that this animal holds the secret to the development of tetrapod ears, and they were the first to develop enameled teeth, the kind we, mammals now have. The lungfish is also superbly adapted to the harsh seasons of the southern African savanna, and can burry itself in the ground and survive for months out of water, hidden from the hot sun in a muddy cocoon (stories abound about African farmers digging out live, large fish from their dry fields.) Its dependence on atmospheric oxygen is so strong that the lungfish will drown if not allowed to breathe above the surface of the water. Its strongly reduced gills are virtually non-functional, but during the lungfish’s larval development are external and feathery, resembling those of the salamander larvae.
A portrait of the Southern African lungfish (Protopterus annectens) from Gorongosa.
In Gorongosa lungfish are common in seasonal pans and rivers of the park, feeding on a wide range of aquatic invertebrates, smaller fish, and frogs. As they are highly territorial, it is likely that almost every body of water on the plains has at least one of these remarkable animal. It is a shame that they are never seen by the visitors to the park – in my opinion the lungfish surpasses in its importance and a fantastical set of features any other vertebrate of Gorongosa, lions and elephants included, and I vow to make it my mission to spread the knowledge of its existence.
Last week, on March 27th, the E.O. Wilson Biodiversity Laboratory was officially opened, a culmination of my and many other people’s dream. More about the Laboratory soon, but I thought it fitting that the first vertebrate sample for our synoptic collection is a small snippet of the lungfish’s caudal fin, preserved for future DNA barcoding. The animal itself is being released back into its habitat, and I hope that this was not the last time that I looked into the eyes of my grand…grandfather’s twin brother.
Resembling an oversized salamander, the lungfish has four distinct limbs, a long tail, and only remnants of gills. The gills are virtually non-functional and the fish will drown if not allowed to breathe above the surface of the water.
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