10— ON THE INFLUENCE OF LIGHT ON THE PERIODICAL DEPTH- 

 MIGRATIONS OF PELAGIC ANIMALS. 



BY JACQUES IX>EB, M. D., 



University of Chicago. 



As is now well known, the animal life in the ocean and of fresh-water lakes is 

 confined chiefly to two regions, one extending from the surface of the sea to a depth 

 of about 400 meters, the other being the ground region of the ocean. We know, 

 moreover, that a good many of the surface animals migrate periodically in a vertical 

 direction, coming up to the surface during the night and going downward during the 

 daytime, but not deeper than 400 meters. For the physiologist the question arises 

 what determines this peculiar vertical distribution and periodical migration of marine 

 animals. My investigations concerning the effects of light on the motion and orien- 

 tation of animals made it certain that this periodical depth migration of sea animals 

 is determined to a certain extent at least by the light. In order to make this clear I 

 must give a short sketch of the way in which the light determines the orientation and 

 motion of animals. 



You all know that many animals, like the moth, for instance, go towards the 

 light. It was believed that this was due to an attraction of the animals by the light, 

 or at any rate that these animals liked the light; other animals show the opposite 

 reaction, they go away from the light, and it was generally believed that these ani- 

 mals were fond of the dark. My experiments, however, showed that these reactions 

 are the outcome of a purely mechanical effect of the light upon the animal, and that 

 the animal, as a rule, is neither fond of the light nor of the dark. The light forces 

 the animal to orientate itself in such a way that its symmetry axis or symmetry 

 plane falls into the direction of the rays of light, and consequently all symmetrical 

 elements of the surface of the animal are met by the rays of light at the same angle. 

 There remain two possibilities: The animal in this orientation can turn either its oral 

 pole or its aboral pole toward the source of light. In the former case I have called it 

 positively, and in the latter case negatively heliotropic. 



In the case of animals which are fixed to the ground, like hydroids, for instance, 

 the only effect of the light consists in this orientation of the animals. But in the case 

 of animals which are free-moving, like insects or copepods, the animal is orientated 

 by the light, and is forced to move in this orientation. The consequence is that if 

 the animal is positively heliotropic it must necessarily move straight toward the 

 light. If the animal is negatively heliotropic it must move straight in the opposite 



65 



F. C. B. 1893 5 



