PART 2. LIFE HISTORY AND PROPAGATION OF FRESH-WATER 



MUSSELS. 



INTRODUCTION. 



The life histories of fresh-water mussels present features in striking contrast to those 

 of other familiar mollusks of our seas and rivers. The American oyster, the clam, the 

 quahaug, and the sea mussel cast the eggs out to undergo development while floating in 

 the water. The pearly mussels of rivers and lakes, on the contrary, deposit their eggs 

 in marsupial pouches which are really modified portions of the giUs, and there they are 

 retained until an advanced stage of development is attained. This particular feature of 

 breeding habit is not, however, unique to mussels. There are clams in coastal waters 

 that incubate the eggs in the gills, and the common oyster of Europe displays a similar 

 habit; but with all these the larvae when released are prepared for independent life. 

 Such is not the case with fresh-water mussels. When the larval mussels are discharged 

 from the marsupial pouches, the mother has done all that she can for them, but they 

 still want the services of a nurse or foster parent, as it were. Lacking the structure and 

 appearance of young mussels, they display a peculiar form designated as glochidium, 

 and (with few exceptions) they will not continue to live unless they become attached to 

 some fish, upon which for a certain time they will remain in a condition of parasitism. 



During the period of parasitic life the glochidium undergoes a change of internal 

 reorganization, or metamorphosis, with or without growing in size. After the change 

 is complete and a form somewhat similar to the adult is attained, the young mussel 

 leaves the fish to enter upon its independent existence. At this time, or soon thereafter, 

 some mussels, but not a great number, differ distinctly from the adult form in bearing a 

 long, adhesive, and elastic thread, or byssus, by which they attach to plants, rocks, or 

 other anchorage. 



The life history, then, comprises the following five stages: (i) The fertilized and 

 developing egg retained in the marsupial pouches of the mother mussel ; (2) the glochid- 

 ium, which, before liberation, is often retained for a considerable further period in the 

 gills; (3) the stage of parasitism on fish (or water dogs) ; (4) the juvenile stage, which may 

 or may not be marked by the possession of threads for attachment to foreign objects; 

 and (5) the mussel stage, with the usual periods of adolescence and maturity. 



Such in brief is the typical story of the life of a pearly mussel. And yet each 

 species of mussel, and there are many, has its own characteristic story, which differs in 

 more or less important respects from those of other species. One kind of mussel will 

 pass through the stage of parasitism only upon a particular species of fish, while another 

 kind acquires the aid of certain other fish. The diversity in life histories also manifests 

 itself in such details as in the season of spawning, in the part of the gills in which the 

 glochidia are carried, in the duration of the incubation period, in the matter of growth 

 in size during parasitism, and in many other particulars. There are even some mussels 

 which, like exceptions that prove the rule, undergo complete development without being 

 parasites upon fish at any stage. It is advisable, therefore, to treat the several stages 



