Roly-poly ocean creature leaves a trail of clues
Biologists exploring the ocean floor for new sea creatures have stumbled upon one of the largest known single-celled creatures, a bland, grape-sized distant cousin of the amoeba that may solve a thorny evolutionary question that has puzzled researchers for decades.
Researchers have seen similar blobs on the ocean floor before, but what distinguishes the new one, called a Bahamian Gromia, is that it moves -- albeit very slowly -- by rolling itself along the ocean floor, leaving behind distinctive trails.
It is the first time that a single-cell organism has been shown to leave such animal-like tracks.
Those trails are virtually indistinguishable from fossilized tracks from the Earth’s pre-Cambrian era more than 530 million years ago, tracks that suggested the existence of multicellular life in a period before such organisms were thought to exist.
Many researchers had argued that such trails could not possibly have been made by a simple organism, meaning that complex body plans had to have existed despite evidence to the contrary.
“We now have to rethink the fossil record,” said biologist Mikhail V. Matz of the University of Texas at Austin, one of the authors of a paper appearing today in the journal Current Biology.
“We used to think that it takes bilateral symmetry to move in one direction across the sea floor and thereby leave a track,” he said. “You have to have a belly and backside and a front and back end. Now, we show that [single-celled organisms] can leave traces of comparable complexity.”
The surprising creatures were discovered by Matz and biologist Sonke Johnsen of Duke University. They were trolling the sea floor on the eastern side of the Bahamas, near Little San Salvador Island, in the National Oceanic and Atmospheric Administration’s research submersible, the Johnson-Sea-Link.
When researchers first sucked the creatures onboard, they thought they were simply soft balls of excrement. “We called them doo-doo balls,” Johnsen said.
Eventually, however, they realized that the balls, which were as large as an inch in diameter, were alive. Matz studied their genetics and concluded that the Gromia were related to giant, immobile amoebas found in the Gulf of Oman, near Antarctica, off Guam and in the Mediterranean.
After much debate, the researchers concluded that the Gromia were, in fact, leaving the tracks. “We watched the video over and over . . . and we argued about it forever,” Johnsen said. The creatures’ motions were too slow to be captured on the video. Johnsen guessed that they move about an inch a day or less.
Because the trails move in several directions at the same spot, and even change course, the researchers concluded that the Gromia were not being rolled by currents.
Closer examination showed that the surface of the creature, which the researchers also called a sea grape, is covered with tiny ports, and its interior is a fluid.
They apparently move by pulling sediment in one side and pushing “pseudo feces” out the other, Matz said.
If the Gromia is like other amoeba, Matz said, they rely on resident bacteria to ferment their food for them, living off the byproducts. Unfortunately, the creatures are too delicate to study in captivity.
It is not clear how the sea grapes reproduce. “They obviously do, because there sure were a lot of them,” Johnsen said. There did not appear to be any smaller, young Gromia.
The Cambrian explosion occurred more than 530 million years ago. Before then, according to fossil records, the only living organisms were single-celled creatures that sometimes clumped together in colonies. After that time, for reasons still not clear, life began changing rapidly, with complex plants and animals evolving swiftly.
The appearance of the fossilized trails in earlier rocks had been a thorn in the side of biologists because they could not explain how they arose. The new find provides a simple explanation that doesn’t require any extraordinary occurrences, Matz said.
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