NO. 7 A NEW COPEPOU HABITAT WILSON 3 



there for any length of tin-ie. There must be plenty of food and the en- 

 vironment must be such as to allow the copepods more or less freedom 

 of locomotion. The former is readily explained by the presence in the 

 sand or mud of such organisms, especially diatoms, as ordinarily serve 

 for copepod food. This would constitute a sort of cold storage supply 

 amongst which the copepods could browse with much less danger from 

 outside interference. But can the copepods move about in the sand or 

 mud with anything like freedom of locomotion? Consider the sand 

 first. 



What is commonly designated as sand may be derived from several 

 sources^ and its constituent grains may vary greatly in size, with con- 

 siderable resultant differences in the sand itself. If derived from the 

 geologic weathering and erosion of crystalline rocks, the sand is made 

 up very largely of rounded grains of quartz. Such is the sand of 

 Cape Cod and the Maine coast, and it cannot be compressed sufficiently 

 to obliterate or even greatly diminish the interstices between the grains. 

 These open spaces make an ideal forage ground for copepods small 

 enough to move about within them, and there is little danger of being 

 crushed. Such sand always contains copepods even on exposed beaches 

 like those of the south shore of Marthas Vineyard, where a heavy 

 surf breaks almost continuously. Such sand also frequently collects 

 in the tide pools along the Maine coast and often contains a good 

 assortment of copepods. One pool at Sea Wall on Mount Desert 

 Island, about the size of a small room, yielded more than 20 copepod 

 species, including calanids, harpactids, and cyclopids. 



If the sand is largely made up of broken shells, as it often is in 

 the Tropics, its grains are not spherical but more or less flattened, and 

 when the flattened surfaces come together, which is the usual tendency, 

 the interstices are entirely obliterated. Any minute organism that 

 tried to live in such sand would be in constant danger of being crushed. 

 This kind of a sand beach never contains copepods, and the bathing 

 beach on the eastern shore of Mount Desert Island just south of Bar 

 Harbor is an excellent example. Two-thirds of the sand of that beach 

 is broken shells, and it is the only sand beach examined on the island 

 that yielded no copepods. 



A third source of sand is coral disintegration, and this is the preva- 

 lent kind of sand beach everywhere in the Tropics. The coral rock 

 is so soft that the resultant grains tend to become extremely small and 

 to vary considerably in size. Here again the interstices between the 

 grains can be practically obliterated by pressure, and if any are left 

 they become so small and irregular as to be uninhabitable. Only rarely 



