464 CLARKE AND DENTON [CHAP. 10 



lower layers of the sea, where the currents differ from those at the surface, may 

 help to distribute the animals horizontally. Russell (1927) and Gushing (1951) 

 have made important reviews of the very extensive literature dealing with this 

 subject and have particularly well described the work done on animals living 

 in coastal waters. Typically animals are thought to be deepest in the daytime, 

 to ascend at dusk, to sink or become more scattered during the night, and to 

 rise again at dawn before descending for the new day. One hypothesis is that 

 animals simply move so as to remain at an optimal illumination. The night 

 "fall" could then be explained by the animals moving more randomly when the 

 light intensity is very low and below the optimal value at all depths. This is 

 a useful but generally too simple hypothesis. Clarke and Backus (1956) have 

 found that deep scattering layers (the sound is almost certainly scattered by 

 animals) sometimes rose before sunset at a rate greater than that needed to 

 remain in a region of constant light intensity ; sometimes these animals moved 

 into light one hundred times brighter than that existing at the level which 

 they had kept in the middle of the day. Again, Southern and Gardiner (1932), 

 studying crustaceans in Lough Derg, have found that an upward movement 

 brought about by the disappearance of light after sunset may be continued in 

 complete darkness. We may emphasize, however, that although other stimuli, 

 e.g. pressure and temperature, may affect the vertical migrations of animals 

 (Moore, 1958), these are more closely correlated with the strength and rate of 

 change of penetrating daylight than with any other stimulus. 



There have been vigorous attempts to account for the complex behaviour of 

 animals in natural light in terms of a few simple responses. After some early 

 difficulties about nomenclature, the following terms which are taken from 

 Fraenkel and Gunn (1961) (largely after Kuhn) are now widely used and their 

 definition will give a good idea of the kind of analysis which is attempted. 



The term phototropism is now restricted to the bending responses of sessile 

 animals and plants. The term photokinesis is used to describe a response in 

 which velocity and rate of change of direction of an animal are changed with 

 change in intensity of light regardless of the direction from which the light 

 comes. Photokinesis can lead to apparently directional behaviour and we may 

 give as a simple example that of an animal which is stationary in a dim light 

 but moves quickly in a bright light ; such an animal will, by purely random 

 movements in the light, eventually find a dark place where it comes to rest. 



Responses by which animals can orientate themselves with respect to the 

 direction of the light are known as phototaxes. These may be subdivided as 

 follows : klinotaxis when an animal with directionally sensitive receptors 

 makes bending movements, successively samples the light reaching it from 

 different directions, and so can orientate itself with respect to the direction of 

 the light ; telotaxis when an animal can move directly towards or away from 

 one of a number of light sources ; and tropotaxis when a bilaterally symmetrical 

 animal moves to maintain an equal stimulation of paired receptors. Two other 

 taxes have been given the special names of dorsal light reaction and compass 

 reaction. In the first of these, an animal tends to orientate itself with its dorsal 



