52 ABIOLOGYOFCRUSTACEA 



given the choice of several seaweeds a young Hippolytc will choose 

 the one which its own pattern matches hest. This shrimp can also 

 change colour to a remarkahle extent, particularly at night, when, 

 no matter what the daytime pattern may he, the whole hody 

 becomes a beautiful translucent blue. When daylight returns the 

 characteristic daytime pattern of the individual is again apparent. 



Most crustacean colours are due to pigments. These are chemical 

 substances which absorb certain of the colours present in white 

 light and reflect others. The colour of a pigment is that of the 

 light it reflects or transmits. There are other ways of producing 

 colour, which depend upon light being split into its component 

 colours by various physical systems, such as thin films of certain 

 substances, or layers of transparent materials which bend the rays 

 of light to varying degrees. These structural colours can often be 

 recognised by the fact that they change according to the direction 

 of the light falling on the surface bearing the colour. In general, 

 structural colours are not very important in Crustacea, but they 

 can be seen, for instance, in the iridescent eyes of some species and 

 on the shiny hairs or bristles on some crustacean limbs. 



Some crustacean colours are, in a sense, accidental. The gut of 

 transparent species is often visible and is coloured by its contents, 

 particularly in plant-eating species, where it may be bright green 

 due to the accumulation of chlorophyll and its breakdown products. 

 The red colour of haemoglobin in the blood can be regarded in the 

 same way. The pigment is important in the physiology of Daphnia 

 and certain other Crustacea (p. 40), but the resulting red colour 

 has no function in concealment or advertisement; it is merely 

 incidental to the respiratory physiology of its possessor. 



Other pigments which appear to be accidents are the bile pig- 

 ments found in the roots of some parasitic cirripedes, such as 

 Peltogaster (see p. 116), and in the liver of an American crayfish 

 Cambarns. Bile pigments are formed from the breakdown of haemo- 

 globin in mammals and other vertebrates, but some of the Crustacea 

 in which bile pigments are found have no haemoglobin, so that 

 they may be formed in some other way, probably by the breakdown 

 of other haem compounds, such as cytochrome, which plays a part 

 in the respiratory processes within cells. Some ostracods accumulate 

 bile pigments in the wall of their gut. The pigment is only found 

 when the animals have been feeding on blue green algae, which 

 have bile pigments as part of their light-absorbing system. When 

 an ostracod with bile pigment in its gut wall is transferred to a diet 



