DISTRIBUTION 1 29 



In the coral reefs the crab family Xanthidae is particularly well 

 represented; Carpilius maculatus is a widespread example of this 

 family, being found in South Africa, throughout the Indian Ocean 

 and over a large part of the Pacific. This crab is buff or orange in 

 colour, with eleven large blood-red spots on the carapace. In the 

 Barrier Reef of Australia it is called the Barometer Crab because of 

 a local belief that the spots alter in density with the weather (see 

 p. 60 for the possible truth of this). Some of the Crustacea become 

 intimately associated with the corals and live in galls which they 

 cause the corals to form (see p. 98). 



Mangrove swamps provide shelter for a great variety of mud- 

 burrowing crabs and shrimps. Related species often inhabit different 

 parts of the swamp : Sesarma taeniolata is found higher up the 

 shore than S. bataviana in the mangroves of the East Indies. Other 

 crabs, of the genus Uca, are often very abundant among the roots 

 of mangrove trees; and burrowing shrimps, such as Thalassina and 

 Callianassa are sometimes present in large numbers so that the 

 whole surface of the mud is pitted with their burrows. 



The warm-water shelf has three major divisions: Indo-West Pacific, 

 East Pacific, and Atlantic. Oddly enough the last two have more in 

 common than the first two. The vast expanse of deep water in the 

 Pacific has been an effective barrier between the two shelves to the 

 east and west. Some shelf Crustacea are circumtropical and are 

 found in all three areas. One such crab is Grapsus grapsus, but it is not 

 really a typical example of a shelf animal because it often clings to 

 floating weeds and turtles and so becomes widely distributed. 



Quite a high percentage of the larger Crustacea are very widely 

 distributed in the Indo-West Pacific region : thirty per cent of the 

 crabs found in the Red Sea are also known from Hawaii. Lupa 

 pelagica is a typical example of a widespread Indo-Pacific species, 

 ranging from the east coast of Africa to Japan and New Zealand, 

 and is one of the species that has migrated from the Red Sea 

 through the Suez Canal into the Mediterranean. 



The similarity between the warm-water shelves of the two sides 

 of the Americas can be illustrated by reference to certain crab 

 genera. About 18 per cent of all the crab genera of America are 

 found on both sides of the continent. Mithrax, for instance, has 2 1 

 species on the Atlantic side and 1 1 on the Pacific side. The two 

 sides have however been separated long enough for most of the 

 species to be different. Often one can find species pairs, which are 

 closely allied species of the same genus occurring on opposite sides 



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