FRESH-WATER MUSSELS. lOI 



ing water and fish into the newly excavated pond. Eighteen species which have been 

 accidentally introduced are listed on page 165 below. 



Few of these mussels are of commercial value, but it has been attempted to intro- 

 duce several useful species by artificial infection upon fish, and success has been at- 

 tained with the Lake Pepin mucket, a lacustrine mussel of high commercial value, 

 which thrives well in the ponds and has attained a size and quality of shell suitable for 

 commercial purposes at the age of 4^ years. 



In canals mussels frequently thrive (PI. XI, figs. 3 and 4). A mill race from a well- 

 stocked stream seems to present a favorable environment for them. Clark and Wilson 

 (191 2, pp. 19-22) describe a luxuriant development of mussels in a canal at Fort 

 Wayne, Ind., as follows: 



Toward the upper end of the canal, in a place where the bottom was 15 feet wide, the mussels were 

 counted for a stretch of 10 feet along the canal bed and the following species noted : Quadrula rubiginosa, 

 11; Q.cylindrica, i; Q. undulata, 86; Anodonta grandis, 6; Piyclwbranchus phaseolus, i; Lampsiiis ligamen- 

 iina, 5 ; L. luteola, 6. The width taken was the total width of the bottom of the canal and was consider- 

 ably wider than the space occupied by the mussels. 



About a mile farther down the canal a space of 10 feet square was measured off in the bottom of the 

 canal, and the following species were foimd: Quadrula rubiginosa, 6; Q. undulata, 60, all rather small; 

 Pleurobemaclava, i; Alasmidonta truncata, 2; Symphynota complanata, 2; S. costata, 5; Anodonia grandis, 

 15; Obovaria circulus, 4; Lampsiiis ligamentina, 5; L. luteola, i; L. ventricosa, 4. This gave a little over 

 one shell per square foot. In 1908, in a square meter of bottom near the Rod and Gun Club, the follow- 

 ing species were noted: Quadrula rubiginosa, 9; Q. undulata, 36; Symphynota complanata, i; Anodonta 

 grandis, 17; Obovaria circulus, 11 ; Lampsiiis iris, 2; L. ligamentina, 2; L. luteola, 3, giving a total of 81 

 per square meter. In addition to these shells there were many small Sphaeriums, the ground being 

 paved with them, 34 Campeloraas, and 23 Pleuroceras. The square meter referred to above repre- 

 sents, as nearly as could be judged, an average number rather than either extreme. 



It would appear from a general comparison of the aspect of mussels in lakes, ponds, 

 and rivers that the effect of currents or circulation upon the growth of mussels is variable 

 according to the relative proportions of organic and mineral foods present. In rivers, 

 where the circulation of water is constant, mussels may grow to large size and possess 

 thick shells, but when circulation is reduced, as in inclosed bodies of water, the mussels 

 may be small and relatively thin-shelled, or they may attain a large size with thin shells 

 (suggesting relative deficiency of mineral food), or else, with heavier shells, they may be 

 dwarfed in size (suggesting a relative deficiency of organic food). 



BOTTOM. 



Most mussels are normally embedded in the bottom from one-half to three-quarters 

 of their bulk." That they may thus establish themselves, a firm but not impenetrable 

 soil is required. The character of the bottom is, therefore, of especial significance to 

 fresh-water mussels, though it has important relations to all bottom-dwelling animals. 

 With regard to the bottom, consideration must be given both to its topography and 

 to the materials of which it is composed. Major inequalities in topography, such as 

 waterfalls and rapids, are discussed elsewhere. Minor inequalities are of importance 

 because of the effects upon currents, sedimentation, light conditions, growth of food, 



o The cases of deep embedding mentioned by Wilson and Danglade (1913). where they give a depth of i foot or more for living 

 mussels in Shell River (p. i5),andtlie report of a fisherman of 2 to 3 feet at Lake Bemidji, seem to be cases of "digging in" because 

 of drought. Unio tetralasrnus (Isely, 1914) and Quadrula plicaia (Howard, 1914) seem to have a remarkable power of resistance 

 under these conditions. 



