I20 BULLETIN OF THE BUREAU OF FISHERIES. 



for existence of the animate world. Since mussels feed upon suspended matter, living 

 or dead, which they filter from the water, and since water once filtered must be less 

 richly supplied with food for other mussels, an actual competition for food undoubtedly 

 exists. Clark and Wilson (191 2, pp. 19-20) give an account of a measured area of i 

 square meter (10.76 square feet) in which they counted 81 mussels and 57 other moUusks, 

 making a total of 138 individuals, or about 13 per square foot; and there were present, 

 of course, many other animals, some of which took the same kind of food as the mussels. 

 This recorded determination of numbers per given area illustrates the possibilities of 

 competition. As indicated on pages 91 and 93, above, a detrimental competition for 

 organic food probably does not occur ordinarily with mussels. 



Symbiosis and commensalism exist in such communities. A few supposed instances 

 affecting mussels are afforded by small forms that live within the shells in the mantle 

 cavity of the mussel where they receive food and protection. A small bristle worm, 

 ChcBtogaster limn<ei, frequently observed in the mantle cavity of mussels, is supposed 

 by some to be merely a commensal, but it may be considered a predacious species since 

 it has been seen with juvenile mussels within its digestive tract (Howard, paper read at 

 meeting of American Fisheries Society, 1918). The leech, Placobdella montifera, enters 

 living mussels, but is not known to feed upon them (Moore, 1912, p. 89). 



Bryozoa and other sessile forms are found attached to the exposed portions of 

 live-mussel shells. Doubtless there are many cases of commensalism to be revealed 

 by closer study of mussels in their natural habitat. 



An interesting symbiotic relation exists between a mussel and the bitterling, a 

 small European fish, which lays eggs in the mantle cavity of a fresh-water mussel which 

 in turn infects the fish with glochidia (Olt, 1893). A difi'erent relation, which shows 

 some reciprocity, however, is that of the fresh-water drum {AplodinoUis grimniens) 

 of the Mississippi Basin, that eats fresh-water mussels but pays for the privilege, 

 in part at least, by nourishing the young of several species parasitically enc3'sted 

 on its gUls. (Surber, 1913, p. 105, and Howard, 1914, pp. 37 and 40.) The same is 

 true of other fish that eat mussels, as the catfishes. 



Parasitism is a phenomenon of community relations, and it is of double significance 

 in the case of mussels, because not only have the mussels parasites to prey upon them, 

 but they with few exceptions depend for existence upon the opportunity to become 

 parasites of fish or, in one case, of an amphibian. A rather close relationship of fish 

 to the mussel community is essential, and there may be a particular interrelation of 

 given species of fish and of mussels. Questions arise as to when and how this special 

 and intimate relationship came about and to what extent the habits of host and 

 mussel interlock in such cases as the gar pikes and the sand-shells (Howard, 1914a), 

 the river herring and the niggerhead, the shovel-nose sturgeon and the hickory-nut, 

 the catfiches and the warty-back, the mud puppy and the little salamander mussel. 

 In the last-named case, the peculiar habit of the mussel which lives beneath flat stones 

 conforms evidently to the habits of the host, for the mud puppy is well known to 

 frequent such situations. 



One feature of certain mussels that possibly ser\^es to decoy fish is the elaborate 

 development of the mantle flap in gravid females of the pocketbook mussel, Lampsilis 

 venlricosa, and others. (See p. 85.) These flaps in their form and coloration, includ- 

 ing an eyespot, resemble a small fish, and the motion of these in the current still further 



