88 A BIOLOGY OF CRUSTACEA 



has for instance been shown that the copepod Calanus finmarchicus 

 is killed by prolonged exposure to bright sunlight. Tf this is so why 

 do not the Crustacea stay permanently in the lower layers? The 

 answer to this lies in the fact that it is in the upper layers of the 

 sea that food, in the form of minute planktonic plants, is most 

 abundant. It has been suggested that vertical migration is con- 

 trolled by feeding reactions. The crustaceans come up at night and 

 feed, then sink down when full, but a creature like Daphnia can 

 fill its gut in half an hour when food is abundant, and empty it 

 just'as quickly. The time scale of such events is of the wrong order 

 to be a dominant factor in controlling diurnal migration. Neverthe- 

 less starvation can act as one of the fine adjustments in the whole 

 complex; Daphnia tends to swim towards a light more when 

 starved than when it is well fed. 



One of the simplest reactions to light is that known as the dorsal 

 light reaction. A free-swimming crustacean, such as Triops, 

 normally keeps its back upwards towards the light. If a light 

 is placed below it, the creature turns over and swims back 

 downwards. A prawn, Processa caniculata, behaves in a similar 

 manner. 



Triops keeps its body on an even keel by balancing the light 

 received by the two compound eyes. If one of these is painted over 

 the creature swims in spirals, rolling over towards the functional 

 eye. The change to swimming upside down when lit from below is 

 started by the small median eye, which has a window through to 

 the underside of the head. Once Triops has turned over the two 

 compound eyes take over to maintain its balance. 



Artemia salina reacts to light in precisely the opposite way. The 

 normal swimming position is with the back downwards. If lit from 

 below it rolls or somersaults into the reverse position. 



As well as keeping their back to the light most Crustacea keep 

 their bellies towards the earth; they lie across the pull of gravity. 

 The organs which tell a crustacean which way up it is are called 

 statoevsts, and are usually found near the base of the antennules. 

 Statocysts are relatively simple in structure, usually just a rounded 

 sac lined with special sense cells which often have hair-like projec- 

 tions from their surfaces. One or more small stone-like structures, 

 the statoliths, lie inside the sac. The statoliths will naturally always 

 fall downwards towards the earth and will press on different groups 

 of cells according to the position that the crustacean takes up with 

 respect to gravity. The impulses set up by the movements of the 



