THE ILLUMINATION OF THE EYE NECESSARY FOR DIFFERENT MELANOPHORIC RESPONSES OF FUNDULUS HETEROCLITUS

¹ From the Biological Laboratory, Hamilton College, and the Mount Desert Island Biological Laboratory, Salisbury Cove, Maine

Only a small area of the lower region of the eye of Fundulus needs be illuminated to induce a darkening of the body, since as little light as 2 footcandles coming from a source 1 cm. in diameter and 18 cm. above the fish elicits the darkening response. Darkening cannot be induced by illuminating the lower region when there is much illumination to the upper region of the eye.

For eliciting paling of the body, a large area of the upper region of the eye must be illuminated. This is shown by experiments with fish over circles. Regardless of the brightness of a circle 3 cm. in diameter beneath the fish, those 50 mm. in length failed to pale. When the illuminated circle was increased in size, fish paled readily. Paling, therefore, depends greatly upon the size of the white area seen by the fish. A Mazda lamp arranged below a fish so as to illuminate the upper region of the eye thus fails to induce paling because its image does not fall upon enough retinal receptors. Paling is more easily elicited when the lower region of the eye is not illuminated at the same time that the upper region is illuminated.

The ratio between the direct and the reflected light, known to exist with a white background below the fish, has been created with gray bottoms by illuminating them both from above and below. Gray backgrounds illuminated in this way have caused fish to pale readily. The shade assumed by Funduli, therefore, depends upon the ratio between the direct and the reflected light entering the eye.

As the percentage of reflected light to the upper region of the eye is increased, there is induced a proportional increase in the inhibitory reaction which causes a reduction in the degree of darkening of the fish.