Elephantnose fish's unique retina helps it see through mud

June 29, 2012 by Lin Edwards report
Elephantnose fish

An international group of researchers has been studying the Peters’ elephantnose fish to try to find out how it can see in its murky habitat, and have discovered it has a unique system of crystal-lined cups within its retina.

The Peters’ elephantnose fish (Gnathonemus petersii) is a freshwater elephantfish around nine inches long. It lives in dark, slow-moving, heavily vegetated waters in central and west Africa. The elephantfish family all have extended mouths that resemble the trunks of elephants, and which generate weak electrical fields that help them sense objects in the water.

The eyes of vertebrates have a system of rods and cones in the retina, with the rods being most sensitive to dim light and cones being most effective in bright light and more sensitive to colors and fine details. In most vertebrates either rods or cones are working at any time depending on the light intensity, and so in humans, for example, cones are working in the daytime and rods are working at night. In animals active in the daytime cones predominate, while nocturnal animals have more rods than cones in the retina.

The research team measured the electrical signals produced in the of the fish and found that both types of photoreceptors work simultaneously all the time. They also discovered that the cones are grouped in unusual structures — light-reflecting cups with a mirror-like surface made of photonic guanine crystals — and the rods are clustered beneath these reflecting cups.

The research team worked out that the cup arrangement in the elephantnose fish amplifies the light falling on the cones to enable them to work even in dim light, and allows just enough light through to the rods to provide optimal light conditions for them to function.

The group tested the vision of the fish and compared it to that of goldfish, by using a monitor on which they displayed images such as a black expanding circle against a white background. The goldfish and elephantnose fish spotted the object in around the same time. When the images were presented with a grey noise pattern superimposed over them, the elephantnose fish responded to the images more quickly than the goldfish (which are renowned for having good vision).

The elephantnose fish are only capable of seeing relatively large objects because all the cones in each cup see the same image. Lead author, Prof. Dr. Andreas Reichenbach of the Paul Flechsig Institute at the University of Leipzig in Germany, likened their vision to humans being able to see objects only if they were larger than six times the diameter of the full moon. The fish can therefore not see bubbles or small particles of mud and dirt, but they can discriminate large objects moving against a cluttered background, which is exactly the kind of vision they need to avoid predators in their muddy habitats.

The paper is published in the journal Science.

Explore further: New research eyes off colourful reef fish

More information: Science 29 June 2012: Vol. 336 no. 6089 pp. 1700-1703 DOI: 10.1126/science.1218072

Related Stories

New research eyes off colourful reef fish

February 11, 2010

(PhysOrg.com) -- Most people wouldn't give a second thought to the inner workings of the fish eye. But research by University of Queensland scientists is unlocking the secrets hidden behind these fishy lenses.

Strange Moonlight

September 28, 2006

Not so long ago, before electric lights, farmers relied on moonlight to harvest autumn crops. With everything ripening at once, there was too much work to to do to stop at sundown. A bright full moon—a "Harvest Moon"—allowed ...

Chickens 'one-up' humans in ability to see color

February 16, 2010

(PhysOrg.com) -- Researchers at Washington University School of Medicine in St. Louis have peered deep into the eye of the chicken and found a masterpiece of biological design.

Fruit Bats are not 'Blind as a Bat'

June 12, 2007

The retinas of most mammals contain two types of photoreceptor cells, the cones for daylight vision and colour vision, and the more sensitive rods for night vision. Nocturnal bats were traditionally believed to possess only ...

Recommended for you

Finding a lethal parasite's vulnerabilities

December 15, 2017

An estimated 100 million people around the world are infected with Strongyloides stercoralis, a parasitic nematode, yet it's likely that many don't know it. The infection can persist for years, usually only causing mild symptoms. ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

2 / 5 (1) Jun 29, 2012
The fish's mouth is not extended. It is at the main body, where the protrusion attaches.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.