Copepod parasites of fresh-water fishes and their economic relations to mussel glochidia

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COPEPOD PARASITES OF FRESH-WATER FISHES AND THEIR ECONOMIC RELATIONS TO MUSSEL GLOCHIDIA : ::::::: By Charles Branch Wilson

From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXXIV, 1914 Document No. 824 : : : : : : : : : : : : : : : Issued June 28 ^ 1916

Introduction

Under an appointment by the Commissioner of Fisheries, during the summer of 1914, at the United States Fisheries biological station at Fairport, Iowa, an extended examination was made of the parasitic copepods which infest our fresh-water fishes in the Mississippi River and its tributaries and of the mussel glochidia which are also parasitic upon fish during their term of metamorphosis. Several of the early American naturalists became interested in the copepods found upon fresh-water fish, and many new species were described. This was especially true of Le Sueur and Dana, and singu larly enough the Danish investigator, Kr0yer, also obtained a number of American species from fish sent to the Copenhagen Museum. But in every instance the species described were isolated, they were sometimes founded upon single specimens, and manv of them have never been seen since their original discovery.

Prof. S. I. Smith published in the Report of the United States Commissioner of Fish and Fisheries for 1872-73 a list of the crustacean parasites of the fresh-water fishes of the United States (p. 661-665). This list included two argulids, one caligid, one ergasilid, six lemaeopods, three of which were new to science, and two lemseans, 12 species in all. With true scientific foresight. Prof. Smith stated that the few species he enumerated were "doubtless only a small fraction of those which really prey upon our common fishes," and that his principal object was to "call attention to the subject and furnish a basis for future investigation" (p. 661). But his suggestion did not meet with the response it deserved and beyond the investigations of Smith himself, Packard, Kellicott, Wright, Fasten, and a few others, all widely scattered, no attempt has been made to increase the list up to the time of the present investigation.

About 1895 Mr. R. R. Gurley, at that time in the employ of the United States Bureau of Fisheries, gathered together all the available data with reference to the copepods parasitic upon fresh-water fishes, translating the descriptions given by Kr0yer and other foreign investigators and identifying both hosts and parasites amongst the material in possession of the Bureau. He made no attempt to establish new species, but only to bring together all that had been previously described, and he accumulated

334 BUI^I^ETIN OF THE BUREAU OF FISHERIES.

a manuscript of about 150 pages, which was subsequently turned over to the present author. This has proved of great value on other occasions as well as the present, and Gurley's original identifications and additions to the work of previous authors are acknowledged in the following pages.

The specimens and other material were derived from several sources. First, the work of the biological station involves the handling of large numbers of fish, and several of the regular staff, notably Mr. H. W. Clark, Mr. T. Surber, and Dr. A. D. Howard, have saved such parasitic copepods as they found while examining the gills for glochidia. These were generously turned o^•er to the present author, who had also accumulated a large number of specimens during the surveys of the mussel fauna of various regions of the United States under the auspices of the Bureau of Fisheries.

These collections were augmented during the present investigations by a careful examination of all the preserved gills of fish in the possession of the biological station, of the gills of five fish caught by the regular seining crew or brought to the station for glochidial infection, and of a large number of dead fish caught by local fishermen.

In these different ways, and including chiefly the waters of the Mississippi Valley, the original list has been increased to 46 species, 10 of which are new to science; i of Kr0yer's and i of Le Sueur's species have been rediscovered, and there have been added the larvae of 4 other species in various stages of development.

During the investigation it early became apparent that certain economic relations existed between the copepod parasites and the mussel glochidia, which are also parasitic on fish. Although the broad fact that parasitized fish do not take or hold glochidia as well as the nonparasitized ones was observed early in the work at the station, nevertheless the existence of particular mutual relations betv/een copepods and glochidia had never been suspected. Of all the authors above mentioned Fasten is the only one who has ever treated the copepods from an economic standpoint, and his excellent papers deal chiefly with the artificial propagation of a single species. It is at once evident, however, that the interrelations between the fish and the two kinds of parasites must exert considerable influence upon the artificial propagation of mussels, as well as upon an intelligent study of the parasitism of the copepods. Accordingly these economical discussions are placed first in the present paper, and the description of the species is left until the last.

RELATIONS BETWEEN THE COPEPODS AND THEIR HOSTS.

As has elsewhere been stated, both by the present author (Proceedings of United States National Museum, vol. 25, p. 654) and by other investigators, it is not probable that the copepod parasites of fresh-water fishes become under natural conditions a serious menace to the life of their host. But it must be remembered that their presence upon the fish is always injurious to the latter and can never be beneficial nor even indifferent.

I . There is a notion prevalent in certain quarters that a limited amount of dirt and vermin is wholesome rather than harmful. It is needless to say that this is erroneous, and that there is no truth also in the idea that a few of these creatures do their host no real harm, but that a considerable number must be present in order to become really injurious. Even a single parasite withdraws from its host enough blood for its own sustenance. That amount may be small, but it is nevertheless a loss, and it weakens the fish's vitality by just so much. The simple fact that a sufficient number of parasites can weaken or even kill a fish is enough to prove that each one does his share toward that end and is therefore harmful. And here in the Mississippi Valley there are other considerations which tend to greatly increase this influence of parasitism.

