494 THE BIOLOGY OF MARINE ANIMALS 



excretory cells beneath the cuticule, as the result of tissue changes and 

 metabolic activity associated with spermatogenesis (72). Sexual dimorph- 

 ism in coloration is associated with characteristic differences in the be- 

 haviour of the two sexes during breeding. Relatively few marine inverte- 

 brates have reached the mental level consonant with such complex activities 

 as courtship display. Sexual coloration has been reported in various 

 Malacostraca. Darwin described a species of Squilla from Mauritius in 

 which the male is a beautiful bluish green, with red on some appendages, 

 while the female is rather dull in colour. In certain fiddler crabs of the 

 genus Uca the males develop gaudily coloured chelipeds and ambulatories, 

 and a light carapace, whereas the females remain rather dark (9). 



Nuptial colours are especially characteristic of teleosts where the males 

 of many species become brightly coloured during the breeding season. 

 Wrasse (Labridae), in particular, show pronounced sexual dimorphism 

 in colour. The male cuckoo wrasse Labrus ossifagus is yellow or orange 

 tinged with red, and marked with blue bands and patches on body and 

 fins. The female is reddish in colour, without blue bands, but bearing two 

 or three large black spots on the back. In Halichoeres poecilopterus, a 

 common Japanese fish, both males and females are brightly coloured, but 

 the colour patterns of the two sexes are quite dissimilar (Fig. 11.8). The 

 adult female is bright reddish in colour, with dark longitudinal stripes, 

 whereas the adult male becomes bright brownish green at maturity (46). 

 Sexual colours of these kinds are utilized in nuptial displays leading to 

 spawning. 



Adaptive and Cryptic Coloration 



The adaptive significance of much animal coloration is now securely 

 established. It is apparent that many marine animals resemble in a general 

 way the colour of their environment and are suitably concealed. In the 

 open sea the transparent condition of the hosts of pelagic organisms — 

 coelenterates, polychaetes, gastropods, crustaceans, tunicates, larval 

 fishes — renders them difficult to detect. Cryptic coloration is common 

 among animals living in inshore waters, coral reefs and floating weed, 

 where every conceivable manner of concealing device and deceptive colora- 

 tion can be found. The drab greens, browns and greys of many shore 

 animals resemble the weeds and stones among which they shelter; for 

 example, Carcinus and Ligia. Very effective are the dull mottled tones 

 of many animals living on sandy and gravelly bottoms ; for example, the 

 skates Raja, pleuronectids and sand eels Ammodytes (Fig. 1 1.9). In contrast 

 with these sombre liveries are the reef fishes which are notable for the 

 brilliance and variety of their colouring. But even the bold hues and colours 

 of these animals can be considered as cryptic against the brightly coloured 

 background of coral reefs (57, 79). 



Adjustments of colours and tones to conform to different backgrounds 

 are quite remarkable in various higher animals. The physiological bases 

 of these changes are treated in the following chapter. Cott (8) describes 



