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ADAPTATION TO ENVIRONMENT AS ILLUSTRATED 

 BY A SURVEY OF ANAHEIM SLOUGH 



In few groups of aiiinials is there such a close eorrelation between 

 the habits, the structures and the physical conditions under which 

 they live as is found in tlie l)ivalves. Being in most cases fixed or 

 sedentary they must meet and satisfy the conditions of their immediate 

 environment or they can not survive even for a day. This fact was 

 pointed out by Thompson in his work on the British Columbia shell- 

 fish* and has proved equally clear in the present work. Not only does 

 this striking adaptation appeal to every one interested in living things, 

 and to whom the clam usually appears as an unresponsive obstructionist 

 opposing its shell to all outside influences, but a knowledge of these 

 relationships enables the clam digger, amateur or professional, to 

 know where and how to find his supply, even in regions which are 

 new to him. 



We will first consider in a little more detail the basis of this nice 

 adaptation to the physical surroundings and then attempt to put the 

 facts in a more tangible form by studying the distribution in a typical 

 bay. The primary thing with clams as with other animals is their 

 relation to the food supply. For all bivalves this is the same — the 

 microscopic animals and plants that abound in sea w^ater. Of these 

 the most important are the diatoms, tiny plants with beautiful shells 

 of silica, so small that only giants among them may be seen with the 

 naked eye, yet so numerous that in past geologic ages great beds 

 thousands of feet thick have been laid down of their remains. The 

 clam draws through its mantle cavity, by means of the current set up 

 by the cilia lining it, a current of sea water; whatever organisms the 

 w^ater may contain are strained from it and used for food by the 

 clam. The oxygen needed by the clam is also obtained from this stream 

 of water. It is therefore clearly evident that the clam must have free 

 access to the water. But tliose positions that assure a good supply of 

 water are also those that expose the clam to the violence of the surf, 

 if on the ocean beach, and to attacks by enemies. The wdiole stor}^ of 

 its adaptation, then, is a recital of the ways that have been developed 

 of getting protection from waves and foes without sacrificing the 

 necessary supply of water. 



In regard to habits, bivalves may be divided into four groups. 

 They will be found eitlier fixed, free, actively burrowing or in rela- 

 tively permanent burrows. The mussel is a form found fixed to rocks 

 exposed to the full force of the surf, yet so firmly anchored by its 

 byssus of tough threads and protected by its firm shell that it seldom 

 suft'ers from the violence of the waves. Here it is safe from all 

 enemies that can not also weather the surf and is sure of an abundant 

 supply of fresh water, which it takes' into the open mantle cavity at 

 almost any place. Other fixed forms are the oyster, which is found in 

 more sheltered waters, the rock oyster and the rock scallop. 



Among the bivalves the truly free forms are few — as a whole the 

 group is not specialized for crawling or swimming, and those species 

 that are active enough to maintain themselves without burrowing or 

 attachment are the exceptioii. The scallop is, however, free living. It 



♦Report of the B. C. Commissioner of Fislieries for 1912 p. I 37, 1913 p. R 103. 

 2 — 6453 



