Millions of years ago, fish crawled out of the water and onto land. But this first foray onto terra firma didn't immediately result in amphibians, but rather fish that were able to move rather clumsily on land yet still survive. The transition from swimming in water to ‘walking’ on land must be a difficult one for fish, especially if they don't have modifications to their fins or bodies to make it easier. Such is the case for a number of fish species that spend some time out of the water, including the grey bichir (Polypterus senegalus), a long eel-like fish from western Africa. So just what does a fish do when the water gets too shallow for swimming, and they need to start walking? Keegan Lutek and Emily Standen from the University of Ottawa (Canada), along with Kathleen Foster of Ball State University (USA), believe that there must be a continuum of movements and behaviours that happen as the water gets shallower and the fish transition between swimming and walking.
To discover how the bichirs moved during this transitional period, Lutek and colleagues filmed the fish from three angles at various water depths. This technique allowed them to see what the fish were doing in three dimensions. In deeper water, the bichirs used their pectoral fins (located near the head) along with slight undulations of their body to swim. These fins normally act as a pair during swimming, performing a flapping motion and keeping the fish balanced in the water. However, once the water got shallow enough, and the fins were touching the bottom of the tank, the fish started placing one fin on the ground and rolling their bodies to the side while lifting the other fin. While doing this, they rotated the fin that was not touching the ground forward and then repeated this process, essentially using their fins to ‘walk’. The team noted that when the bichirs were in shallower water, the fins moved in a bigger arc than they did while the fish were swimming, suggesting that this would help the fish pull themselves further forward. Also, the fish were bending their body much more than they were while swimming in order to move. But as the water levels drop, the balance between the forces of gravity and buoyancy acting on the bichirs changes, and they become heavier. So, how are they able to walk while having to support more of their body weight?
Lutek and colleagues thought that the answer to this may have something to do with how the muscles are working when the bichirs transition to land. Using tiny electrodes to measure the fish's muscle activity, the team again placed the bichirs in water of various depths. Surprisingly, the pattern of muscle activity in the body was similar regardless of how deep the water was, but the muscle contractions used to make similar movements were stronger while the bichirs were in shallower water or on land. The team suggests that the fish's muscles do this to overcome the additional obstacles of gravity and lack of buoyancy when the water isn't deep enough to support the fish's weight. So while these bichirs may be able to ‘walk’ on land, it's not easy for them to do so, and this difficulty starts long before they've hauled themselves from the water.