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Eyespot

An eyespot, often called a stigma (derived from Greek for "spot"), is a heavily-pigmented organelle located near the cell membrane that seems to function in light reception in certain single-celled organisms. (It may also refer to a region of light-sensitive cells in the epidermis of certain invertebrates like worms and starfish that performs the same function.)

An example of single-celled organisms that have an eyespot are euglenids. These organisms have bright red eyespots, which structurally are pigment-filled shields near the flagellum. Because they are next to the locomotive organelle, stimulation of the eyespot with light also stimulates the flagellum, enabling the euglenid to move in response to light so as to find better light conditions for photosynthesis.

Chlamydomonas are another green algae that have an eyespot, this one composed primarily of carotenoids and calcium ions. Considering that photosynthesis and eyespot functions are based on many of the same chemical principles, it's not surprising to find these two things together in many single-celled organisms.

Whether eyespots are related to the development of higher-level eyes is currently an open question. A number of evolutionary scenarios suppose that once one gets light-sensitive cells, the further development of complex eyes is a fairly smooth transition.

Web Resources On Eyespot

Eye Evolution
Eyespot-Assembly Mutants in Chlamydomonas reinhardtii

Book Resources On Eyespot

The Chlamydomonas Sourcebook by Elizabeth Harris
Morphology and division of Chlamydomonas by John McAllister Kater

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