Ecology of Pelagic Amphipoda, I — Brusca 
TABLE 13 
Day and Night Depth Distributions for 
Paraphronima gracilus 
DEPTH IN 
METERS 
AVERAGE 
NO. PER 
TRAWL 
HOUR 
PER CENT 
POSITIVE 
SAMPLES 
Night 
0-400 
72 
83 
400-1100 
15 
37 
Day 
0-400 
44 
38 
400-1100 
28 
82 
in a great number of samples and contamina- 
tion is suspected in certain deep night trawls. 
Density variations, however, support evidence 
of an ascent toward the surface during the dark 
hours. Table 13 shows the average numbers per 
trawl-hour for the positive samples, and the 
percentages of positive stations at different 
depth ranges during the day and night. 
Paraphronima eras sipes Claus 
Paraphronima eras sipes Stephensen, 1924: 
77-78; Chevreux and Fage, 1925:390-391, 
figs. 393-394; Pirlot, 1929:105-106; Barnard, 
K. H., 1930:409-410; Barnard, K. H., 1932: 
267-268; Shoemaker, 1945:234; Hurley, 1956: 
13. 
Specimens of this species were found in 54 
samples ranging in depth from 80 to 1100 m. 
A total of 922 individuals was sorted from 
pint aliquots. A movement toward the surface 
at night was apparent (Table 14). 
SUMMARY 
This study treats 5 species of the suborder 
TABLE 14 
Day and Night Depth Distributions for 
Paraphronima crassipes 
PER CENT 
PER CENT 
POSITIVE 
POSITIVE 
DEPTH IN 
DAY 
NIGHT 
METERS 
SAMPLES 
SAMPLES 
0-200 
0 
50 
200-1100 
75 
65 
391 
Gammaridea and 16 species of the suborder 
Hyperiidea collected from the waters off the 
coast of southern California. Two additional 
genera ( Seina and Orchomenella) were col- 
lected, but are not discussed here because of 
problems in specific identification. Two species 
of the family Oxycephalidae ( Oxycephalus 
clausi and Calamorhynchus pellucidus ) not 
previously reported from California waters were 
taken during this study, raising the known 
number of local pelagic hyperiids from 43 to 
45. 
With the exception of Cyphocaris anonyx, 
all of the gammarids were noted to be deep- 
living forms common only in samples taken 
at depths greater than about 650 m. There ap- 
pear to be diurnal movements in some species, 
involving a rise during the daylight hours and 
a sinking at night. The controlling factors here 
are not clear, and may involve an endogenous 
rhythm. 
With the exception of a few species, the 
upper depth limit of the hyperiids was defined 
by the thermocline, and captures were uncom- 
mon at depths of less than 50 m. From re- 
peated sampling and analysis of abundance, it 
was found that most of the hyperiids exist in 
a thick band down to depths greater than 1000 
m during the daytime, with the greatest con- 
centrations recorded between 200 and 600 m. 
Most of these species rise to shallower levels 
during nightly migrations toward the surface. 
Although it is easy to speculate on the ad- 
vantages of such vertical movements, it is very 
difficult to establish the actual causes. The re- 
sults of this study indicate that, in addition to 
the barrier imposed by the thermocline, light 
is the most important factor influencing the 
migrations of the hyperiids. The controlling 
effect of light intensity is illustrated by the 
massing of populations at shallow levels dur- 
ing periods of dim light and by the vertical 
spreading-out of individuals at times of total 
darkness. 
A great deal of work remains to be done 
on midwater ecology in general. The use of 
more refined sampling methods will enable 
workers to establish accurate depth distribu- 
tions and will offer more precise information 
on the habits of vertical movements. 
