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BULLETIN OF BUREAU OF FISHERIES 
DAPHNIA MAGNA 
The second test animal, Daphnia magna, which is found in the quiet backwaters 
of various river systems (Surber, 1936), and which is raised extensively as food for 
young fish, was chosen because of its high sensitivity to most stream pollutants. 
Comparative tests showed this animal to be more sensitive even than trout, so that 
the minimal lethal concentration for daphnia of a given pollutant gave approximately 
the maximal dilution from which damaging effects on fresh-water fishes and many 
other fresh-water animals could be expected (exclusive of certain cumulative effects). 
Consequently, between the minimal lethal concentration for daphnia on the one 
hand and the minimal lethal concentration for goldfish on the other, the gamut of 
lethal concentrations of any pollutant or effluent under conditions comparable to 
those of particular pollution cases could be defined rather definitely. 
Daphnia proved to be an excellent test animal with which to obtain a quick 
orientation concerning the relative toxicity of effluents of unknown lethality, since 
this animal is quite free from the protective mucous secretion which safeguards fish 
for a tune against many pollutants. 
Daphnia were also valuable in studying the cumulative effects of some pollutants, 
as under favorable controlled conditions a new parthenogenetic generation could be 
expected every 5 to 8 days. 
Like that of the goldfish, the scientific literature on daphnia is extensive, and the 
general physiology and reactions of this animal are well known . The use of daphnia 
as a bioreagent is also well established (Munch, 1931; Adams, 1927; Billiard, 1925). 
The daphnia used in these tests were raised from original stock secured in Texas, 
pedigreed strains being established and lines of parthenogenetic clones secured for the 
assay work. The daphnia colony is maintained in a series of large glass jars, each 
containing four liters of water; and the animals fed bacterial nutrient material pre- 
pared from cottonseed meal as described by Chipman (1934). 
All assay tests were made in a constant temperature cabinet (see fig. 22) at 25° C. 
One hundred fifty daphnia of the same age (usually 4 days old) from the same strain 
were placed in each jar containing 3 liters of the solution to be tested, i. e., in a known 
dilution of the pollutant, using water from the same source as that in which the 
daphnia were living as the diluent. No abrupt changes in temperature were per- 
mitted, and the solution was well aerated before the daphnia were placed in it. 
Every 48 or 72 hours, depending upon the type of test, the animals in each jar were 
concentrated in a pyrex glass trap devised for this work, counted, and returned to the 
same or fresh solution as desired. When young appeared they were removed and 
carried in separate jars containing the same concentration of the pollutant as that of 
the original jar. In this way the mortality of each succeeding generation as well as 
the rate of reproduction could be followed. 
WATER TYPES 
When determining the specific lethality of any given pollutant for a particular 
case, water from the stream into which the effluent was being poured, taken just up- 
stream from the point at which the pollutant entered the stream and filtered through 
bolting cloth to remove large masses of suspensoids and the macroplankton was used 
as the diluent water. In this way the pollutant was mixed with the same water as 
