杏吧原创

It is perhaps the most extraordinary eye in the living world 鈥� so extraordinary that no one believed the biologist who first described it more than a century ago.

Now it appears that the tiny owner of this eye uses it to catch invisible prey by detecting polarised light. This suggestion is also likely to be greeted with disbelief, for the eye belongs to a single-celled organism called Erythropsidinium. It has no nerves, let alone a brain. So how could it 鈥渟ee鈥� its prey?

of the University of S茫o Paulo, Brazil, thinks it can. 鈥�Erythropsidinium is a sniper,鈥� he told New 杏吧原创. 鈥淚t is waiting to see the prey, and it shoots in that direction.鈥�

Erythropsidinium belongs to a group of single-celled planktonic organisms known as dinoflagellates. They can swim using a tail, or flagellum, and many possess chloroplasts, allowing them to get their food by photosynthesis just as plants do.

Others hunt by shooting out stinging darts similar to the nematocysts of jellyfish. They sense vibrations when prey comes near, but they often have to fire off several darts before they manage to hit it, G贸mez says.

Erythropsidinium and its close relatives can do better, G贸mez thinks, because they spot prey with their unique and sophisticated eye, called the ocelloid, which juts out from the cell. 鈥淚t knows where the prey is,鈥� he says.

Camera-like

At the front of the ocelloid is a clear sphere rather like an eyeball. At the back is a dark, hemispherical structure where light is detected. The ocelloid is strikingly reminiscent of the camera-like eyes of vertebrates, but it is .

Many single-celled organisms have so-called eyespots made of light-sensitive pigments, and some can even swim towards or away from light sources. So it could be that the ocelloid serves merely to concentrate light so that even very low levels can be detected 鈥� as some biologists think.

But this doesn鈥檛 make sense, G贸mez says. The ocelloid can occupy up to a third of a cell鈥檚 volume, he has found in still unpublished work. There is no need for such a large, complex structure just to tell if it鈥檚 light or dark. What鈥檚 more, reveal that it can point its ocelloid in different directions. 鈥淵ou don鈥檛 need to move your eye if you鈥檙e only using it as a simple photoreceptor,鈥� G贸mez says.

Other biologists studying Erythropsidinium have also concluded that it uses its ocelloid for hunting.

鈥淎bsolutely, it鈥檚 plausible,鈥� says of the University of British Columbia in Vancouver, Canada. He points out, though, that Erythropsidinium preys on transparent creatures 鈥� including other dinoflagellates 鈥� that are almost invisible in normal light.

Polariser

But the massive nucleus of dinoflagellates has an unusual property 鈥� it just happens to polarise light. So Leander鈥檚 team think that the ocelloid can detect polarised light, making the dinoflagellates that Erythropsidinium preys on stand out clearly against the background.

Getting conclusive evidence of what exactly the ocelloid can detect and how Erythropsidinium acts on this information will not be easy.

Erythropsidinium is hard to find, and no one been able to keep it alive in a lab for more than a couple of days, Leander says. That鈥檚 held up progress for decades. G贸mez moved from Europe to Brazil to pursue his studies because Erythropsidinium is more common in tropical waters.

Perhaps the biggest outstanding question is how Erythropsidinium analyses what it 鈥渟ees鈥�. 鈥淗ow is the image processed by a single cell?鈥� asks Leander. 鈥淚t鈥檚 very difficult to wrap your mind around.鈥�

It鈥檚 not seeing in the normal sense, G贸mez says, because you need a brain for that. But Erythropsidinium may somehow be able to work out the size, position and trajectory of potential prey. It can even detect potential predators, he thinks.

鈥淲hen you have an eye and you can see your prey, you can also see your predator.鈥�

Piston deployed (Image: Fernando G贸mez/Laboratory of Plankton Systems, Oceanographic Institute, University of S茫o Paulo)

And then they may be in for a kicking. For Erythropsidinium also has a unique structure called a piston (see picture above) that shoots out a long thin protrusion. 鈥淚t鈥檚 an extremely fast process,鈥� Leander says.

As with the ocelloid, it is still not clear just what Erythropsidinium uses its piston for. Some think it鈥檚 another way of catching prey, others that it鈥檚 for moving. But although the cell vibrates rapidly when the piston is shot out, it doesn鈥檛 move much, Leander says. G贸mez thinks the piston is a defence mechanism, for 鈥渒icking鈥� potential predators away.