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You can take a fish out of water.

You can take a fish out of water

From dinosaurs to dogs, all land vertebrates trace their ancestry back to the water. Sometime around 350 million years ago, fish-like amphibians crawled out of the water and established for the first time a foothold, or perhaps a fin-hold, on terra firma. While paleontologists believe these amphibians evolved from some primitive fish that had developed the ability to breathe air, they have debated for years which kind or kinds of fish spawned the earliest amphibians. Most theories have held that the ancestral fish must have had special organs, such as lungs, to help them breathe air directly. Yet a new study suggests this argument may not hold water.

Physiologist Karen Martin reports that some modern fish lacking special organs can breathe air quite well. Martin, of the University of California, Los Angeles, placed different near-shore marine fish in a dry chamber and passed air over them while measuring the levels of oxygen and carbon dioxide in air leaving the chamber. She found that for up to an hour, many types, including sculpin and a form of toadfish, called midshipman, could pull in oxygen and discharge carbon dioxide -- the two key elements of respiration.

Modern lungfish and certain fish with gas bladders are well known for their ability to breathe air, but the sculpin and toadfish apparently use the tissue in their gills or mouths to respire, according to Martin. She chose to test these fish because they are found in tidal pools or lagoons, a logical place to look for fish that might be able to breathe, she says. During the day, when tides drop and the sun warms these shallow areas, levels of dissolved oxygen can fall drastically, making it more difficult for fish to absorbe oxygen from the water.

Martin's findings do not reveal which ancient fish actually evolved into amphibians, but it opens up the possibilities in the search, she says. Air breathing does not seem to be a unique adaptation, and amphibians may have evolved from fish that lacked special breathing organs.
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Title Annotation:research on how fish evolved into amphibians
Author:Monastersky, Richard
Publication:Science News
Date:Oct 22, 1988
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