THE FAUNA ASSOCIATED WITH THE CRINOIDS OF A 

 TROPICAL CORAL REEF: WITH ESPECIAL REFER- 

 ENCE TO ITS COLOUR VARIATIONS. 



BY F. A. POTTS, M. A. 



INTRODUCTION. 



Although so much attention has been devoted to the phenomena of 

 mimicry and protective resemblance displayed by land animals, in only 

 one case has the colour resemblances of a marine animal been exhaus- 

 tively studied. I refer to the classical instance of Hippolyte varians, 

 illustrated by a long series of ingenious observations made by Gamble 

 and Keeble. Briefly stated, the story is as follows: The young Hippo- 

 lyte is free-swimming and colourless, but it becomes virtually a seden- 

 tary animal, anchoring itself to a seaweed or hydroid in the Laminarian 

 zone, on which it finds both food and shelter. The prawn has the 

 power of forming red, yellow, and blue pigments and by altering their 

 relative proportions in the chromatophores it can acquire a green, 

 brown, blue, or red ground-colour, and is thus able to adapt itself to 

 the varied colours of the seaweeds and hydroids. The pigment may be 

 laid down in longitudinal stripes or horizontal bars and in this way a 

 colour scheme can be formed matching whatever seaweed the prawn 

 shelters in. In early life a change in habitat is followed by a readjust- 

 ment of the pigment altering the colour scheme, but this power is 

 soon lost. 



There are, however, a great number of cases where species of small 

 marine animals are associated with an environment not varying, as in 

 the case of Hippolyte, but definitely fixed for the species for instance, 

 some particular kind of sedentary animal, sponge, alcyonarian, or 

 crinoid, as the case may be, which it frequents for shelter and commonly 

 resembles in colour. Sometimes the first is definitely a parasite on the 

 second, as in an example of the phenomenon often noted at Murray 

 Island, where the bright blue starfish Linckia Icevigata, so widely spread 

 on the Indo-Pacific reefs, was a source of food to multitudes of tiny 

 copepods (Linckiomolgus cceruleus Stebbing), whose colour exactly 

 matches that of the host, though the pigment is of a different chemical 

 nature. 



7:3 



