26 November 2008

Giant amoebas found rolling on sea floor

By
Cosmos Online
Marine biologists in the Bahamas have discovered large numbers of an unknown, grape-sized, single-celled animal slowly rolling across the sea floor.
Sea grapes

Living fossil?: Sea grapes and the furrows they leave photographed next to a shrimp on the sea bed. Credit: Sönke Johnsen

SYDNEY: Using a research submarine, marine biologists in the Bahamas have discovered large numbers of an unknown, grape-sized, single-celled animal slowly rolling across the sea floor.

“[It's] huge for a single cell. If I had cells that big I’d be six kilometres tall and weigh three trillion kilograms,” said Sönke Johnsen, a biologist at Duke University in North Carolina, and the expedition’s chief scientist.

Single-celled animals, known as protists, are usually the size of a pin-head or much smaller, but the size of this “sea-grape” isn’t the most unusual thing about it.

“We watched the video over and over,” said Johnsen. “We argued about it forever… [we thought] these things can’t possibly be moving. There are other large protists, but none of them move.”

Evolutionary debate

But these large protists do move, and more importantly, the tracks they leave behind are very similar to fossil tracks that date back to before the Cambrian Explosion, around 530 million years ago, when many different types of complex animal first appeared.

Because simple, single-celled animals were previously thought to be incapable of leaving tracks, the established theory is that these grooves and furrows were left by complex multi-cellular animals, and the date back to 1.8 billion years ago.

“We’re confident that drawing attention to these bizarre mega-protists will provide a powerful new spin to the debate,” said Mikhail Matz a biologist at the University of Texas in Austin and lead author of a study detailing the find in the journal Current Biology.

With DNA testing, the sea grape has been cautiously identified as a close relative of another giant amoeba, Gromia sphaerica from the Arabian Sea – though that species is not known to be mobile.

Slow roll

The tracks left by them, as they feed on sediment in the Bahamas, are up to 50 cm long, and it’s estimated that they roll at a rate of just 2.5 cm a day. See a video slide show of the animals here.

The researchers said that it’s possible that the sea grape may be a descendent of the creature that made the tracks that are well known from the fossil record. Or – like the tuatara or the coelacanth – the protist could be a living fossil, that has changed little for as many as 1.8 billion years.

“This description of similar [sea floor tracks] made by the giant single-celled amoeba is very important,” commented Jim Gehling, a palaeontologist at the South Australian Museum in Adelaide. “It shows that we can never rely on one piece of evidence to demonstrate the origins of motile animals.”

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