2. The parasites, especially the ergasilids, are more numerous upon young fish; one can scarcely examine a young crappie or calico bass 3 to 5 inches in length without finding it infested with Ergasilus caeruleus, its particular parasite, and the same may be said of the hosts of the other ergasilids. It is not quite as noticeable in the case of the argulids and lemseopods, although even here the smaller fish are the ones most frequently infested. These young fi.sh are like the young of all animals, including even man. They are growing rapidly; they need all the vital energy they can produce to carry on this growth successfully, and hence they are more susceptible to the injurious effects of parasitism than the matured adult. We thus find a maximum of numbers of parasites at that very stage of development when there is a minimum of resistance on the part of the host, and this greatly increases the influence of the former upon the latter.

3. Again, the parasites are more numerous in the slews and cut-offs (so-called lakes) than in the main river. This is due partly to the absence of a current, thereby enabling the parasite larva to swim about freely, and partly to the crowding together of the parasites and fish, which materially aids the former in their search for the latter; but in these shut-off bodies of water the conditions are not as favorable to the fish as in the open river, especially late in the season. There is not as much food, the water is not as well aerated, and there is a keener struggle for existence. Furthermore, in these slews the young fish far outnumber the older ones; these are the very places to which they resort to escape their enemies. Scarcely a fish can be found in these "lakes" and slews which is free from parasites, and towing reveals the presence of large numbers of parasite larvae swimming about in search of a host. Thus the parasites attack their hosts not only at the stage of development when they are most susceptible, but also in the places and under the conditions when they are least able to withstand the attack, again greatly augmenting the influence of parasitism.

4. With the time, the place, and the conditions thus favorable to the parasites, the latter respond quickly and show an abnormal increase in development. A far greater number reach maturity than under less favorable conditions; these in turn breed, and the number of larvae is increased a hundredfold ; a considerable percentage find hosts, thus crowding the gills of the young and already weakened fish. In this way parasites that are comparatively harmless under ordinary conditions may, and often do, become a serious menace to the life of the fish.

These considerations are enough to show that the presence of even a few parasites is not a matter of indifference. Fortunately, under ordinary conditions the parasite has an even harder struggle for existence than its host. In this struggle the different kinds of parasites are affected differently, while the ultimate issue is the same for them all.

The ergasilids swim about freely until they reach maturity. The male never becomes a parasite, but completes its life as a free swimmer, while the female seeks a particular host. During this comparatively long free-swimming period both sexes have to contend with many enemies. They are then a part of the plankton and as such have to contribute their share toward the support of all the varied life which feeds upon the plankton. There are many animals which eat copepods and none of them are at all particular as to the species. These free-swimming ergasilids are fully as toothsome as other kinds and are as often eaten. The male never escapes this danger, but the female does when she has once fastened to the gills of a fish. It sometimes happens, however, that when the female is ready to fasten to a fish all the fishes suitable for hosts have left the vicinity. Under such conditions the female parasite must die unless she can swim far enough to find a host.

The argulids swim about freely, even after reaching maturity, especially the males. During this swimming they also become part of the plankton and share in its dangers and vicissitudes. Being external parasites, they are not compelled to find a particular host, for they can remain temporarily upon almost any fish until their true host is found. They are thus much less susceptible to the dangers of the plankton than the ergasilids, and when they have once reached maturity they are thenceforth free from such dangers. Their much larger size also operates in their favor, for they are too bulky to be caught by most of the creatures which eat ordinary copepods.

The lerngeopods have but a very short free-swimming period, a few hours at the most, and during that time they, too, are subject to the dangers of the plankton. They must not only survive these dangers but they must also find a particular host within this brief period or they perish; and the same disaster often overtakes them that happens to the ergasilids, namely, when they are ready to attach themselves there are no suitable hosts available.

The lernaeids also become a part of the free-swimming plankton at two separate periods in their development. First during the nauplius and metanauplius stages, when they are indistinguishable from all other copepods in the same stages, so far as the dangers of the plankton are concerned. Then they spend the copepodid stages as parasites upon the gills of some fish, apparently any that happens to be available. On leaving this intermediate host they again enter the plankton and swim about freely while a union of the sexes takes place. The male develops no farther, but the female must seek a permanent host, and this time it must be a particular species of fish. During this latter period, therefore, they are in the same condition as the lernaeopods and often experience the same trouble, namely, when they are fully developed there are no suitable hosts available.

It follows that the parasites are ordinarily held in check by these means, and if they are to become anything of a menace to the fish there must be peculiar conditions favorable to them and unfavorable to their hosts. The custom practiced by the biological station of seining the fish out of the "lakes" and slews that are likely to go dry and putting them back into the main river is the best thing that could be done to get rid of the parasites. We have just seen that the latter breed rapidly under the conditions obtaining in the slew and that everything works together in their favor. By removing the fish such breeding is at once stopped ; all the parasite larvae and adults left in the slew die, and the new conditions in the main river are such as to keep subsequent breeding within due bounds.

COPEPOD PARASITES AND MUSSEIv GLOCHIDIA ON FRESH-WATER FISHES. 337 RELATIONS BETWEEN THE COPEPODS AND THE GLOCHIDIA.

We have just discussed the relations between the fish and the copepods, but both copepods and glochidia infest our common fresh-water fishes. Consequently, in view of the efforts which are being put forth by the United States Bureau of Fisheries for the success of artificial mussel propagation it becomes imperative to know whether the habits of these two kinds of parasites are harmonious or antagonistic. Does the presence of copepods upon our common fishes influence in any way their susceptibility to infection by mussel glochidia ? This problem can be most intelligently discussed in the form of a series of questions and answers.

I. Are the fish that serve as hosts for the copepods those which are naturally susceptible to infection by glochidia?

This question can be best answered by arranging in tabular form a list of the fishes with their glochidia and copepod parasites in parallel columns.

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