VERTICAL MIGRATION RECONSIDERED 34I 



the more congenial environment of the lower layers, then the more abundant the food 

 in the upper layers the less time need they stay there; similarly the poorer the food the 

 longer would they have to remain to gain sufficient sustenance. The food supply might 

 be so poor that the animals might have to remain in the upper layers for the whole 

 twenty-four hours. Clarke (1932) provides important experimental evidence in favour 

 of this view from his study of fresh-water Daphnia. He writes : 



If the culture medium is allowed to "run out" or to become too dilute, it is found that the 

 majority of adult Daphnia exhibit a positive phototropism, whereas when the culture medium is 

 fresh and rich most of the animals are primarily negative to light. 



It is well known that in the daytime the young stages of planktonic animals, 1 par- 

 ticularly Copepoda, tend to be higher in the water than the older and adult stages (cf. 

 Russell, 1927a; Clarke, 1933; Gardiner, 1933, 1934; Nicholls, 1933). Again for the 

 fresh-water Daphnia Clarke (1932) shows that the young "almost always exhibit a 

 strong positive phototropism which becomes progressively weaker as the animals grow 

 older"; the adults being usually primarily negatively phototropic. Young growing 

 animals will need a more constant supply of food than will the adults, so that they may 

 remain altogether in the upper layers, or may remain there longer than the older 

 specimens. 



Thus the food requirements of the animals may be a factor governing their depth 

 distribution in relation to varying degrees of phytoplankton density. The condition of 

 the animal, whether it is well fed or starved, may determine the phototropic sign of the 

 animal, or in other words whether the animal is trying to reach the more illuminated 

 upper layers for feeding. It is a common observation that from a sample of animals 

 caught in a tow-net some individuals may be negatively phototropic, and others of the 

 same species positively phototropic ; there may be those which have had sufficient food 

 and those which require yet more. Likewise we find that not all the individuals of a 

 species come to the surface together; there are often a good proportion left below, at 

 times sharply separated from those which have ascended (see Figs. 107 and 114). Some 

 individuals in areas of rich phytoplankton may have been able to obtain sufficient food to 

 enable them to remain below for two days. If this were so, then, by the differing speeds 

 of the water layers, these animals would be carried more quickly away from the denser 

 phytoplankton. In connection with this ecological conception of vertical migration 

 being a journey from and return to a more congenial environment for purposes of 

 feeding, mention should be made of the more mechanical ideas and experimental work 

 of Eyden (1923) suggesting that changes in the specific gravity of the animal due to 

 feeding in the upper layers must be taken into consideration when investigating the 

 causes of vertical migration. 



If the phytoplankton is a closely grazed crop, as Harvey's results indicate, then the 

 withdrawal of the herbivores from the pastures during the hours of photosynthesis must 

 be an important factor in maintaining the yield of the crop. Gran (1933) at Woods Hole 



1 Velella is an interesting exception. 



nxi 44 



