THE REACTIONS OF THE PLANKTONIC COPEPOD, CENTROPAGES TYPICUS, TO LIGHT AND GRAVITY

¹ From the Department of Physiology, McGill University, the Biological Laboratories, Harvard University, and the Woods Hole Oceanographic Institution

Experiments on phototropism and geotropism in adult female Centropages typicus were conducted. The following conclusions were indicated:

A Experiments with experimental tube horizontal.

1. The copepods are primarily photopositive and constant exposure does not modify this reaction except at very high intensities in the neighborhood of that of bright sunlight (ca. 12,000 foot-candles) when a large number exhibited negative phototropism after continual exposure for about an hour.

2. The lowest intensity at which there were always more copepods in the brighter than the darker half of the tube was ca. 0.005 f.c.

3. There are two types of individuals. One type, after continuous exposure to light, becomes indifferent. In the other type, the animals are persistently attracted.

4. Decrease in light intensity, at a variety of rates and at a wide range of intensities, always results in increased attraction.

5. Increase in light intensity, at a variety of rates and at a wide range of intensities, has no effect on the behaviour. Only prolonged exposure at high intensities repels the animals.

B. Experiments with experimental tube vertical.

1. With the light from above the animals stay mainly at the top of the tube through a wide range of intensities, a distribution which is probably the result of positive phototropism, negative geotropism, or both.

2. Increases in intensity have no effect on the animals except when sunlight is used. A fair percentage of the animals is then repelled.

3. With the exception of decreases as low as 10 per cent per hour, decreases in intensity result in increased attraction.

4. The animals are strongly negatively geotropic in darkness. When geotropism and phototropism are opposed, the reactions depend upon the intensity of the light.

5. The possible bearing of these conclusions on the vertical distribution and diurnal vertical migration of adult female Centropages typicus is discussed.