104 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Although many copepods have been obtained from great depths in the ocean, 
they are, for the most part, surface or pelagic animals. This even applies to shallow 
waters. Thus, in Buzzards Bay, which is probably nowhere over 20 fathoms deep, 
the copepods, according to Peck (1896, pp. 356-368), are about twice as abundant 
in the more superficial waters as in the middle depths or near the bottom. Those near 
the top show a remarkable daily rhythm in their movements. During the night they 
swarm in great numbers on the immediate surface, but from sunrise to sunset they 
desert the surface almost completely. This change makes itself felt in their availa- 
bility as a food supply, for Peck (1894, p. 117) has observed that menhaden taken in 
the daytime contain fewer copepods than those caught at night. Thus the question of 
what determines the daily migrations of copepods is a problem not only of general 
interest to the naturalist, but also of practical importance to the fish-culturist. 
According to Giesbrecht (1892), Weismann (1877) was the first to suggest a solu- 
tion for this problem. He believed that light determined the upward and downward 
migration of pelagic animals and that most animals, being adapted to light of medium 
intensity, retreated from the surface when the light increased and moved upward 
when it diminished. An explanation essentially like this was accepted by F uchs (1882). 
Chun (1887) opposed this view and maintained that temperature changes were probably 
the means of inducing daily as well as yearly migrations. The first to approach the 
question from an experimental standpoint were Groom and Loeb (1891), who studied 
the reactions to light of the free-swimming young of barnacles (Balanus perforates). 
Their experiments showed that the animals were not constant in their reactions. 
When placed in a glass vessel in bright daylight they eventually deserted the side of 
the vessel next the window and swam away from the light, i. e., became negatively 
phototropic; when, however, they were placed in dim light they sooner or later 
reversed and swam toward the light, i. e., became positively phototropic. Since the 
barnacle larvae in the aquarium, where the only changes were in the intensity of 
illumination, went down by day and came up by night as they did in the open sea, 
Groom and Loeb believed that this reaction in nature was conti'olled by light in essen- 
tially the same way as it could be controlled in the laboratory. Thus, while they did 
not deny that temperature changes might have some influence on the movements of 
the animals, they concluded that light, not heat, was the chief factor in controlling 
the daily migrations. In copepods studied by Groom and Loeb (1891, p. 176) no 
change in reactions to light such as that observed in the barnacle larva 1 , could be 
discovered. Subsequently, however, Loeb (1893, p. 96) studied at Woods Hole certain 
copepods, most of which were probably of the species Temora low gicornis / these 
could be made negatively phototropic by increasing the temperature of the water or 
by decreasing its density, and positively phototropic by decreasing the temperature 
or increasing the density. Although Loeb does not state that these factors play any 
important part in daily migrations, he does call attention (Loeb, 1893, p. 105) to gravity 
as an effective element in this respect. Animals that respond negatively to the force 
of gravity, i. e., are negatively geotropic, would, so far as this factor alone is con- 
cerned, continually move toward the surface. Thus one of the migratory movements 
might be accounted for. 
From the preceding survey it is evident that the daily migration of pelagic 
animals is not the result of as simple a combination of circumstances as was at first 
supposed, and, further, that what may be effective in bringing about migration in 
