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Seeing in the dark

Last week, we explained how some creatures in the animal kingdom have extraordinary eyes. This week we examine how different animals have evolved different ways to see in the absence of light.

There are many animals that use infrared radiation and far red light to see in the dark....But, before we proceed any further, it's important to know what infrared and far red is, and for that we need to enlighten you about the Visual Spectrum.

Electromagnetic radiation plays a key role in the Visual Spectrum. It consists of the energy waves associated with electric and magnetic waves, and results from the acceleration of an electric charge. Are you aware that there are several types of electromagnetic radiation which includes radio waves, visible light, ultraviolet, x-rays, gamma rays and of course infrared (heat)?. All these are distinguished by their wavelength and also frequency bands.

The longer the wavelength of a type of electromagnetic radiation, the shorter the frequency; and the opposite in the frequency when the wavelength is shorter. The arrangement of these different types in a continuous spectrum, with the longest wavelength at one end and the shortest at the other, makes up the Visual Spectrum.


Gold fish and Piranha

Humans can see only one small band of wavelengths - the Visible Spectrum - which consists of seven colours; red, orange, yellow, green, blue, indigo and violet. Red has the longest wavelength and violet the shortest. Some animals can see electromagnetic radiation wavelengths beyond the Visible Spectrum.

Using infrared vision means the ability to see the body heat of other animals. Some snake species such as the rattlesnakes and pit vipers and other animals including goldfish can see infrared radiation (IR) wavelengths.

This enables them to track warm-blooded prey in total darkness.Radiant heat or infrared radiation (IR) is invisible to the human eye. But it has been recorded that the pit viper's ability to sense heat is so sensitive that it can feel the temperature variation produced by a mouse from about 15 cm (6 inches) away. And do you know where these great heat sensors are located? In pit-shaped holes which are positioned on each side of the snake's head, between the eye and nostril. It is these holes that give the snake its name.

The tiny, pinhole openings are supplied with a grid of 7,000 nerve endings from a branch of the trigeminal nerve, leading to the head and face. Towards the base of this pit is a membrane which is similar to the retina of the eye. There are 500-1,500 minuscule thermoreceptors per square millimetre in the membrane.


Pit viper

And because the fields of sensitivity of the two pits overlap, a pit viper can see heat in stereo. As a result of this bifocal thermal vision, the snake is presented a fiery infrared image of its prey, and it is also able to judge how far it is. It is said that due to this, a pit viper can respond to a heat signal in under 35 milliseconds.

The ability to see infrared radiation is an advantage for pit vipers. They could hunt at night too, not only using their highly developed sense of smell to locate prey, but also their heat sensitive pits. They also have additional thermoreceptors located in their mouths.With all these special senses to track warm-blooded prey, it is not a wonder that a pit viper is greatly feared. Although not as well-known as pit vipers for their infrared vision, boas and pythons too have heat sensors that enable them to track warm-blooded prey. However, instead of pits, these two types of snakes have up to 13 pairs of thermoreceptors around their lips.

Apart from snakes, there are certain species of fish too that are endowed with special vision to see in the dark. The black, loose-jawed dragon fish is a predatory deep-sea species. Living at depths of 915 to 1,830m (3,000 to 6,000 ft) in the sea, these creatures hardly see any light.


Green boa

It is their ability to see far red light that aids them in their hunt for food. Light at the far-red end of the so-called Visible Spectrum has the longest wavelengths of all. Although invisible to the human eye, this type of light can be seen by the dragon fish and also by some other animals. The dragon fish is believed to have got the ability to absorb far-red light due to its diet of small crustaceans (hard shelled animals).

Thanks to far red light, many fish such as the piranha, swimming in the murky waters of the Amazon River can see in the dark. The rotting vegetation in the Amazon River absorbs most of the wavelengths of visible light, except for far-red light, which is reflected back and can be seen by the piranha. This allows the fish to see the prey it's hunting in the dark, murky waters.

Another fish that is able to detect far-red light is the goldfish which often lives in murky, vegetation choked stretches of freshwater in its natural habitat. Its visual range exceeds that of the piranha. It can not only see far-red light, but also true infrared.

If you have goldfish in your aquariums at home, watch out....It can see more than you think. Even the infrared beams emitted by common household electronic items, which are invisible to our naked eye are visible to your pet goldfish.

It is the composition of the eye that helps the goldfish to see infrared beams emitted even from television remote controls and intruder alarm systems. The retinae in its eyes contain cone cells that are sensitive to various colours. The red sensitive cones are dominant and as a result, the red signals override other signals to the brain. This shifts the goldfish's vision to the red end of the Visual Spectrum.

ANCL TENDER- Platesetter

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