Deep sea researchers have just released a paper [PDF] on the apparently motile habits of Gromia sphaerica, a giant deep-sea amoeba the size and shape of a grape. Previously known only in the Red Sea, the specimens in question here were found at Little San Salvador Island in the Bahamas.
Aside from the obvious reason (single-celled deep-sea grape-animals, man!), the photos and videos taken from the Johnson Sea-link submersible are interesting because of the tracks in the sediment left behind these little critters. The sea grapes are more or less neutrally buoyant, so any currents that move them tend to lift them up and bounce them along, not drag them through the sediment. The scientists also saw tracks going uphill, so they weren’t just rolling with gravity. After much head-scratching, they decided that the only plausible explanation was that the sea grapes were rolling, very slowly, under their own power.
So big whoop. The deep sea protozoans are capable of rolling as far as several inches per day. But the tracks they leave in the mud are dead ringers for certain fossil tracks from the Precambrian, about 1.8 billion years ago, interpreted by many as evidence of the first bilaterians—that is, bilaterally-symmetrical animals. The reasoning was that in order to leave tracks in the mud, you needed to be able to crawl or wriggle through it in a particular direction. The problem with this explanation, however, is that molecular clock analyses put the first bilaterally-symmetrical animals closer to 0.5 billion years ago, when the Cambrian Explosion was in full swing. If the first bilaterian—called the ur-bilaterian, and probably some kind of flatworm-type creature—only appear five or six hundred million years ago, where did those fossil tracks, fully twice as old, come from? A possible answer, it appears, is some kind of prehistoric sea grape.
Here’s some of the video taken of Gromia sphaerica in the Bahamas. The shrimp is about 20cm long.