Africa is, among other things, the land of weird fish. It has a strange combination of modern and ancient lineages and extremely odd ecologies. There are fish with lungs, fish that live underground, blind fish that look like cavefish but live in rivers instead. There are fish swimming upside down, fish flying in the air and fish singing love songs in electric pulses.
And then there are bichirs.
The twelve or so species of bichirs form the sister group of all other ray-finned fishes. In other words, they are the oldest still living branch of the lineage leading to modern bony fish.
Bichirs have retained many characteristcs of the common ancestors of ray-finned fish, lungfish and tetrapods, maybe the most obvious being functional lungs. That is not to say they are primitive. Their 400 or so million years of independent evolution has produced some quite unique oddities, like a row of dragonlike dorsal finlets and scales that are so hard they have been studied as a model for lightweight body armors.
While the 11 species in genus Polypterus are closely related and rather similar to each other, the reedfish (Erpetoichthys calabaricus) diverged from the others when the dinosaurs were still alive, and looks more than anything else like a little smiling snake with elephant ears.
Along with lungfish (obviously), bichirs are among the few fishes that actually have lungs. Many fishes, especially in oxygen-poor tropical waters, can breathe air, but most of them use some alternative organ for breathing. For example, labyrinth fish such as bettas and gouramis have a complex chamber called the labyrinth organ in their heads, in which they gulp air to breathe. Many catfish swallow air into special pockets in their digestive tracts and belch it out when they are done with it.
Bichirs, however, have real lungs, which they seem to have inherited from the same air-breathing common ancestor that gave tetrapods our lungs. Interestingly, swim bladders of most bony fish are apparently modified lungs, not the other way around. It could be said that in this regard, humans are more primitive than, say, minnows. And bichirs are about as primitive as we are. Primitive and advanced are tricky words.
Bichir lungs are simple bags of air compared to the complex tube systems in tetrapod lungs, but their basic structure is practically identical. They are also proportionally pretty large. Their gills, in turn, are rather rudimentary and act mostly as complementary breathing organs. These fishes will actually drown of they are denied access to the surface.
Sorry for the gruesome images, but I think someone might find these photographs of the lungs of my recently perished female Senegal bichir (Polypterus senegalus) informative.
Interestingly, bichirs do not breathe air solely through their mouths, but also have a spiracle on both sides of their heads, though it’s hidden under their gill covers. Spiracles are openings that lead into the mouth, and though bichirs apparently mostly use them to exhale, at least one of the specimens I have kept also takes air in through them without having to raise its mouth over the surface. Both reedfish gulp air using their mouths, however.
The story behind spiracles is an interesting one. The first gilled fishes had a series of gill slits opening on the body surface, each supported by a gill rod, much like lampreys and sharks still have today. When jaws evolved from the first gill rod, the associated gill opening also migrated and found a new use as a spiracle. In addition to bichirs, most sharks and rayfish, coelacanths and sturgeons still have spiracles. It has also been suggested that the ear opening of tetrapods might be a highly modified spiracle.
At least one species of bichir, the snake-like reedfish (Erpetoichthys calabaricus) is amphibious, which means it spends a proportion of its life on dry ground, where it slithers around much like a snake. It’s an useful skill in reedy lowland habitats where water level varies. Unfortunately, I haven’t been able to find any information whether the reedfish also forages on dry land, as some amphibious catfish do, or if it only uses this skill to move from one body of water to another. I wouldn’t be surprised if other species of bichirs moved around on dry land as well. They have strong, muscular pectoral fins that resemble the fins of coelacanths or the Australian lungfish. They use these almost leg-like appendages to stand up looking around on the bottom, or “walk” through vegetation. It’s not as unusual as it sounds: there are also anglerfish that walk long distances with a disturbingly tetrapod-like gait. Bichir hatchlings are not as well equipped when it comes to lungs. Newly-hatched fish are actually larvae and go through a metamorphosis before becoming simply smaller copies of their parents. In most freshwater fishes, the transition is subtle, but some marine fish, such as eels, sunfish and flabby whalefish have larvae that look so different they could be easily mistaken for completely different species, and in many cases, have been classified as such for decades.
In bichir larvae, lungs are not yet fully developed. Instead, they have a neat little pair of external gills. These gills are also a relict shared with lungfish and larval salamanders. When coupled with an unusually flexible neck and muscular, leg-like fins, larval bichirs look extraordinarily similar to axolotls. In the sketch above are presented, from top to bottom, the larvae of a bichir, a lungfish and a salamander. In the wild, bichirs are opportunistic hunters with a row of formidable, shark-like teeth and very strong jaws. Depending on the species, most of their diet might be composed of fish or aquatic invertebrates. One researcher studying the stomach contents of wild Senegal bichirs actually found a bird foot. The fearsome image is completed by a crocodile-like death roll the bichir performs to rip apart a food item too big to swallow whole. My finger, for example.
As their home rivers are often turbid and they forage partly during nighttime, bichirs have a rather dull eyesight, though they do have tapetum lucidum, a reflective layer in their eyes that gives them an edge when navigating in dim light. It also means they have a demonic red eyeshine when viewed in the right lighting.
Bichirs have a set of interesting senses that more than compensate their bad eyesight. They have a good sense of smell with tube-like nostrils that look somewhat like tentacles. The tubes can be jiggled around to better pinpoint the direction interesting smells are coming from. Like most fishes, they have a sensitive lateral line organ, which detects tiny changes in water pressure, such as those caused by another fish swimming by. The lateral line system can be thought as a fish-long ear, or maybe a biological radar. Even more interesting are the little crater-like organs scattered over the bichir’s face and jaw. They are ampullae of Lorenzini, same kind of electroreceptive organs sharks have on their snouts. Ampullae of Lorenzini are used to detect weak electric fields the muscles and nervous systems of all living creatures produce. The ability to sense bioelectricity produced by other animals is called passive electroreception.
There is also some evidence that bichirs may have even more sophisticated way of sensing electricity, known as active electrolocation. That is, they have specialized tissues that produce a weak electric field around the fish. The fish can then sense the changes in that electric field caused by other animals as well as inanimate objects that have a different conductivity from the surrounding water, such as stones and driftwood. Active electrolocation of bichirs is probably much more rudimentary than that of the really specialized electric fishes, such as elephantnoses and knifefishes, but still it’s an extraordinary way to sense the surrounding world.
References and further reading:
Near et al. 2012: Resolution of ray-finned fish phylogeny and timing of diversification. PNAS.
Divergence time estimates are from TimeTree.org
Liem et al. (2001) Functional Anatomy of the Vertebrates: an Evolutionary Perspective. Thomson Learning, Inc.
Bruet et al. 2008: Materials design principles of ancient fish armour. Nature Materials.
Lechleuthner et al. 1989: Lungs of Polypterus and Erpetoichthys. Journal of Morphology.
A. M. Abdel Magid 1967: Respiration of air by the primitive fish Polypterus senegalus. Nature.
Pace & Gibb 2011: Locomotor behavior across an environmental transition in the ropefish, Erpetoichthys calabaricus. Journal of Experimental Biology.
Hederick Dankwa: Biology of Polypterus senegalus (Pisces: Polypteridae) in the Pru River, Ghana. INCO Publications (number four here).