A fish out of water — walks and morphs

‘Walking’ may point to changes ancient fish took in their evolution to life on land

This fish, a Senegal bichirs can walk — and breathe on land. If forced to grow up on land, its body develops differently, closer to that of land animals, not fish. 

Antoine Morin

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Scientists have just forced some fish to grow up on land. That experience really changed these animals. And how the animals adapted hints at the way their prehistoric ancestors might have made their big move out of the sea.

The scientists worked with the Senegal bichir (Polypterus senegalus). Normally it swims in African rivers. But this elongated fish has both gills and lungs, so it can live on land if it has to. And that’s what Emily Standen forced her bichirs to do for much of their youth.

While working at McGill University in Montreal, Canada, she created tanks with a special floor. These tanks let only a few millimeters of water seep across their bottoms, where the fish would move. Grocery-store produce aisles provided additional inspiration for her tanks’ design. (“We need misters, lettuce misters!” she realized.) Then, for eight months, those tanks housed crowds of young fish, each roughly 7- to 8-centimeters (2.8 to 3.1 inches) long. And the bichirs took to these land homes well, moving around actively, she says.

Having too little water to swim, these animals used their fins and tails to dart around, looking for food. Scientists refer to these movements as walking.

A Senegal bichir wriggles forward on land, shown at its actual brisk speed.

E.M. Standen and T.Y. Du

As the walkers matured, certain bones in their heads and shoulder regions began developing differently than in bichirs that grew up swimming. The skeletal changes matched what scientists had predicted for animals beginning to transition to life on land, says Standen. (This biologist now works at the University of Ottawa, in Canada.)

Land-reared fish also moved in ways that appear more efficient than water-reared bichirs that they forced as adults to walk, Standen and her colleagues note. They described their findings online August 27 in Nature.

Young fish forced to walk, not swim, developed a sturdier build. The clavicle bone in their chests also was more strongly attached to the bone next to it (in the shoulder area). Such changes mark a step toward a skeleton that could bear weight instead of relying on water to support the animal. The gill area enlarged a little and bone connections loosened slightly at the back of the head. Both represent small steps toward a flexible neck. (Fish in water can dart stiff-necked at food from above, below, or elsewhere. But a bendy neck would help for feeding on land.)

Bichirs who grew up on land had less drag when they walked. These landlings kept their front-stepping fin close to their bodies. Using that fin almost like a crutch, this gave them a little extra height when their “shoulders” rose upward and forward. Because that close-in fin temporarily hoisted more of the fish’s body into the air, there was less tissue to rub along the ground and be slowed by friction.

Bichirs don’t belong to the broad group of lobe-finned fishes that gave rise to land-dwelling vertebrates (animals with backbones). But bichirs are near relatives. The changes observed in the land-reared bichirs suggest how some prehistoric fishes or no-longer-quite fishes might have moved, Standen says.

The speed with which the fish in the experiment changed — over three-quarters of a year — was lightning fast. At least in evolutionary terms, it is. This suggests that quirky conditions early in life similarly might have given ancient fish a little head start in adapting to life out of water.

This ability of a species to make adaptive changes based on early-life effects is called developmental plasticity. And it has triggered interest among evolutionary biologists in recent years, says Armin Moczek. He works at Indiana University in Bloomington. Changing environments can use the genes an organism already has to create new forms. If this plasticity played a major role in the colonization of land by marine vertebrates, that would be a big deal, he says.

Still, showing that a modern fish has the flexibility to cope with land doesn’t prove that prehistoric fish also had it. But, he says, this experiment “raises the possibility that preexisting developmental plasticity provided the first baby step [toward life on land].”

Power Words

developmental plasticity    (in biology) The ability of an organism to adapt to its environment in unusual ways based on conditions it encountered when its body (or brain and nervous system) were still growing and maturing.

drag    A slowing force exerted by air or other fluid surrounding a moving object.

evolution  A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolutionary   An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

friction  The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage the surface of the materials rubbing against one another.

gills  The respiratory organ of most aquatic animals that that filters oxygen out of water, which fish and other water-dwelling animals use to breathe.

marine  Having to do with the ocean world or environment.

plasticity   Adaptable or reshapable. (in biology) The ability of an organ, such as the brain or skeleton to adapt in ways that stretch its normal function or abilities. This might include the brain’s ability to rewire itself to recover some lost functions and compensate for damage.

tissue  Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.

vertebrate  The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes all fish, amphibians, reptiles, birds, and mammals.

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