FRESH-WATER MUSSELS. 



141 



that have discharged the glochidia quite early in the summer may already have the 

 brood pouches filled with eggs for the next season, while other mussels of the same spe- 

 cies are still retaining the glochidia developed from eggs of the past year. This fact is 

 obviously favorable to the work of artificial propagation, rendering it possible to obtain 

 glochidia of certain species of mussels at any time during the year. Thus in Lake Pepin, 

 a widened portion of the Mississippi River between Minnesota and Wisconsin, where the 

 Lake Pepin mucket or fat mucket is being propagated on a large scale by the Bureau, 

 a sufficient number of gravid mussels can be obtained for carrying on the operations from 

 the time they are commenced in May until they are terminated in October or November. 



In the case of the short-term breeders the breeding activities are restricted to a 

 season of about five months, from April to August, inclusive. The period of incubation 

 for any individual mussel of this class is undoubtedly very much shorter, although tem- 

 perature or other conditions may cause the period of incubation to be lengthened or 

 shortened. 



In Tables 1 5 and 1 6 there are listed the more common species of mussels with indi- 

 cation of the months in which females have been foimd with mature glochidia. The 

 lack of a record of gravidity may, of course, be due in some cases not to an actual gap 

 in the breeding season but to the want of opportunity for sufficient observation of the 

 species during a particular month. (See also Ortmann, 1909; Lefevre and Curtis, 1912; 

 and Utterback, 1916.) 



The commercial and noncommercial species are grouped in different tables, not 

 only because the records are more complete for the former but because those who are 

 concerned with the conduct or regulation of the mussel fishery will be interested almost 

 exclusively in the mussels of direct economic importance. 



Table 15. — The More Important Commerciai, Mussels, with Indication of Months During 

 Which Females Have Been Found with Mature Glochidia. 



75412°— 22- 



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