Traces of ancient lungs in fish bladders

CORNELL (US) — New research that uses computed tomography (CT) technology goes a long way toward showing that lungs and gas bladders in fish are variations of the same organ.

Researchers say one of the great problems of evolution is to understand how the major features of organisms have changed over great swaths of time. How did limbs evolve from fins? How did bird feathers arise from scales?

The lung is a major organ of great functional importance for vertebrates (animals with backbones). For 150 years biologists have debated the idea that in living fishes, gas bladders—internal bags of air to which fishes can add or eliminate oxygen to control buoyancy—are simply a modified version of an ancestral pair of lungs. Some have argued that the two are completely different organs.

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By proving that several ray-finned fishes, namely sturgeons and paddlefishes, as well as bowfin, have pulmonary arteries like those that supply the lungs of vertebrates, researchers show that the common ancestor of all these fishes must have originally had lungs supplied by a pulmonary artery.

Micro-CT enabled researchers to look at resolutions of 25 microns, about the width of a human hair.

“This is biology as we’ve never been able to see it before,” says Amy McCune, professor and chair of ecology and evolutionary biology at Cornell University and co-author a study published online in the Journal of Morphology. “We’re studying biological diversity from the inside out.”

“[W]e noninvasively created 3D datasets that could be examined from multiple perspectives,” says co-author Mark Riccio, director of the Cornell University’s Multiscale CT Facility in the Institute of Biotechnology.

By studying the CT scans of blood vessels filled with X-ray opaque barium in lungfishes, bowfin, and several other related fishes, the researchers showed that other fishes in the lineage that includes bowfin actually have tiny vestigial pulmonary arteries, which branch off from a parent vessel in the same way that pulmonary arteries branch in tetrapods and lungfishes.

The arteries in gas bladder fish, therefore, are actually vestigial pulmonary arteries that have been co-opted for new functions. The researchers hypothesize that this evolutionary change occurred either by the loss of respiration or by dorsal shifts in the anatomical structures of these fishes.

Scientists have known about the vestigial arteries for a long time, but because traditional dissection and corrosive casting techniques lose detail, no one made the connection with pulmonary arteries, says Sarah Longo, the paper’s lead author, who conducted the research as a student at Cornell. Longo is now a graduate student at the University of California, Davis.

Source: Cornell University