THE REACTION OF CERTAIN CRUSTACEA TO DIRECT AND TO DIFFUSE LIGHT

¹ From the Woods Hole Oceanographic Institution and the Biological Laboratories, Harvard University, Cambridge

When the copepod Acartia tonsa is placed in a tall glass cylinder illuminated from above, the animal swims upward. When the cylinder is illuminated obliquely, the animal sinks downward.

Measurement of the light distribution inside the cylinder shows that the behavior of A. tonsa depends upon the angular distribution of the light. In highly directional illumination, the animal reacts phototropically, and swims toward the light. In less directional (diffuse) illumination, the animal stops swimming and sinks passively downward.

Measurement of the light distribution in the air and in the sea shows that it is generally more diffuse than the conditions in the cylinder. The reaction of A. tonsa to diffuse light in the laboratory accords with its downward movement in the ocean during the day. Its reaction in the direct light in which phototropism experiments are usually performed has no bearing on its behavior in nature.