446 
PACIFIC SCIENCE, Vol. XXI, October 1967 
to the sea surface (Sheard, 1953; Ponomareva, 
1959, 1963; Zelickman, 1961). Although these 
theories are still controversial, the last can prob- 
ably be applied to the swarming of E. pacifica 
in Japanese nearshore waters. 
The author observed that more than 50% of 
the females in a few swarms in the Sea of 
Japan had spermatophores in the thelycum. No 
specimens from the Kinkazan area were carry- 
ing spermatophores. However, the females 
were full-grown and the degree of maturation 
of the ovaries corresponded to the stage 3 de- 
fined by Ruud (1932) or to the stage 5 or 6 
established by Bargmann (1945) for Euphausia 
superb a. As is shown in Figure 2, offshore 
populations of E. pacifica seem to come close 
to the coast around Kinkazan with a pulselike 
rhythm. This may suggest that the population 
of E. pacifica can be divided into several stocks 
in accordance with the phase of maturation, 
and that, as stocks reach a certain degree of 
maturity, they approach the coast in succession. 
The tendency for swarming to occur earlier 
in the southern part of the Sea of Japan than 
in the northern part, as shown in Figure 3, 
may be understood in relation to the geo- 
graphical difference between the maturing or 
reproduction phases of euphausiids and to sea- 
sonal differences in temperature of the two 
areas. 
Kun’s opinion (1955) on the daytime ascent 
of Calanus tonsus, which may be related to a 
biochemical process (e.g., transformation of 
vegetable carotenoids into vitamin A by ultra- 
violet radiation) may be applied to the swarm- 
ing of euphausiids. During certain periods of 
their maturing or reproductive process, eu- 
phausiids might need rather strong daylight, 
regardless of the usual daytime level of their 
vertical distribution. As listed in Table 1, 
nearly all species whose swarming has been 
reported hitherto are cold water forms. This 
suggests that their physiology should be ana- 
lyzed, with the objective of solving the mech- 
anism of this peculiar behavior of euphausiids. 
ACKNOWLEDGMENTS 
The author is indebted to Dr. Yoshiyuki 
Matsue, former professor at the University of 
Tokyo, who kindly oriented and supervised the 
initial phase of this work. Special appreciation 
is due to Dr. Joyce C. Lewin and Dr. Karl 
Banse, Department of Oceanography, Univer- 
sity of Washington, and to Dr. Michael Mul- 
lin, Institute of Marine Resources, University 
of California, for their critical reading of the 
manuscript. 
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