The deft wrigglings of fish forced to grow up on land could offer a glimpse of how ancient vertebrates started to make the big move out of the sea.
The modern fish species called Senegal bichir (Polypterus senegalus) normally swims in African rivers. But the elongated fish possesses both gills and lungs and can walk on land if it has to. And that’s what Emily Standen forced bichirs to do for much of their youth.
While working at McGill University in Montreal, she created tanks with special bottoms that let only a few millimeters of water seep across the surface where the fish would move. Grocery-store produce aisles provided additional inspiration for the tanks’ design. (“We need misters, lettuce misters!” she realized.) For eight months, the tanks housed crowds of roughly 7- to 8-centimeter-long young fish, which used their fins and tails to dart around the bottom looking for food.
As the fish matured, certain bones in their head and shoulder regions began developing differently than in bichirs that grew up swimming. The skeletal changes matched what scientists have predicted for beginning to transition to life on land, says Standen, now at the University of Ottawa. The land-reared fish also moved in ways that should be more efficient than water-reared fish forced as adults to walk, Standen and her colleagues say August 27 in Nature.
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Young fish forced to walk, not swim, developed a sturdier and more strongly attached clavicle bone in their chests. The change marks a step toward a skeleton that could bear weight instead of relying on support from water. Also, the gill area enlarged a little and bone connections loosened slightly at the back of the head, both small steps toward a flexible neck. (Fish in water can dart stiff-necked at food from above, below, or elsewhere, but having a bendy neck helps terrestrial feeding.)
Land-reared bichirs also were better than water-baby ones at reducing drag in walking. Landlings kept the front stepping fin close to their bodies, gaining a little extra height when their “shoulders” rose upward and forward, as if the fin were a crutch. The close-in fin hoisted more bichir body temporarily into air, leaving less to rub along the ground.
Bichirs don’t belong to the broad group of lobe-finned fishes that gave rise to land-dwelling vertebrates. But bichirs are near relatives. The changes observed in the land-reared bichirs suggest how some of those early fishes or no-longer-quite fishes might have moved, Standen says.
The speed with which the fish in the experiment changed — as one generation grew up in bizarre circumstances — raises the possibility that quirky development might also have given ancient fish a little head start in adapting to life out of water.
This developmental plasticity, as it’s called, has triggered interest among evolutionary biologists in recent years, says Armin Moczek of Indiana University in Bloomington. Changing environments can reorchestrate the genes an organism already has and reveal new forms without mutations. A role for plasticity in such a major evolutionary event as marine vertebrates’ colonization of land would be a big deal, he says.
Still, showing that a modern fish has the flexibility to cope with land doesn’t prove that ancestral fish had it. But, he adds, this experiment “raises the possibility that preexisting developmental plasticity provided the first baby step.”
Shown at its real speed, a Senegal bichir wriggles forward on land quite briskly. Credit: E.M. Standen and T.Y. Du