Contributions from the Biological Laboratory of the U. S. Fish Commission, 
Woods Hole, Massachusetts. 
THE REACTIONS OF COPEPODS TO VARIOUS STIMULI AND THE BEARING 
OP THIS ON DAILY DEPTH-MIGRATIONS. 
By G. H. PARKER, 
Assistant Professor of Zoology , Harvard University. 
INTRODUCTION. 
Although the marine copepods are almost microscopic crustaceans, they are so 
numerous and so widely distributed that they form one of the most important con- 
stituents in the surface life of the ocean. As a food for fishes, especially the newly 
hatched fry, they have been justly regarded by fish-culturists as almost indispensable, 
and their remarkable daily migrations have been a matter of growing interest and 
speculation for the naturalist. 
From the standpoint of oceanic food supply, the copepods form an important 
link between the microscopic plants and the larger animals. In the ocean, as on the 
land, animals feed either directly on plants or on other animals that in turn feed on 
plants, and, though this regression may sometimes go back several steps, all animals 
are ultimately dependent on vegetable life for their food. Since plants are the only 
forms that have the capacity of elaborating food from strictly inorganic materials, they 
are in reality toe only independent organisms. The vegetation of the ocean, except 
on the very shore, differs from that of the land chiefly in being composed almost 
entirely of plants of microscopic size. A vegetation of this kind can not give sup- 
port to large herbivorous animals such as find easy nourishment in the luxuriant 
plant growth of the land, but it affords an abundant supply of food for small animals 
like the copepods. Many copepods feed almost exclusively on the minute plants of 
the sea, and then fall a prey to small fishes, which in turn are eaten by larger ones. Thus 
the copepods form a link in the chain of food supply that reaches from the independ- 
ent microscopic plants to the largest marine animals. Their place in this chain is 
admirably illustrated by Peck (1S96, p. 353) in his account of the food of the squeteague. 
In the stomach of a large squeteague Peck found an adult herring in which were two 
young scup besides many small crustaceans. The stomach of the scup contained 
copepods, and the alimentary canals of these showed remains of microscopic plants 
(diatoms). Thus plant substance served as food for copepods, copepods for scup, 
scup for herring, and herring for squeteague. This sequence makes evident the 
importance of copepods as collectors of vegetable food which after assimilation serves 
as nourishment for the larger fishes. 
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