INFLUENCE OF LIGHT ON MIGRATIONS OF PELAGIC ANIMALS. ()7 



Investigations of Forel and others have shown that at a depth of about 400 meters 

 below the surface the intensity of the light is already so small that on a bright day 

 it hardly affects longer the most sensitive photographic plate. Thus the uauplii 

 which, under the influence of intense light, have become negatively heliotropic on 

 their migration downwards, very soon must reach a depth where the light is so weak 

 that they become positively heliotropic again, and necessarily must begin to move 

 upward. But as soon as they come back into stronger light they become negatively 

 heliotropic, and must go downward again, and so on. So it happens that these animals, 

 by the influence of the light, have to migrate periodically — during the night toward 

 the surface, during daytime downward, but no deeper than 400 meters below the 

 surface. 



Later on, at Woods Boll, I tried to tind out whether it was not possible to make 

 other pelagic animals at desire positively and negatively heliotropic. I found that 

 this could be done easily in copepods and larvae of Polygordius in different ways; for 

 instance, by the influence of the light itself. Direct sunlight made positively helio- 

 tropic larva? of Polygordius negatively heliotropic within one or two minutes, but 

 only as long as the temperature was above 10° O. In these experiments great care 

 was taken to keep the temperature of the animals constant. But when the tempera- 

 ture of the water in which the animals were kept was brought down to 7 degrees or 

 less the most intense sunlight was not more able to make the animals negatively 

 heliotropic. On the other side, at a temperature of about 30 degrees, the animals 

 remained permanently negatively heliotropic, even in the weakest light. I found 

 similar phenomena in copepods. These facts, it seems to me, must have some bearing 

 upon the depth migration of sea animals. At the surface of the Mediterranean, for 

 instance, the temperature rises to a considerable height in summer. The consequence 

 is that animals of a similar heliotropism, like the larva' of Polygordius or copepods, 

 can not more come to the surface even at night, for the high temperature at the 

 surface makes them negatively heliotropic even toward weak light, which in winter 

 time would make them move towards the surface. But on the other side it is clear 

 that these animals can not go down to the bottom of the sea. The temperature of the 

 water decreases with increasing depth, and as soon as these animals on their migra- 

 tion downward come to water which is sufficiently cool they become positively helio- 

 tropic again and now have to go upward; but this brings them back to warmer water, 

 where they become negatively heliotropic again, etc. The investigations made at the 

 zoological station at Naples have shown, indeed, that certain pelagic animals which in 

 winter come up to the surface during the night, in summer always remain at a certain 

 depth below the surface. These few examples may suffice to show how the light can 

 determine the periodical depth migrations of pelagic animals. 



It is unnecessary to say that it is impossible to exhaust this subject in so short a 

 time. But in order to prevent misunderstandings I must mention that I do not 

 believe that light is the only physical influence which determines depth migrations of 

 sea animals. 



As I have shown in former papers, gravitation, for instance, cooperates with light 

 to bring about these periodical depth movements of pelagic animals as well as the 

 constant distribution of sea animals. 



It has been found that the jelly-fish which had been carried by the Gulf Stream 

 from our latitude to the region of the midnight sun, continued to migrate to and from 



