Video: Single-celled sniper uses one eye to hunt

It is perhaps the most extraordinary eye in the living world – 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 “see” its prey?

Fernando Gómez of the University of São Paulo, Brazil, thinks it can. “Erythropsidinium is a sniper,” he told New Scientist. “It is waiting to see the prey, and it shoots in that direction.”


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, Gómez says.

Erythropsidinium and its close relatives can do better, Gómez thinks, because they spot prey with their unique and sophisticated eye, called the ocelloid, which juts out from the cell. “It knows where the prey is,” 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 actually a modified chloroplast.

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 – as some biologists think.

But this doesn’t make sense, Gómez says. The ocelloid can occupy up to a third of a cell’s volume, he has found in still unpublished work. There is no need for such a large, complex structure just to tell if it’s light or dark. What’s more, his videos of Erythropsidinium reveal that it can point its ocelloid in different directions. “You don’t need to move your eye if you’re only using it as a simple photoreceptor,” Gómez says.

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

“Absolutely, it’s plausible,” says Brian Leander of the University of British Columbia in Vancouver, Canada. He points out, though, that Erythropsidinium preys on transparent creatures – including other dinoflagellates – that are almost invisible in normal light.

Polariser

But the massive nucleus of dinoflagellates has an unusual property – it just happens to polarise light. So Leander’s 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’s held up progress for decades. Gómez 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 “sees”. “How is the image processed by a single cell?” asks Leander. “It’s very difficult to wrap your mind around.”

It’s not seeing in the normal sense, Gómez 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.

“When you have an eye and you can see your prey, you can also see your predator.”

Piston deployed (Image: Fernando Gómez/Laboratory of Plankton Systems, Oceanographic Institute, University of São 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. “It’s an extremely fast process,” Leander says.

As with the ocelloid, it is still not clear just what Erythropsidinium uses its piston for. Some think it’s another way of catching prey, others that it’s for moving. But although the cell vibrates rapidly when the piston is shot out, it doesn’t move much, Leander says. Gómez thinks the piston is a defence mechanism, for “kicking” potential predators away.