Forrest Gump asks, “What’s normal anyways?” While he may not be talking about fish, his question rings through all our minds when we think of them. There is truly no such thing as a “normal” fish, especially when it comes to the beautiful Bichirs (family Polypteridae, with two genera of Bichirs underneath that category). Bichirs, however, are an especially interesting type of fish because they show many mechanisms of vertebrate evolution, including walking on land and breathing air. This paper is going to give some background on Bichirs, explain the evolution and physiology of their breathing and mechanisms of movement, and give insight to their ties to tetrapods.
|Saddled Bachir (Polyuterus endlicher) Source|
Bichirs are ray-finned fish in class Actinopterygii, subclass Cladista, family Polypteridae. There are 2 genera and 12 species of Bichirs within this family (Hastings 2014). Bichirs are believed to be a sister group to all other fishes that are in Actinopterygii due to the fact that they share many of the same characteristics (Hastings 2014). While they do have many similarities with the rest of their class, they also have many characteristics that are strange. They possess lungs, a spiracle valve in their intestine, two gular plates, and they have a skeleton that is mostly made of cartilage (Hastings 2014). There has been consideration to put Bichirs in their own separate group among teleosts, but this has not yet occurred (Zaccone et al. 2009). Bichirs are carnivores—their main prey of choice are fish, mollusks, and crustaceans (Hastings 2014).
Of all of the strange characteristics that Bichirs have, there are two that particularly stand out—one of these are the lungs. Lungs were present in the majority of the late Paleozoic fishes, so lung-breathing has not been as uncommon of a method of respiration as most would think (Zaccone et al. 2009). Today, there are some teleost fishes that depend on modified lungs for oxygen uptake when the oxygen conditions in their aquatic environment are poor. The reliance that these fishes have on their air-breathing modifications depends heavily on their adaptive radiation and on the evolutionary capacity the populations have to produce gas bladders (Zaccone et al. 2009). Bichirs are among these fishes. They are referred to as “dual breathers”, so gills and lungs are the two mechanisms they use to take in oxygen (Zaccone et al. 2009). When the oxygen levels in the water are too low, Bichirs resort to taking oxygen in from the air because gills cannot function well in low-oxygen environments (Zaccone et al. 2007).
|Internal organs of the Bichir, Photo by Maija Karala. Source|
A study was done to look at how the evolution of lungs in Bichirs is related to the evolution of lungs in tetrapods. In history, there are many similarities in the development of lungs between Bichirs and tetrapods (Tatsumi 2016). There are three genes that are very important in the early stages of lung development that were found in both Bichirs and tetrapods (Tatsumi 2016). This study also produced results that suggest that one of the genes (the lung enhancer Tbx4) was most likely already existent in the common ancestor of fishes in the classes Actinopterygii and Sarcpterygii (Tatsumi 2016). These findings further provide evidence of the evolutionary connections between tetrapods and Bichirs.
Another strange adaptation of Bichirs is their ability to walk on land. Baker conducted an experiment to see how Bichirs raised on land for eight months would walk in comparison to those who were raised in a usual aquatic environment. He found that the Bichirs that walked on land were more successful at doing so than the ones that had grown up in the water (Hutchinson 2014). They walked much faster and their fins moved much easier across the land than the water-reared group of fish (Hutchinson 2014). Baker also looked at how the skeletal structures in the two groups differed and found that the neck and shoulder bones developed differently so that they could move much more easily on land (Hutchinson 2014). There was surprisingly no difference in the way the two groups swam, which showed that there was no trade-off of traits (Hutchinson 2014). These outcomes suggest the modifications that allowed fins to become limbs 400 million years ago when some fish switched from water to land (Baker 2014). According to Baker (2014), the evolution of Bichirs’ ability to walk on land is thought to give rise to tetrapods, which range anywhere from amphibians to mammals. This experiment was done in order to look at how evolution affected those tetrapods so long ago (Baker 2014).While Bichirs are a quite strange group of fishes, they have significantly helped to explain two very important evolutionary patterns in tetrapods. The comparison of the development of lungs between Bichirs and tetrapods, as well as how Bichirs helped to give rise to tetrapods by their ability to walk on land, has given scientists more insight on the mechanisms that tetrapods possess and how they acquired them. The studies by Baker and Tatsumi help to provide the most evidence of how these amazing fishes have helped to explain the puzzling methods of how tetrapods became.
Baker, N. 2014. How Fish Can Learn to Walk. Nature.
Hastings, P. A., Walker Jr., H.J., Galland, G. R., editors. 2014. Fishes: A Guide to Their Diversity. University of California Press, Oakland, California.
Hutchinson, John. 2014. Evolutionary developmental biology: dynasty of the plastic fish. Nature 513: 37-38.
Tatsumi et al. 2016. Molecular developmental mechanism in polypterid fish provides insight into the origin of vertebrate lungs. Scientific Reports.
Zaccone, et al. 2007. Innervation and Neurotransmitter Localization in the Lung of the Nile bichir Polypterus bichir bichir. The Anatomical Record 290: 1166-1177.
Zaccone et al. 2009. Innervation of lung and heart in the ray-finned fish, bichirs. Acta Histochemica 111, 3: 217-229.