Rolling 'Sea Grape' Rocks The Fossil Record

The grape-like animal, tentatively named the Bahamian Gromia, is actually a single-celled organism, fully one inch long. But what makes it really fantastic is that it moves -- very slowly -- by rolling itself along the ocean floor.

"At first, we assumed they were snails, because they had trails," said Sönke Johnsen, an associate professor of biology at Duke University. But after sucking up a few with the tools aboard the NOAA research submarine Johnson-Sea-Link and having a look, they figured the soft, nondescript blobs were simply some kind of elaborate poop. "We called them doo-doo balls," Johnsen said.

"We watched the video over and over," Johnsen said. The trails couldn't be the result of currents because they went in several directions at the same spot, and sometimes they even changed course. And they weren't the result of rolling downhill. In fact, one trail was found that went down into a small depression and came back up the other side.

"We argued about it forever," Johnsen said. "These things can't possibly be moving!" But they are, at a rate too slow to be captured on the sub's video. Johnsen guesses they move maybe an inch a day or less.

The distinctive trail that the Gromias leave is identical to mud tracks found in the fossil record, which throws a big wrench into one long-standing argument in biology. The fossil tracks pre-date the so-called "Cambrian explosion" 530 million years ago, which was a blossoming of multicellular life and complex body plans from what had previously just been simple, blobby life forms. Many paleontologists and evolutionary biologists have argued that such a trail couldn't possibly have been made by a simple organism, meaning complex body plans were around before the Cambrian explosion. But the Gromia show that simple blobs can indeed move and make tracks in the light, silty bottom.

We're confident that drawing attention to these bizarre mega-protists will provide a powerful new spin to the debate," said biologist Mikhail Matz of the University of Texas at Austin, who is first author on the paper in Current Biology. Matz worked out the genetics of the new creature and found it's a giant amoeba closely related to similar blobs found in the Gulf of Oman, near Antarctica, off Guam, and in the Mediterranean. None of them are known to move.

The surface of the cell is covered with tiny ports. Its interior is just a fluid; the important working parts of the cell are all near the surface. Think of the working cell as a very thick balloon, Johnsen said.

These sea grapes are almost neutrally buoyant, so they barely rest on the ocean floor 800 meters down. It's possible, Johnsen said, that they're sort of eating and rolling at the same time, pulling new sediment in on one side, and pushing "pseudo feces" out on the other, leaving the distinctive trail.

What "eating" they do is pretty rudimentary, he added. If they're like other large amoebas, they're really just relying on resident bacteria to ferment their food for them and living off the byproducts. Unfortunately, the Gromia are too fragile to study in captivity.

Johnsen also couldn't guess how the critters reproduce. "They obviously do, because there sure were a lot of them," he said. They all appeared to be about the same size however, so it wasn't obvious if there were any young Gromia.

The Gromia were found on a relatively sterile area of sea bottom on the eastern side of the Bahamas, near Little San Salvador Island, where the current is extremely slow. "It's actually kind of a spooky habitat," Johnsen said. The expedition made a series of dives on the area in August 2007, with support from NOAA's Office of Ocean Exploration and Research. Johnsen was chief scientist on the expedition.