It’s a single cell, it’s the size of a grape, and it propels itself across the ocean floor: Behold the Bahamian Gromia — one of the strangest beasts yet discovered in the briny deep.

Gromia sphaerica, as the organisms are known, are superbig amoebas, growing up to 1.5 inches in diameter. They were first discovered in 2000 in the Arabian Sea, and have since been found in various locations around the world. Then last year a team of biologists were diving near Little San Salvador Island in the Bahamas, when they discovered a new form of Gromia — the “Bahamian” Gromia, as they’re calling it.

The weird thing is, the Bahamanian Gromia were all found at the end of a trail — as if they’d been somehow pushed or dragged along the seafloor. This didn’t make sense, because the currents at that depth either weren’t strong enough or were irregular, so they wouldn’t push the Gromia in single, uniform paths, the way the trails lay. That left only one possibility: Somehow, these wee blobs are propelling themselves across the ocean floor, at a pace so slow it cannot be readily observed. In a paper published a recent issue of Current Biology — “Giant Deep-Sea Protist Produces Bilaterian-like Traces” (PDF here)— the scientists argue this is precisely what’s happening.

As they said in a press release:

“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.

Here’s the even crazier thing: If these guys are right, this discovery could completely upend our ideas about the “Cambrian explosion.”

Remember those tracks the Bahamian Gromia left? They’re found in the pre-Cambrian fossil record. For years scientists assumed that only organisms with complex body plans that are symmetrical down the middle — “bilateria,” as they’re called — could possibly move in a fashion that would leave such trails. Thus, biologists have argued that bilateria were around before the Cambrian explosion, which sort of primed the pump for that crazily rapid diversification of bilateria into all the major animal groups we have today. But now it looks as though all those pre-Cambrian seabed trails could have been left by rolling, grape-sized ameobas. Maybe — who knows? — the Cambrian explosion happened even more psychotically quickly than we think. Maybe bilateria weren’t kicking around for millions of years later than we suspect.

The Rolling Grape That Rocked The Fossil Record. As I’ve said before, you can’t make this stuff up, and thankfully you don’t need to.

Check below the jump for pictures of the trails the Bahamian Gromia leave!