FRESH-WATER MUSSELS. I51 



Before taking up the history of the mussels in independent juvenile life, we must 

 discuss the very significant facts which have been discovered concerning the special 

 relation between mussel species and fish species, and refer also to the rare instances known 

 of mussels which complete their development without the aid of fish. 



HOSTS OF FRESH-WATER MUSSELS. 



As has previously been indicated in a general way, mussels do not attach to fish 

 indiscriminately, but for each species there is a restricted choice of hosts. Some are 

 more catholic in their tastes than others, yet for any mussel there is a limited number 

 of species of fish upon which it will attach and complete its metamorphosis. The Lake 

 Pepin mucket has nine known hosts, while the niggerhead has apparently but one; 

 the yellow sand-shell is restricted to gars, and the pimple-back to catfishes. It is, of 

 course, employing language in a loose sense to refer to this selection of hosts in terms of 

 taste or choice; it is a matter of physiological reaction. When fish and glochidia are 

 artificially brought together, glochidia will sometimes attach to the wrong fish, but in 

 such cases they soon drop off, or even if partial or complete encystment ensues, the glochi- 

 dium does not develop normally and after a time cyst and glochidium are sloughed off 

 and lost. It seems evident, then, that successful encystment and development depend 

 upon appropriate reactions on the part of both glochidium and fish, and that failure 

 ensues upon the lack of a favorable reaction on the part of either parasite or host. The 

 reaction may depend in part upon the condition of the individual glochidium or fish, but 

 primarily it def>ends upon the species of mussel and the species of fish. 



It is evident that the artificial propagation of mussels can not be conducted success- 

 fully and economically unless we have accurate knowledge of what species of fish serve 

 as hosts for the several species of mussels. Such knowledge has been gained by following 

 two methods of inquiry, the observational and the experimental. 



By the observational method, fish taken in the rivers are subjected to careful 

 examination for the presence of glochidia on the g^Us or fins. Preliminary to and 

 attendant on such studies, glochidia have been taken from as many species of mussels as 

 could be found in gravid condition, these have been studied with the microscope, meas- 

 ured, and figured, so that in most cases the species of mussel can be identified in the 

 glochidium stage as well as in the adult. (See text figs. 9 to 12.) This method of deter- 

 mining the natural hosts is exceedingly laborious. Infection in nature is a matter of 

 chance, and only a small proportion of fish bear infections. If it were otherwise, artificial 

 propagation might not be necessary. One must, therefore, examine large numbers of fish 

 from different localities and at different seasons, and even then the glochidia of some 

 species may not be encountered, or they may not be found upon all the hosts to which 

 they are adapted. During the calendar year 1913, for example, 3,671 fish of 46 species 

 were examined for natural infections principally during the warmer months from April 

 to October. Of these, 324, or 8.9 per cent, were found to be infected with glochidia of 

 some species, but only 104 of these, or less than 3 per cent, were infected with glochidia 

 of commercial species of mussels. The fishes infected with commercial mussels belonged 

 to 12 species, and the glochidia represented 20 species. The average number of glochidia 

 of a given species on infected fish ran from i to 416, with a mean of 125.° 



o In August, 1912, s examples of the river herring were taken and found to bear glochidia of niggerhead mussels in numbers 

 ranging from 1,89s to 3,740 per fish (Surber, 1913, p. no). Similarly, heavy infections are frequently foimd on the fresh-water 

 drum, but the glochidia are not usually those of commercial mussels. 



