The Ecology of Pelagic Amphipoda, II 
Observations on the Reproductive Cycles of Several Pelagic Amphipods 
from the Waters off Southern California 
Gary J. Brusca 1 
The purpose of this study was to utilize mid- 
water samples from off the coast of southern 
California to determine the general patterns 
of the reproductive cycles of the local pelagic 
Amphipoda. All of the materials analyzed dur- 
ing this project were collected in the area of 
the Outer Santa Barbara Passage at approxi- 
mately 33°20'N, 118°4(/W. All samples were 
collected with an Issacs-Kidd Midwater Trawl 
(Issacs and Kidd, 1953) from the R/V "Velero 
IV” of the Allan Hancock Foundation, Univer- 
sity of Southern California. Partial support for 
this work was furnished by National Science 
Foundation Grants (G10691 and G23467). 
Complete information on the reproductive 
cycles of amphipods is scarce and, in the case 
of pelagic species, difficult to obtain. Some data 
can be gained from various expedition reports 
in which the presence of ova and young in the 
brood pouches is recorded. Some of these past 
records are mentioned in this paper. 
METHODS AND MATERIALS 
Samples were taken at various depths and 
at different times of the day and night from 
the summer of 1962 through the spring of 1963 
using a 10 X 1 0-ft Issacs-Kidd Midwater 
Trawl. A few samples were used from later in 
1963 for qualitative confirmation of the data 
gathered earlier. Species accounts and analyses 
of vertical distributions and migrations are pre- 
sented by Brusca (1967). 
For comparative information on fluctuations 
in population densities throughout the year, 
counts made from pint aliquots were converted 
to the number of individuals captured per hour 
trawling time. Such values are only approximate 
1 Assistant Director, Pacific Marine Station, De- 
partment of Biology, University of the Pacific, Stock- 
ton, California. Manuscript received October 13, 1966. 
and do not account for the suspected gregarious 
nature of these pelagic amphipods, but they 
probably do reflect the general trends in the 
population size. Complete raw data are on file 
with the author. 
OBSERVATIONS 
Presented in this section are only those data 
which specifically pertain to the reproductive 
cycles of these animals. For a more detailed 
species account and reference lists see Brusca 
(1967). 
Suborder gammaridea 
Family eusiridae 
Rhachotropis natator (Holmes) 
A total of 77 specimens was taken from pint 
aliquots. None of these individuals was noted 
to be carrying ova or young in the brood 
pouches, but there was some variation in the 
size range and in the density of the population. 
Table 1 illustrates these changes. 
Since a rise in population density accom- 
panied an extension of the lower limit of the 
size range during the winter months, this time 
probably represents the entrance of young into 
the adult or "catchable” population. The data 
TABLE 1 
Seasonal Changes in Size Range (SR), and 
Average Number per Trawl Hour (n/th) 
Based on Positive Samples for 
Rhachotropis natator 
SEASON 
SR (mm) 
n/th 
Summer 
20 (l spec) 
4 
Fall 
13-17 
22 
Winter 
9-16 
32 
Spring 
9-17 
24 
449 
