598 



MAN AS A CONQUEROR 



Shellfish 



What has been said with regard to the great destruction of fish is 

 equally true of shellfish. The oysters in Chesapeake Bay were 

 thought to be inexhaustible until they were almost exterminated, when 

 the state of Maryland found that in order to preserve this great 

 natural asset, oyster culture was necessary. Oysters are now con- 

 served here and in other states by cultivation. In certain areas of 

 shallow water old oyster shells, broken stone, or bunches of fagots 

 are placed on which the young, free-swimming larvae may attach 

 themselves. After these "seed oysters" have grown to a sufficient 

 size, they are removed and placed in beds in shallow water, where 

 they are later harvested. 



There are two general aspects of the question dealing with the 

 shellfish industry, namely, as it affects marine or fresh-water forms. 

 Among the problems of fresh-water biology those relating to the 



life history of the fresh- 

 water mussel should be 

 noted. 



It is known that the 

 propagation of fresh-water 

 mussels, the shells of which 

 are used in the pearl button 

 industry, depends upon 

 whether the larval stage, 

 or glochidium, finds the 

 proper temporary fish host. 

 This tiny larval mussel 

 attaches itself to the fins or gills of certain species of fish. The 

 host builds a protecting cyst wall about it, when it undergoes fur- 

 ther development. After reaching a sufficiently advanced stage, 

 it drops from its host, which by this time may have reached 

 quite a different locality, and continues its own battle for life. 

 The rivers of the mid-west, especially the Mississippi and Ohio 

 systems, are the great producers of fresh-water mussels. The con- 

 struction of dams and the addition of pollution to these streams have 

 killed off a large percentage of these mussels. Here the United States 

 Bureau of Fisheries came to the rescue and used the fish they were 

 salvaging from the back waters of the Mississippi system as hosts for 

 the glochidia of the mussels. While the effectiveness of this type of 



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Life history of fresh-water mussel. 



