COPEPOD PARASITES AND MUSSEIv GI<OCHIDIA ON FRESH-WATER FISHES. 34I 



A careful reading of this table shows us : 



1. The fish which carry the copepods are also those which serve as hosts for the 

 glochidia. 



There are a few exceptions on either side — some fish, like the eel and the shovel- 

 nosed sturgeon, which carry only glochidia, and others, like the dogfish and the bull- 

 head, which carry only copepods. But these are simply the exceptions that prove the 

 rule, and we must also remember that not all the fishes in the list have been thoroughly 

 examined for both copepods and glochidia. Future investigations are very likely to 

 reduce these exceptions and possibly to eliminate them entirely. This is exactly what 

 would be expected, for the temporary parasite, the glochidium, is not so very different 

 in some respects from the permanent parasite, the copepod. The conditions which are 

 favorable to the one would favor the other also, and the conditions which are adverse to 

 the one would be adverse to the other. Hence we may go a step further and affirm : 



2. The species of fish which are ordinarily free from copepod parasites do not furnish 

 conditions favorable to infection by glochidia. 



The numerous species of buffalofish, carp, suckers, lampreys, minnows, shiners, 

 dace, chubs, and darters are excellent examples. The above table includes all the fresh- 

 water fish at present known to serve as hosts for either copepods or glochidia, and prac- 

 tically none of these fish appear in either list. Nor are they likely to appear in any 

 numbers, for these fish have been as thoroughly examined as any others, but nothing 

 has been found upon them. Lefevre and Curtis mention some of the mechanical factors 

 which tend to render a fish immune to infection by glochidia, such as the smallness of 

 the gill openings, the rapidity of the fin movements, and the texture of the gills. They 

 mention as the most striking instances of immunity the German carp, certain minnows, 

 and the darters, three of the above-named fish. By means of artificial infection they 

 exposed these fish to glochidia, a few of which fastened upon their gills and fins; but 

 these were quickly sloughed off, and none could be carried through the parasitic period. 

 "The disappearance of the hookless glochidia of Lampsilis from both gills and fins of 

 the carp * * * suggests rather that there may be some reaction of the host's tissues 

 comparable to the processes which confer immunity against parasitic bacteria in higher 

 vertebrates." (Lefevre and Curtis, Bulletin Bureau of Fisheries, vol. xxx, p. 163.) 



We can readily understand how an immunity of this character could operate against 

 the parasitic copepods as well as against the glochidia. Extensive examination in the 

 future may, and probably will, reveal straggUng copepods and glochidia, but in such 

 small numbers that they must be regarded as accidental infections. " 



3. The fish which make the best copepod hosts are also those which are naturally 

 infected with the greatest number and variety of glochidia. 



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. In the copepod parasites these two 

 criteria are usually separated and must be considered independently. In the mussel 

 glochidia they are nearly always united, and may therefore be treated conjointly. 



Keeping these facts in view, we notice first that the crappie, Pomoxis annularis, 

 stands at the head of both lists. It serves as the host of at least 13 species of mussel 

 glochidia, and yields often as many as 500 or 800 specimens of some particular species 



o Ar^lus foliaceus and Ergasitus sicboldii have been fotmd once or twice on the carp (Cyprinus carpio) in Europe, while 

 Lemaocera pecloraUs was reported by Kellicott from the red-fin shiner (Nolropis coniutus) in the Shiawassee River, MicU. 



