Ecology of Pelagic Amphipoda, I — Brusca 
TABLE 1 
Depth Distribution of Day and Night Samples 
Taken in the Outer Santa Barbara Passage 
DEPTH 
IN 
METERS 
NO. 
QUANTI- 
TATIVE 
SAMPLES 
NO. 
QUALI- 
TATIVE 
SAMPLES 
TOTAL 
NO. 
SAMPLES 
Night 
0-50 
2 
2 
4 
50-100 
2 
0 
2 
100-200 
5 
2 
7 
200-300 
3 
1 
4 
300-400 
4 
2 
6 
400-500 
2 
0 
2 
500-600 
4 
3 
7 
600-700 
2 
0 
2 
700-800 
1 
0 
1 
800-900 
0 
1 
1 
900-1100 
1 
2 
3 
Day 
0-50 
0 
2 
2 
50-100 
0 
2 
2 
100-200 
0 
2 
2 
200-300 
3 
1 
4 
300-400 
3 
1 
4 
400-500 
4 
1 
5 
500-600 
5 
0 
5 
600-700 
3 
1 
4 
700-800 
3 
0 
3 
800-900 
4 
0 
4 
900-1100 
7 
1 
8 
Certain hydrographic data were utilized in 
determining the effects of oceanographic con- 
ditions on the habits of the pelagic amphipods. 
Some of this information was obtained from 
data collected from four stations in the Outer 
Santa Barbara Passage occupied by the Cali- 
fornia Cooperative Fisheries Investigations 
(ccofi) research vessel. In addition to the 
ccofi reports, several bathythermograph read- 
ings were taken from the "Velcro IV” at times 
of biological collecting. 
From the information gathered, Table 2 was 
constructed to illustrate the variation in ther- 
mocline depth and intensity throughout the 
year. 
Although fluctuations in surface water salin- 
ities were noted in the ccofi reports, there was 
no indication that the halocline had any effect 
on the vertical distribution of the amphipods. 
The identification of water masses off the 
southern California coast is difficult due to the 
383 
great amount of mixing and the formation of 
complex eddy systems as the California Cur- 
rent passes Point Conception. Emery (I960: 
97-115) presented a description of the cur- 
rents of the local shelf waters and suggested 
that a zone of mixing between the northern 
water of the California Current and a deeper 
layer of southern water exists at depths of from 
200 to 300 m. Analysis of temperature-salinity 
diagrams drawn from the ccofi data suggests 
the possibility that the water of the Outer Santa 
Barbara Passage below about 100 m is an area 
of mixing of east and west North Central Pa- 
cific water. In some cases T-S values approach 
readings indicative of Pacific equatorial water. 
These measurements may reflect the incorpora- 
tion of southern water as the eddy systems 
turn northward below Point Conception. The 
surface waters are subject to a great deal of 
seasonal variation, especially in temperature. 
The surface salinity values (above 100 m) are 
typical of those reported as Pacific subarctic 
water. This top 100 m may be subarctic water 
which has been heated by solar radiation with 
very little salinity change as it is brought into 
lower latitudes by the California Current. 
SPECIES ACCOUNTS 
Suborder gammaridea 
Family eusiridae 
Rhachotropis natator (Holmes) 
Gracilipes natator Holmes, 1908: 527-529, 
figs. 32-34; Thorsteinson, 1941: 85, pi. 6, 
figs. 67-70. 
Rhachotropis natator Barnard, J. L., 1 954,3 : 
54-56, pi. 6. 
This species occurred in 15 samples taken 
in this study at depths of 600 to 1100 m. A total 
of 77 individuals was recorded in quantitative 
samples. Table 3 illustrates the day and night 
distributions of this species and shows some 
evidence of vertical movement. The population 
appeared to be of about uniform density dur- 
ing the daylight hours between 600 and 1100 
m, but during the night captures were made 
only at depths greater than 900 m. Indicated 
here is a movement into shallower water dur- 
