COLOURS 



5' 



flat so that when pressed against the sand it does not cast a shadow. 

 In this way the crab can apparently disappear. 



As a contrast to concealing coloration some Crustacea have 

 strikingly conspicuous patterns which act as warnings to their 

 predators. The best example is the African Land Crab, Sesarma 

 meinerti, which has a purple back bordered with orange, and 

 bright red chelae which it waves threateningly in the air. If caught 

 away from cover this crab does not flee in the way that the Ghost 

 Crab would, but stands its ground and will fight with considerable 

 pugnacity. The contrasting pattern and conspicuous chelae adver- 

 tise its powers of retaliation if attacked, and in fact it is rarely 

 attacked. There are records of an allied species successfully defend- 

 ing itself from the attacks of small dogs. 



The pattern of coloration often varies within a species, the Shore 

 Crab Carcinus maenas is particularly variable when young; patches 

 of green, yellow, white or red form a variety of patterns on the 

 backs of crabs about an inch across, but when the crabs reach 

 larger sizes they become more uniform in colour, with dark backs 

 and paler undersides. 



One of the most striking examples of variation in pattern and 

 colour within a species is found in the small shrimp Hippolyte 

 varians. A number of variants, all captured on the same day at 

 Brighton are illustrated in fig. 27. The young specimens in 

 particular show great variation in pattern, and as a rule the pattern 

 is effective in concealing the shrimp on a particular seaweed. If 



Fig. 27. Hippolyte varians (Decapoda Natantia, Caridea), showing 

 variation in colour and pattern. All the patterns have been 

 imposed on a standard outline. A, uniform pale green; uniformly 

 brown specimens were also found. B, transparent, with pattern 

 dominated by two longitudinal red lines. C and D disruptive 

 brown patterns. 



