Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Step from a sunlit hillside into the darkness of a cave, and you immediately have a problem: you can't see. It's best to stand still for a few minutes until your eyes adjust to the dimness, otherwise you might blunder into a hibernating bear that doesn't appreciate your presence.
The same thing will happen when you leave again: the brightness of the sun will dazzle you at first. That's because your eyes have two types of receptor: one set works in bright light and the other in dim light. Barring a few minutes around sunset, only one set of receptors is ever working at any given time.
Peters' elephantnose fish has no such limitations. Its peculiar eyes allow it to use the two types of receptor at the same time. That could help it to spot predators as they approach through the murky water it calls home.
Peters' elephantnose fish belongs to a large family called the elephantfish, all of which live in Africa. They get their name from the trunk-like protrusions on the front of their heads. But whereas the trunks of elephants are extensions of their noses, the trunks of elephantfish are extensions of their mouths.
To find a Peters' elephantnose fish, you must lurk in muddy, slow-moving water. Look closely, because the fish is brown and so is the background.
It finds its way through the murk using its trunk, which generates a weak electrical field that helps it sense its surroundings and even discriminate between different objects. The fish's electric sense allows it to hunt insect larvae in pitch darkness.
The fish has paid a price for its electrical sensitivity. Processing the signals takes brainpower, so it has an exceptionally large brain. As a result, 60 per cent of the oxygen taken in by the fish goes to its brain. Even humans, with our whopping brains, only devote 20 per cent of our oxygen to them.
Now for its eyes. Most vertebrates, including humans, have two types of light receptors on their retinas: rods and cones. Rods can sense dim light, but become bleached in bright light and stop working. Cones can't see in dim light, but given enough light they can see fine details and colours.
Most animals' eyes are specialised for one or the other. Animals that are active during the day tend to have more cones than nocturnal animals such as foxes. In the human eye, the cones are clustered in a central region called the fovea, where the light is sharply focused, and the rods are outside it. As a result, we have excellent daytime vision and rather poor night vision.
The retina of the Peters' elephantnose fish looks completely different. It is covered with cup-shaped depressions. Around 30 cones sit inside each cup, and a few hundred rods are buried underneath.
Because of the peculiar design of the fish's retina, it was thought to be blind until about 10 years ago, says Andreas Reichenbach of the Paul Flechsig Institute for Brain Research in Leipzig, Germany. Reichenbach has now worked out what the cups are for.
Each cup has a layer of massive cells that are full of guanine crystals. These form a mirrored surface that amplifies the light intensity within the cups, ensuring that the cones have enough light to work with.
At the same time, because the cups are eating up so much of the light, only a small amount reaches the cones. As a result, both sets of receptors are supplied with the right amount of light.
Yet when Reichenbach tested the fishes' vision, they didn't seem to do very well. For instance, they could only see objects that covered a big swathe of their visual field. If humans had vision that bad, we would miss any object whose width was less than one sixth of a full moon.
However, the Peters' elephantnose fish were very good at spotting large moving objects against a cluttered background – essential for fish that live in dirty water. Presented with a monitor displaying a black stimulus on a white background, they took as long to spot it as goldfish. But when a grey noise pattern – like an untuned TV – was superimposed, the elephantnose fish spotted the stimulus faster than the goldfish.
The fish's ability to see the wood for the trees probably helps it spot incoming predators like catfish. So Reichenbach thinks its oddball visual system isn't a mistake. "It's the right type for this fish," he says.
Journal reference: Science, DOI: 10.1126/science.1218072
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Thu Jun 28 22:15:54 BST 2012 by Freederick
If I understand correctly, each cup-shaped depression serves as a single aggregate receptor, combining the output of all the individual light-sensitive cells comprising it.
In effect, the fish is trading resolution for sensitivity. This is the same sort of effect as used to be employed in high-ISO photographic film, where the larger, flattened grains of photosensitive chemicals resulted in high sensitivity, at the cost of a coarse-grained image.
The fish employs an even more effective method, effectively combining many smaller "grains" into one huge hypersensitive receptor.
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