Fish species snake their way on land

By John Whiteman, Department of Zoology & Physiology, University of Wyoming

As the tropical sun sets over central Africa, a fish wriggles from the edge of a shallow body of water, moving onto a muddy surface speckled with rocks. Leaving the aquatic environment, it becomes pinned to the ground by gravity and now faces the paramount challenge of finding a way to keep moving.

This scene unfolded 350 million years ago when lobe-finned fishes first made their way onto land – but is also common today. It is estimated that at least 100 living species of fish can breathe air via lungs or similar specialized organs, or exchange gases through the skin. Individuals of many air-breathing species make frequent trips onto land, hunting or seeking new bodies of water. But traveling through a terrestrial environment is much different than traveling through water; how can one animal successfully navigate both? One solution relies on snake-like body shapes and movements, and may allow one fish species to out-maneuver others on land.

To accommodate the difference between water and land, many fish species switch between types of locomotion. The ropefish (Erpetoichthys calabaricus) is an elongate, eel-like fish that abandons drying pools during droughts, crossing land to find new aquatic habitat. In water, ropefish tend to beat their tails side-to-side for propulsion; however, once on land, they move more like terrestrial snakes. Instead of holding the anterior region of the body relatively still while the posterior region moves (the “tail-beating” motion of swimming), the entire body moves in lateral undulations. The body slithers through a curve; if you placed dots at regular intervals along the dorsal surface and watched the fish move, each dot would closely follow the preceding dot.

Although ropefish and many other species move like snakes on land, they lack a snake’s unique scales, which can snag on tiny irregularities of the ground surface and provide friction to assist in movement. Instead, when using lateral undulations to weave through a terrestrial habitat, these fishes must push off of rocks, sticks, or other large bumps in the ground, rather than relying on friction alone.  

This ability to push off therefore determines how well a fish maneuvers on land. Robert Aluck, a M.Sc. student at Adelphi University (Long Island, New York), and his advisor, Dr. Andrea Ward, suspected that the long, narrow body shape of ropefish gives them a critical advantage over other fish in pushing off objects. To test this idea, Aluck and Ward compared terrestrial locomotion of ropefish to two other species with different body shapes. Gymnallabes typus, a catfish which frequently ventures onto land to hunt small invertebrates, is elongate as well. However, G. typus has a taller and thinner (more laterally-compressed) tail – in other words, a body shape more like a typical fish. The third species, Polypterus senegalus, is slightly shorter and shaped more “fish-like” as well. Aluck and Ward predicted that the body shape of the latter two species would not be as effective for pressing against objects.

Aluck and Ward filmed the fish from above with a high-definition camera, as they moved through an artificial habitat. At first, the habitat was simply a sheet of plain plexiglass, with no objects for pushing off. As expected, Aluck observed that the fish had little success. “It was pretty hectic,” he noted. “There was a lot of whipping back and forth, as if they were trying to reach some kind of contact point to push off.” But the fishes did not flop and jump like trout in the bottom of a fishing boat – they maintained lateral undulations and simply could not travel without contact points.

However, after attaching objects for the fishes to push on – a grid of small wooden pegs, spaced 3 cm apart – they had no problems navigating. Filming revealed that as individuals of the shortest species (P. senegalus) passed a peg, about 45% of the total body length would come into contact with it. In contrast, as ropefish and the other elongate species (G. typus) passed a peg, about 70–80% of their body would come into contact. In addition, the more elongate species managed to stay in contact with the pegs for longer, with ropefish holding contact for about a second longer than G. typus and more than five times longer than P. senegalus. This suggests that ropefish, with their long and narrow body shape, were best able to take advantage of objects in the terrestrial environment for pushing off.

Aluck and Ward next proposed that ropefish not only push off for longer, but are more efficient at doing so. To one peg, they added gauges which recorded the force exerted by a passing fish, and found no difference between ropefish and G. typus as they pushed off. However, if ropefish are more efficient, they may be able to produce that force with less muscle activity. Aluck is currently constructing electrodes the width of a human hair and implanting them into muscles at regular intervals along the length of each fish’s body. As a fish moves through the habitat and pushes off pegs, the electrodes transmit the electrical activity controlling the muscles to a recorder, providing a measure of how hard the fishes are working.

If ropefish are more efficient, they may be simply better suited for terrestrial locomotion than the other species. This could provide an important advantage in portions of their native range in Africa, where water bodies periodically dry up and force the fish to strike out across land for new habitat.

John Whiteman is working on his PhD in physiological ecology at the University of Wyoming, studying how polar bears adjust their physiology during summer months. He has a keen interest in the philosophy and communication of science, within the scientific community and beyond.