That is correct. You read the title correctly: fish can walk. Of course, not all kinds of fish can put on a strut. This special distinction belongs to a fish called the bichir or reedfish, of the family Polypteridae (see photo). This particular type of fish is very old, having originated during the early Cretaceous period. That makes this species over 100 million years old with live specimens today found only in tropical areas of Africa and the Nile River.
So how exactly does this critter walk? The species has evolved very strong pectoral ray fins on either side of its upper body that actually can touch the ground, allowing it to exert significant force on surfaces beneath. This is very different from virtually all other types of fish, which do not have pectoral fins that can reach below the belly. The bichir also has a much flatter belly than other fishes, allowing it to rest on the belly with eyes up. These unique physical adaptations are specifically designed to allow the bichir to move in and out and across the surfaces of shallow water, particularly when pursuing its natural food source: small vertebrates, crustaceans, and insects.
An equally impressive anatomical feature is found in the fact that bichirs possess paired lungs that connect to their esophagus. These lungs make the fish natural air-breathers, another important feature that facilitates their ability to pursue pray outside of water and also within shallow pools, especially because shallow standing pools of water tend to have very poor oxygen supplies, unsustainable for fish.
For those of you familiar with biology, you might not be so surprised with this specimen. No doubt you have read textbooks describing how simple reptiles and amphibians most likely evolved from fish. In fact, when you compare the anatomical structures of early amphibians (think salamanders) and reptiles (think lizards) with bichirs, the physical similarities are quite striking and obvious. For example, the legs sprouting out of either side of a lizard or salamander, placed high above the belly look exactly like the position of the pectoral fins on a bichir. So in many ways, the bichir is living proof of the close evolutionary relationship between fish and early land vertebrates.
So now let's talk about the “running”. Dr. Emily Standen at McGill University did a very interesting experiment. Since she knew the bichir could live with very little water, she was curious to see how the fish’s walking behavior would change if it was raised on a pure land environment where the fish didn’t have enough water to swim and had to get all its food by walking. Until then, all bichirs raised in captivity had been housed in aquariums where swimming was still the primary mode of locomotion. To conduct the experiment, Dr. Standen raised young bichir fish from a very young age in a flat, controlled environment with only a few milliliters of water on the floor to keep the fish moist. The fish were then observed over the course of many months to see how their walking behavior differed from the behavior of bichir siblings who lived in swimming environments.
A number of very interesting differences were observed. First, the land-raised bichirs lifted their heads higher and held their fins closer to their bodies when moving. This raising of the head reduces friction against the ground while the closer fins allow more force to be applied to the ground with each stroke. The land-raised fish also took faster steps than their water-raised siblings, very similar to a running motion in lizards. Undulation of the land-raised tails was also less frequent. This undulation is important because it shifts weight in a way that forces the fins to work harder when moving forward. Think about shaking your butt while walking: the more you shake, the harder it is to walk. The land-based fish even developed slightly different skeletal and muscular structures, emphasizing more frequent use of muscles in the upper body.
So all in all, even though you won’t catch bichirs running in next summer’s Olympic Games, it is still very striking to see how the fish naturally learned how to use its body in a significantly different way by responding to a new environment. This speaks volumes about our underappreciation of other animals. Think twice the next time you say, “as dumb as a fish.” That fish might be able to learn how to run.
Huang Xu / Justin Fendos
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