140 BULLETIN OF THE BUREAU OF FISHERIES. 



With such species as have all four gills, or the entire outer gills serving as marsupia, 

 the sexes are scarcely, if at all, distinguishable from an examination of the shell; but 

 when a distinct portion of the outer gill is used as a brood pouch there is usually a pro- 

 nounced inflation of the shell over the region of the marsupia, so that the female mussel 

 is clearly marked on the exterior. (See also Grier, 1920.) 



It is to be remaiked that the eggs packed into the water tubes or marsupial cham- 

 bers do not usually remain free of each other, but become either attached together by 

 their adhesive membranes or else embedded in a common mucilaginous substance. When 

 the eggs or glochidia are removed from the gills they do not separate from one another 

 unless fully ripe, but remain in large masses which conform to the shape of the tubes 

 from which they have been removed" (PI. XIV, figs. 8-1 1). It occurs frequently 

 when gravid mussels are disturbed that the eggs, in whatever stage of development they 

 may be, are aborted or discharged into the water. This not infrequently happens in 

 aquaria, and doubtless may occur in nature. Abortion is presumed to be due to a de- 

 ficiency of dissolved oxygen in the water; the mussel, beg;inning to suffocate, discharges 

 the eggs in order to employ its gills more effectively for respiration. 



SEASONS OF DEPOSITION OF EGGS. 



We must distinguish with fresh-water mussels the seasons when eggs are matured, 

 passed out of the body, and deposited in the marsupial pouches from the season when 

 the developed glochidia are cast out into the water. The term "spawning season" 

 might be misleading, because it is commonl}' used to refer to the occasion when the 

 glochidia are discharged to the exterior, and this may be weeks, months, or some- 

 times nearly a year after the eggs are actually extruded from the reproductive organs 

 and the young are launched into existence. In general, the deposition of eggs — the 

 actual spawning process, scientifically speaking — occurs with the long-term breeding 

 class (see below) in the latter part of the summer or early fall. In the short-term 

 breeding class spawning usually takes place in June, July, or August, although in one 

 or two species it is known to occur as early as April. One mussel, the washboard, 

 deposits eggs only in the late summer and early fall, August to October. 



It is the experience of the Fisheries Biological Station at Fairport that the spawn- 

 ing seasons of mussels fluctuate to some degree in different years, no doubt because the 

 ripening of mussels is affected by varying conditions of water temperature. There are 

 also, of course, some differences of breeding season corresponding to differing climatic 

 conditions in more northern or more southern waters. 



SEASONS OF INCUBATION OF EGGS. 



Generally speaking, fresh-water mussels may be divided into two classes with re- 

 spect to their breeding seasons — -the long-term breeders and the short-term breeders. 



In the case of the long-term breeders the eggs are fertilized during the middle or 

 latter part of the summer and, passing into the brood pouches, develop into glochidia, 

 which are usually matured by fall or early winter. The glochidia may pass the entire 

 winter in the brood pouches, to be expelled during the following spring and early summer. 

 As might be expected, there is some overlapping of successive breeding seasons; females 



° Exceptions to this rule are noted by Ortmann (1911, p. 299). In sucli cases (the (;enera Anodonta, Anodontoides, Sjin- 

 phynota, and Alasmidonta) the eggs or glochidia are entirely separate from one another and flow out freely when the ovisac is 

 opened. 



