What are the main Biological Effects of Light?
1. Effect on Metabolism
The absorption of light rays of the visible part of spectrum has little effect on organisms. The violet and ultraviolet rays are harmful and produce photochemical changes in the organisms and, therefore, influence their metabolism. The intensity of solar radiation affects living organisms at different latitudes and seasons of the year. Within optimum limit, photosynthesis increases in direct proportion to the intensity of light. Cold blooded animals aestivate or hibernate as the solar radiation increases or decreases. Unicellular organisms including bacteria, algae, protozoa and eggs of vertebrates and invertebrates are killed by exposure to ultraviolet rays.
2. Effect on Reproduction
In many animals light initiates the breeding activities by stimulating their gonads. There appears to be a definite relationship between length of the day and egg lying in birds. Rowan (1931) reported that gonads of some birds become more active with increased illumination during summer. According to Farner (1959 and 64) and Walfson (1960) during short days of winter there is decrease in gonadial activity of birds.
On the basis of response to illumination, animals have been classified as long day, short day and indifferent day length animals. Certain increased by a decrease in the length of the day. Spring breeding birds and certain mammals such as turkeys and starlings are daylong animals as they become sexually active with the lengthening of the day. Ground squirrels, guinea-pigs and stickle-back fish are indifferent to day length, as they are least affected by the short or long day periods.
3. Effect on Development
Since light influences metabolism, it affects the growth and development of organisms. For example, Salmon larvae undergo normal development only when sufficient light is present. In the absence of light their development is not normal and there is heavy mortality. Mytilus larvae in their earlier stages grow larger in darkness than in light.
4. Effect on Pigmentation and Colour
Light induces certain chemical which result in the formation of photoreceptors in the form of pigment spots. It may influence the pigmentation of animals in the following ways-
Characteristic lack of pigment in cave animals is associated with darkness (i.e. total absence of light). Certain aquatic animals lose their colour when shielded from light. Rasquin (1947) showed that cave amphibians (e.g. Proteus) and fishes with little or no colour, when exposed to normal light, developed abundant pigment in the skin. The darkly pigmented skins of human inhabitants of the tropics also indicate the same effect.
The pigmentation of a number of animals acquires a colouration that can protect them from enemies. Such colouration is known as protective colouration. One common type of protective colouration is a simple matching of body colour in respect and pattern to the background. For example, colouration in case of a quail squatting in the grass, moth on the bark of a tree and leaf-insect (Phyllium) among green leaves is exactly like that of the background. It is exceedingly difficult to distinguish them from their surroundings. The arctic hare, weasel and ptarmigan show seasonal colour changes from brown in summer to white in winter. This is clearly related to the conspicuousness of such animals against bare ground or snow-covered landscape.
A second type of protective colouration is obliterative shading in which the birds, mammals or fish display darker colour on the back and lighter colour underneath. This difference counteracts the stronger illumination received from above and the animal blends with its background.
Certain animals have the capability to change their colour according to their surroundings. Frogs and chameleons are well known examples. According to Prosser and Brown (1950) colour changes are brought about by visual stimulus. Visual stimulus is the ability of the animals to change the colour to suit the environment in which they live. Colour changes are widely found among crustaceans, insects, fishes, amphibians and reptiles. These help the animals in concealing them from their enemies, help in thermoregulation and are sometimes, associated with breeding.
Females of certain birds have dull colouration. This is due to the greater need for concealment while brooding the eggs. The brilliant colouration of males in many animals does not have any protective value for the male itself, but its conspicuousness might draw attention away from the female on the nest. In some birds, such as Wilson’s phalarope, the females are brightly coloured and the dart males perform the job of incubating the eggs.
The brilliant breeding plumage of the male is often replaced by the duller drab during the winter season. The light is, thus involved in colouration through its effect on reproduction and through its role in protective resemblance.
5. Effect on Eyes
The degree of development of eyes sometimes depends upon the intensity of light available. In animals living in caves (e.g. Proteus anguinus) and in deep sea fishes, the eyes are absent or rudimentary because these animals live in complete darkness.
In surface dwelling forms such as crustaceans and fishes e.g. Labeo and Catla the ratio of eyes to head is considered to be normal. In the ocean with increasing depth, size of eyes goes on increasing with the progressive decrease in light intensity. However, below the upper limits of lightless zone, there is a gradual decrease in the size of the eyes. Some deep sea (bethel) fishes have well developed and enlarged eyes to see in bioluminescent light. Terrestrial nocturnal animals such as owls and geckos have large eyes to see in the dark.
6. Effect on Vision
Higher animals including man are able to see various objects only in the presence of light. According to Bigelow Welsh (1924) and Clarke (1936) many fishes (e.g. Lepomis) depend on sight to locate their food.
7. Effect on Locomotion (Photokinesis)
In certain lower animals the locomotion is influenced by light. This is known as photokinesis. For example, the blind larvae of mussel crab, Pinnotheres, move faster when exposed to increased light intensity. Movement of flies is considerably influenced by the wave lengths of light. Locusts stop their flight when the sun is hidden by the clouds.
In some animals, light plays a role in the orientation of locomotion. This phenomenon of movements of animals in response to light is known as phototaxis. When an animal moves towards the source of light, e.g. Rantara and Euglena, it is known as positively phototactic. The animals like earthworms, slugs and certain zooplanktons such as copepods are negatively phototactic as they move away from the source of light.
When only a part of an organism moves in response to light. It is known as phototropism. Phototropism is of common occurrence in plants. Among animals, the hydroids or polyps of many coelenterates and tubicolous worms show phototrophic response.
Photoperiodism is the response of an organism to the duration of day-length or length of the day, i.e. the time between sunrise and sunset which is known as photoperiod. Between the equator and polar circles, the photoperiod varies with the season from nearly twenty four hours to nearly no time at all. In temperate region photoperiod ranges from approximately six to eighteen hours with longer day in the summer and shorter in winter. In equatorial regions the day lasts for about twelve hours. But it is always the same for a given season and locality.
The photoperiod is the most important ecological factor which triggers the physiological and reproductive behaviour in both plants and animals, such as flowering in certain plants, moulting, fat-deposition, migration and breeding in birds and mammals and the onset of diapause in insects.