26 INVERTEBRATE PHYSIOLOGY 



If the compound eye of Daphnia magna is denervated and the animal 

 is allowed to recover from this operation the color dances are completely 

 missing. 



One might be tempted to conclude that Cladocera living in water with 

 humic stains would never blue-dance and might hence die of starvation by 

 never reaching food which might be nearby. However, such is not the 

 case, since the color-dance responses to incident wave lengths are over- 

 ruled by hunger or by the presence of food. For example, an animal which 

 is hungry will blue-dance in all wave lengths of the visible spectrum. If 

 food in the form of suspended photoplankton is now added to the hungry 

 population, there will be nothing but red-dancing observable no matter 

 what the incident wave length. 



Fig. 2 summarizes the situation with regard to color dances, showing 

 that these two behavior patterns are mediated by the compound eye which 

 has a receptor for long and for short wave lengths and showing that 

 hunger or the chemical stimulation of food can overrule the light stimulus. 



Vertical Migration 



Vertical migration is essentially a very complex combination of geotaxis 

 and phototaxis which is influenced by a number of parameters of the en- 

 vironment. In various marine and freshwater zooplankters it can be in- 

 duced by light intensity or wave length, pH, redox poising compounds, 

 temperature, or pressure. Sometimes all of these parameters are effective 

 in a single species, such as Daphnia magna (see Fig. 3), and sometimes 

 visible light has little effect, as for example certain marine zooplankters 

 of the Inland Waterway of Florida. Since gravity is a constant force in the 

 environment capable of serving as a behavior cue along with radiant en- 

 ergy which is less constant, it is not wholly unexpected that geotaxic and 

 phototaxic behavior patterns have evolved in response to those parameters 

 of the environment which have diffuse vertical gradients, as for example 

 light, temperature, pressure, pH, and redox potentials. The gravitational 

 field of force of the earth has been well exploited by the Cladocera and be- 

 havior responses to nongradient situations are oriented not to the stimulus 

 per se but to gravity. A nongradient situation has no spatial dimensions for 

 cueing an oriented response but the stimulus serves to set off a gravita- 

 tional response. When one considers thermal, chemical, or radiant energy 

 gradients of the environment in relation to the size of Cladocera, it is clear 

 that the change in intensity of the gradient over the length of the animal 

 is too small to be detected and resolved into directional information on 

 which to base an oriented taxis. Hence, we have behavior patterns like 

 geotaxes initiated by chemical or radiant energy stimuli. Radiant energy 

 sources, on the other hand, may be localized if the animal has a receptor 



