264 
BULLETIN OP THE UNITED STATES FISH COMMISSION. 
their aquatic larvie, aud fresh-water copepods are also taken. Many of these are 
again dependent upou the far more minute protozoan and microscopic plant life about 
them. This is notably the case with Daphnidce themselves, in which certain ai>pend- 
ages are used to sweep the microscopic infusoria and swimming algm into the mouth, 
so that the water in which swarms of daphuids are kept is soon cleared of its micro- 
scopic life. 
As the sturgeon grows larger and its mouth more capacious it becomes capable of - 
capturing still larger prey. When they reach a length of fr.>m 5 inches to 2 feet, in 
some localities at least, they begin to prey in the main upon amphipods and isopods, 
two groups of crustaceans found in great abundance in the waters of the estuaries 
frequented by the sturgeon. These larger organisms in turn, which at this stage be 
come the prey of the sturgeon, must feed upon smaller organisms, so that they become 
accumulators, so to speak, of the food of the fish at this stage, just as were the daph- 
uids during an earlier period. The amphipods and isopods are found in great num- 
bers in the spiral valve of young fish under 2 feet long, and besides occasionally the 
undigested cuticular covering of earth-worms is encountered. The remains of the 
larger organisms are, however, always mixed with more or less mud or ooze, which 
contains diatoms, rhizopods, etc., so that these low forms furnish some nutriment even 
in a relatively advanced stage, if not for the entire life of the animal. Of the amphi- 
pods Amphithoe and Gammarus were most abundant in the stomachs of young stur- 
geons. The commonest isopod found in the intestine is a species of Idotea. The fish 
from which these were taken were caught in brackish water, where these amphipods 
aud isopods are very abundant, often adhering to the gill-nets of the shad fishermen 
operating in the same waters, to the number of many thousands. They are known, 
for this reason, to the fishermen as “shad lice.” 
After the sturgeon becomes adult larger organisms are sought for as food, though 
the writer has been surprised to find how little there remains in the digestive tract 
after death to indicate what formed their principal dietary. Occasionally the shells of 
Mytilus or Modiola are found, thus indicating that the mollusca are laid under contri- 
bution as a source of food. These mollusca, living as they do fixed to one spot, are 
in turn dependent upon the microscopic j»rotozoau and larval life which is found in 
the surrounding waters. 
From the foregoing inventory of the food of thesturgeon at various periods of its life, 
it is obvious that its existence is dependent upon that of a great multitude of diverse 
forms, which serve it merely as accumulators of pabulum to be converted into its own 
tissues Starting with the lowest grade of organization, the iarva can feed tor a time 
only upon forms not over a line in length, aud which are minute enough to be sundered 
and rent by its microscopic teeth. At a later stage larger organisms are captured, 
measuring half an inch or more in length, while during adult life large mollusks and 
other organisms of 1 to 2 inches in diameter are readily taken aud swallowed. 
The story of the life of a sturgeon is therefore seen to be bound up with the lives 
of vast myriads of organisms in no way related to it in the system, but only as sources 
of nutriment. It is quite certain from what has preceded that if the minute life upon 
which the young sturgeons subsist were exterminated, the sturgeon would also be- 
come extinct. It follows from this that whatever affects the relative abundance of 
the minute life of the rivers and estuaries where sturgeons are found, must also affect 
the survival aud abundance of the latter. The importance of a study of all the or- 
