530 THE BIOLOGY OF MARINE ANIMALS 



fiddler crab by altering the timing of illumination (e.g. advancing the time 

 of illumination six hours). Such a rhythm, six hours out of phase, may 

 persist for several days in constant darkness. But repeated exposures to 

 illumination by night and darkness by day are necessary to obtain reversal 

 of the chromatophore rhythm. 



The basic centre responsible for the chromatophore rhythm of Crustacea 

 retains its inherent periodicity even in the absence of rhythmic chromato- 

 phore movements. This factor becomes apparent when fiddler crabs, in 

 which periodic chromatic movements have become weakened by pro- 

 longed illumination, are returned to darkness. The single change, from 

 constant light to darkness in such animals, causes the reappearance of 

 regular chromatophore fluctuations, and the time of change determines 

 the time of occurrence of a given phase in the re-established rhythm. 



Supplementary to the diurnal chromatic rhythm there is a persistent 

 tidal rhythm with a periodicity of 12-4 hours in crabs (Callinectes, Ucd). 

 The simultaneous occurrence of these two rhythms, diurnal and tidal, 

 results in semilunar cycles having a frequency of 14-8 days, at which inter- 

 vals the diurnal and tidal rhythms are in the same phase relative to one 

 another. Under constant laboratory conditions the tidal rhythm maintains 

 phasing which bears a definite relationship to times of high and low tides 

 in the normal habitat (16, 17, 19, 21, 22, 27, 32, 72, 73). 



FUNCTION OF COLOUR CHANGES 



The biological significance of colour changes in adapting the animal to 

 its background has already been discussed in this and the preceding chap- 

 ter. The response may take the form of alterations in tone or shade, colour 

 and pattern, as in many decapod Crustacea and fish. In cephalopods and 

 teleosts there are many species in which colour changes have a disruptive 

 effect in breaking up the body outline and are suited to particular environ- 

 ments. Disturbance and excitement also evoke a range of colour patterns 

 in these animals, and it is suggested that the colour changes serve to dis- 

 tract or confuse predators or prey (Figs. 11.10 and 11.12). Moreover, it 

 appears that in some species, individual animals in particular colour 

 phases are able to choose backgrounds to which they bear colour resemb- 

 lance. The chameleon-prawn Hippolyte varians exhibits manifold alterable 

 colour phases, green, brown, reddish and other tints, and can also assume 

 patterns suitable for different backgrounds. Under experimental conditions 

 it was found that certain colour varieties selected particular coloured plants, 

 thus green prawns favoured green Zostera, brown ones brown Halidrys 

 and Dictyota, and reddish specimens red Gigartina and Griffithsia. 



Experimental verification of the effectiveness of colour changes in con- 

 cealing fish from predators is available in the work of Sumner (65). This 

 investigator employed mosquito fishes Gambusia patruelis, a fresh- and 

 brackish-water species. Specimens were placed on white or black back- 

 grounds for seven weeks to bring about morphological colour changes and 



