COPEPOD PARASITES AND MUSSEL GLOCHIDIA ON FRESH-WATER FISHES. 37I 



of fresh water are relatively seldom the prey of parasitic Eucopepoda under natural 

 conditions" (p. 828\ And he adds: "A study of the literature of the subject confirms 

 our conclusion." 



His judgment is based upon the results of this African expedition, during which he 

 says very large numbers of fish were examined, but only two of them were found infested 

 with eucopepods. 



While such a conclusion seems inevitable from the data he has given, it must be 

 understood as applying to Africa, and perhaps to that portion covered by these Tan- 

 ganyika expeditions and not to the world at large. There has been very Uttle work 

 done on the parasites of fresh- water fishes, as has been already shown (p. 333), and no 

 one can say what the future holds in store. It is possible that other portions of Africa 

 are richer in these parasites, and it is certain that the results of the present investigation 

 are not essentially inferior to those obtained from salt-water fishes. It has already been 

 stated (p. 341) that a fish's efficiency as a host may be measured either by the number 

 of any single parasite it harbors or by the variety of species. If we are comparing 

 fresh-water fish with salt-water fish, or the fish from one region in the world with those 

 from another region, we should take into account both the number and the variety. 

 In variety of forms the salt-water fish considerably surpass those from fresh water, but 

 in number of specimens the latter sometimes surpass the former. The present author 

 never has obtained any salt-water fish that could compare with the two crappies in 

 numbers of parasites. Furthermore, in the variety of species found upon any single 

 kind of fish the fresh-water fish present an average fairly comparable with those from salt 

 water. Three and four from the same fish are the general rule rather than the exception. 

 (See table, p. 338.) 



And if we were to include the mussel glochidia and all other kinds of gill parasites 

 with the copepods, the salt-water fish would be hard pushed for a victory. Not many 

 salt-water fish can compare with the crappie (P. annularis) , which harbors 1 3 species of 

 glochidia, 3 species of copepods, and 3 species of trematode ectoparasites, 19 in all; or 

 with the sheepshead, which acts as the host of 1 1 species of glochidia, 2 species of trema- 

 todes, and two of copepods, 15 in all; or with the sauger, upon which have been found 

 6 species of glochidia, 2 species of trematodes, and 4 species of copepods, 12 in all. And 

 it must be remembered that these are all natural infestations, which have occurred under 

 perfectly normal conditions. When we come to the abnormal conditions which are 

 favorable to the copepod parasites, then their numbers increase to such a degree that 

 they cause serious epidemics in Ihe breeding ponds and often kill off large numbers of 

 the fish; and since it is fresh-water fish only that are bred in this way it follows that 

 this sort of damage is confined to them and does not occur amongst salt-water fish. 



The facts presented in the present paper open up a very fascinating chapter in the 

 bK)ok of copepod parasitology, and one that bids fair to become far-reaching in its prac- 

 tical relations; but it must be remembered that we have as yet scarcely made a begin- 

 ning, and that a vast amount of work is still to be done before we can reach a final solu- 

 tion of the problems. From the facts here presented, however, it would seem as if fresh 

 water presented fully as rich a field to the parasitologist as can be found in the ocean. 



