BEHAVIOUR 87 



of eyeless animals lit from above, and in marked contrast with the 

 responses of intact animals. The reactions of the eyeless specimens 

 account for the dawn rise to the surface. As the light gradually 

 increases they swim upwards, then when the light becomes strong 

 enough to inhibit swimming they sink down again. Other experi- 

 ments have shown that the colour of the light is important in these 

 responses. If the spectrum of the light is shifted towards the blue, 

 without any change in intensity it provokes active down-swimming: 

 shifting the spectrum towards the red, again without change in 

 intensity, can provoke up-swimming. In general, Daphnia is much 

 more sensitive to changes in the blue part of the spectrum than in 

 the red. 



Although the action of light is so clear it is by no means the only 

 factor involved. Changes in temperature can alter the strength of 

 the response to light. Dissolved carbon dioxide and other substances 

 can also influence such responses. The first experiment demonstrat- 

 ing this is reputed to be the occasion when the celebrated biologist 

 Loeb poured beer into his tanks of Daphnia and found that it 

 reversed their responses to light. 



Several Crustacea, such as the isopod Eurydice pidchra, and 

 larvae of the crabs Portunus and Carcinus are sensitive to changes 

 in pressure; they swim upwards when pressure is increased. 



The general impression gained from the various studies on 

 vertical migration is that the major features are controlled by light 

 intensity, but the details and fine adjustments are controlled by 

 other factors such as temperature, pressure and dissolved carbon 

 dioxide. 



Vertical migration in the sea can be very considerable; some 

 euphausiids migrate 200 metres up and down each day. A migration 

 of this magnitude can carry a crustacean into a layer of water which 

 is moving in a different direction from the one in which it started, 

 and it is thought that it may gain certain advantages from this. 

 Instead of drifting in one direction in a limited patch of water the 

 crustacean can change direction and sample a greater volume of its 

 environment. This may well be important if food becomes scarce; 

 if the crustacean drifted along in the same patch all the time it 

 might starve, but by migrating to a lower level moving in another 

 direction it can then come up again in a fresh patch which mav be 

 richer in food. 



Apart from such advantages the downward migration during the 

 day may be essential for some species, at least at some seasons. It 



