34o DISCOVERY REPORTS 



Before proceeding to examine a further factor which might govern depth distribution 

 let us consider the general significance of the phenomenon of vertical migration. No 

 satisfactory explanation appears to have been given to account for it in terms of ad- 

 vantage to the organism. It is one of the commonest of pelagic phenomena, and is 

 found in nearly all groups of planktonic animals. Invariably 1 the animals migrate up- 

 wards at the approach of darkness and downwards at the approach of daylight. There 

 can be no doubt that the animals react in part to the stimulus of light. The photo- 

 tropisms of planktonic animals have long been established, and it is well known that 

 these tropisms may be changed from positive to negative under different conditions. 

 Whilst usually it has been held that the animals are reacting to certain optimum condi- 

 tions of light intensity, Clarke (1930) has shown experimentally that the fresh-water 

 Daphnia reacts to certain rates of change in light intensity rather than to any fixed 

 optimum intensity. Whilst it is desirable and necessary physiologically to study separ- 

 ately this and that factor to which the animal may be reacting, it is also desirable to look 

 at the problem as a whole from the ecological standpoint. In our survey the many 

 different animals which are making vertical migrations spend the greater part of the 

 twenty-four hours at depths below 50, 100 or even 200 m. The herbivorous animals 

 presumably come into the upper layers to feed on the richer phytoplankton, and the 

 carnivores to feed upon the herbivores concentrated in these upper zones. As these 

 animals move upwards into the phytoplankton zones they pass into a different environ- 

 ment, one more alkaline and richer in oxygen. Whilst we saw that the changes in alka- 

 linity and oxygen from station to station within the upper layers may not be very great, 

 the changes encountered in passing from depths below 100 m. to the surface may be 

 considerable. It seems possible that these animals find conditions of living more suited 

 to them down below than in the upper phytoplankton zones, and that they only make 

 diurnal excursions into the upper zones for feeding for a comparatively short time. If 

 conditions were just as suitable for them in the upper zones, where their food is, one 

 might expect that they would remain there. Their preference for living in the lower 

 zones may be due to the different chemical conditions of the environment such as those 

 just referred to or others unknown, or it might possibly be due to the greater protection 

 the darkness of the lower layers affords them against the attacks of predators. If they 

 remained in the upper illuminated layers during the daytime they might more easily 

 become the victims of attack. It would seem that vertical migration is primarily a 

 movement towards and away from the upper phytoplankton pastures. If the animals 

 usually avoid these upper zones because they find them chemically uncongenial, then 

 should the phytoplankton crop become exceptionally low we might expect them to be 

 able to remain longer in these zones, because the alkalinity, oxygen and other conditions 

 set up by the phytoplankton would also be reduced. We have considered such little 

 evidence as we have in this respect from our survey, and found it not very satisfactory. 

 Now there is another possibility. Suppose the animals ascend and remain up for just 

 so long as will allow them to get sufficient food to carry them over a further period in 

 1 With the possible exception of the amphipod Primno discussed on p. 198. 



