74 BULLETIN OF THE BUREAU OF FISHERIES. 



combined with what is apparently a larger quantity of nonliving organic and inorganic 

 material appearing like the mud in which the mussels are embedded when in their 

 natural habitat. Mussels are supposed by some to act as scavengers in consuming 

 sewage. The evidence indicates, however, that, as a rule, they flourish better in waters 

 of natural purity. (Linnville and Kelly, 1906.) It seems not unlikely that mussels 

 may derive considerable nutriment from substances in solution. Churchill's (1915) 

 experiments on the absorption of fat by mussels seem to support such a view. 



Consideration of the finely balanced conditions found necessary for the welfare of 

 other lamellibranchs, including marine clams, to the growth of which considerable 

 study has been given, removes any wonder at negative results with fresh- water mus- 

 sels that have been subjected to highly artificial environments of aquaria and tanks. 

 Complete success in the use of aquaria and such more or less artificial containers can 

 hardly be expected until the factors of growth and their control are more thoroughly 



understood. 



GROWTH IN CEMENT-LINED PONDS. 



The cement ponds (see p. 66 and fig. 61), because of their location, size, and shape, were 

 found very convenient in the experimental work for temporary holding of fish. The 

 perpendicular sides permitted of ready subdivision by screens and easy control of fish, 

 such as removal, transfer, etc. For the planting and culture of juvenile mussels, how- 

 ever, their usefulness is still somewhat a question. Many unsuccessful trials led to the 

 assumption that the cement bottom and sides presented an environment unnatural 

 and unsuited to the life of the mussel ; but later results seemed to indicate that by proper 

 control of conditions in them fair results might be obtained. 



Variations in bottom were tested, together with changes in depth and flow of water, 

 in order to take into account the special needs of given species so far as known. The 

 kinds of bottom employed were gravel, sand, mud or loam, and the uncovered cement. 

 The gravel, sand, or loam were evenly distributed 1 to 3 inches deep over the cement. In 

 addition to this a greater or less deposit of silt always accumulated from the water, the 

 maximum precipitation occurring at the end where the supply pipe entered. 



The plants of juveniles were made from their fish hosts with the following species 

 of mussels: Lampsilis luteola, L. ligamentina, Quadrula plicata, and Q. pustulosa. After 

 one plant of L. luteola on mud bottom at the end of the growing season in November, 

 19 14, an examination was made to determine the results as to growth. The whole 

 bottom contents of the pond were passed through a sieve of 3-millimeter mesh. Two 

 mussels only were present out of a plant of several thousand. These measured only 

 1 1.4 and 15.3 millimeters, respectively, and the appearance of their shells gave evidence 

 of unfavorable conditions. Many tests with the different species were made on a 

 bottom of sand or mud. 



Another variation tried was the narrow cement pond in which large plants of the 

 pimple-back mussel, Quadrula pustulosa, were made. In these ponds, as has been 

 described (p. 66), a current of water over gravel and sand was kept up during the 

 growing season. There was no opportunity for fish to disturb them, as the host fish 

 (channel cat, Ictalurus punctatus) were removed as soon as the mussels had been shed 

 from their gills. 



