What are Photoreceptors



The receptors which are sensitive to light are called photo-receptors. All photo-receptors possess light-sensitive pigments. These pigments. These pigments are carotenoids and rhodopsin. These pigments absorb photons of light energy and produce a generator potential. Different types of photo-receptors are present in different animals.


Stigmata (stigma) are present in flagellated protozoa like euglena. Their chlorophyll possesses stigma. Stigmata are mass of bright red photo-receptor granules. The granules are carotenoids pigments. The actual photo-receptor is the swelling at the base of the flagellum. The stigma acts as a shield. It helps the photo-receptor to detect light coming from a direction. Thus, the photo-receptor and stigma enable Euglena to orient itself towards light. Thus animal remains in water which has maximum light.

Unicellular photo-receptors

Some animals like earthworm have simple unicellular photo-receptor cells. They are scattered over the epidermis. Or they concentrated in particular areas of the body.

Multi-cellular photo-receptors

There are three types of multi-cellular photo-receptors:

  1. Ocelli,

  2. Compound eyes

  3. Complex eyes


It is simply a small cup like structure. It is lined with light-sensitive receptors. It is backed by light absorbing pigment. The light-sensitive cells are called retinular cells. These cells contain a photosensitive pigment. Light stimulates these cells. They cause a chemical change in the pigment. It produces a generator potential.It causes an action potential in the sensory neurons. It carries this potential to other part of the body for interpretation. Ocellus gives information about light direction and intensity. But it does not form an image. Ocelli are common in many phyla like Annelida, Mollusca, and Arthropoda.

Compounds eyes

Compound eye consists of many units called ommatidia (ommatidium). Compounds eyes are present in arthropods. They also occur in some annelids and bivalve molluscs.

Compound eyes in Arthropods

A compound eye may contain thousands of ommatidia. Each ommatidium is arranged in a slightly different direction. It gives the eye a convex shape. The visual field of a compound eye is very wide. Each ommatidium has its own nerve tract. This tract is connected to large optic nerve. The visual fields of adjacent ommatidia overlap with each other. Thus the change of position of one object stimulates several ommatidia. Therefore, compound eyes are very effective in detecting the movements. They form images. Most compound eyes can detect changes in light intensities. Some compound eyes form four color visions. Color vision is important in honeybees. Honeybees recognize particular flowers by color, scent, and shape.

Complex eyes in Squids and Octopuses

Squids and octopuses have complex camera eyes. These are the best image-forming eyes among the invertebrates. The eye of the giant squid is the largest eye of the animal. Its diameter is more than 38 cm. Cephalopod eyes can be compared with the eyes of vertebrates. Both have transparent cornea and a lens. The lens focuses illumination light on the retina. Lens is hanged by ciliary muscles. But there is difference between the complex eyes of squids and vertebrate eye.

Squids (Cephalopods)

  1. The receptor sites are present on the retinal face of the eye of squids. Retinal layer controls the direction of light entering the eye

  2. Their muscles move the lens towards or away from the retina like moving a magnifying glass. It reorganize the shape of the eyeball. 


  1. The retinal layer is inverted and the receptors are the
    bottomless cells in the retina.

  2. Their muscles alter the shape (thickness) of the lens in terrestrial vertebrates. Light is focused by muscles in fishes and cephalopods.

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