86 A BIOLOGY OF CRUSTACEA 



attempts to attack the abdomen through the shell, clearly indicating 

 that it could see the vulnerable abdomen of the hermit. 



Some Crustacea can distinguish between various colours. This is 

 perhaps best shown in Hippolytc varians, whose choice of a match- 

 ing background has already been described (p. 51). Responses to 

 light of different colours are also shown by Daphnia : in red light 

 most of the individuals ' dance ' more or less upright in the water, 

 and do not move about much in a horizontal plane. Under blue 

 light most individuals tend to swim horizontally. This behaviour 

 is not constant, because the responses can be over-ruled by hunger; 

 starved Daphnia swim horizontally no matter what the colour of 

 the light may be. When abundant food is available the water fleas 

 perform their red dance even in blue light. 



These reactions are clearly an aid to keeping Daphnia in an area 

 rich in planktonic green algae, which form its main food. The 

 light transmitted by such algae has had most of the blue removed, 

 so that the water fleas will do their red dance in such areas, and not 

 move much in a horizontal direction. The change to horizontal 

 swimming in blue light helps the creatures to explore a wide area 

 when food is sparse. 



Perhaps the most remarkable feature of crustacean behaviour is 

 the diurnal migration which many of the planktonic species under- 

 go. In its typical form this consists of a movement towards the 

 surface at dusk, a slow irregular downward movement during the 

 night, movement up again towards the surface at dawn, and a 

 marked movement away from the surface as the sun becomes 

 brighter. In some species it has been shown that light intensity 

 is the main governing factor, and in Daphnia magna two distinct 

 mechanisms have been found. One mechanism is mediated by the 

 compound eye, and involves swimming away from intense light 

 and towards dim light. The direction of the light is important in 

 this response. If D. magna is lit from below it can be made to swim 

 upwards towards the surface by a bright light, thus reversing the 

 usual sequence in Nature. The other mechanism does not involve 

 the compound eye, but may involve the ocellus. This second 

 mechanism is not related to the direction of the light, but to its 

 intensity. As light intensity increases from darkness there is at first 

 an increase in swimming rate, but when the light becomes brighter 

 swimming activity is decreased. If specimens of D. magna have the 

 compound eyes removed, and are then lit from below, they sink 

 downwards in bright light. Their responses are identical with those 



