156 BULLETIN OF The BUREAU OF FISHERIES. 



found that some of the very large bass, having doubtless experienced some previous 

 natural infections, become immune after one heavy artificial infection, while small bass, 

 without previous infections presumably, require two or three artificial infections before 

 showing immunity. When immunity is acquired, the fish can not be successfully infected 

 with glochidia of any species of mussel. The period of duration of immunity is not 

 known. 



An earlier significant discovery had been made by C. B. Wilson (191 6, p. 341). His 

 observations and experiments showed that the fish which are most susceptible to glo- 

 chidia are those which are subject to parasite copepods (fish lice) ; that there is a definite 

 connection or fellowship of copepods and mussel parasites, so that knowing the species of 

 mussel for which a given species of fish serves as host, one may often predict what species 

 of copepod fish of that species will carry ; and finally, that the presence of glochidia on an 

 individual fish renders that fish practically or completely immune to the attacks of the 

 fish lice, and vice versa. These conclusions may be stated in another way: While 

 ■glochidia and copepods have essentially identical taste in fish hosts, the presence of the 

 one is antagonistic to the other. 



These observations indicate that artificial infection of fish with glochidia may have a 

 positively beneficial effect upon the fish in giving it protection against a class of parasites 

 which are pernicious in effect ; for copepods are relatively large parasites which sap the 

 vitality of fish and have been known to cause serious mortalities. 



The case of the sheepshead or fresh-water drum, Aplodinotus grunniens, may be sig- 

 nificant. Sheepshead are found to be almost invariably loaded with glochidia upon the 

 gills, carrying infections which would be regarded as highly excessive if caused artificially 

 (PI. XVI, fig. i). They are, no doubt, greatly exposed to infection in consequence of 

 the habit of feeding upon molluscs, which they are well fitted to crush with their strong 

 grinding teeth. By carrying successfully glochidia, which they secure while devouring 

 the parent mussel, they are aiding in the propagation of the mussel which may serve them 

 as food. Indeed, the sheepshead unwittingly engages in growing its own food supply. 

 Now, of the fish which have been examined in numbers, the sheepshead is the one species 

 of fish (besides those of the sucker family, which carry neither glochidia nor copepoda) 

 which has never been found to have copepods on the gills. Its immunity from copepods 

 is now easily understood, and it may be presumed that this immunity is worth the cost 

 of almost continually carrying heavy infections of glochidia. 



METAMORPHOSIS WITHOUT PARASITISM. 



So generally, almost universally indeed, are fresh-water mussels dependent upon fish 

 for the completion of their development, that peculiar interest attaches to the two ex- 

 ceptions which have so far been encountered. Lefevre and Curtis (191 1) discovered that 

 glochidia of one species, the squaw-foot, Strophitus cdcntitlus Rafinesque, may undergo 

 metamorphosis into the juvenile stage without the aid of the fish (PI. XVII, fig. 3). In 

 this mussel, as in others, the eggs when deposited in the gills are packed in a formless 

 mucilaginous matrix, but in the course of the development of the glochidia, the matrix 

 becomes changed into the form of many cylindrical cords, in each of which a few glo- 

 chidia are embedded. There is evidently in this case a special provision for the nour- 

 ishment of the embryo from materials supplied by the mother, so that metamorphosis 



