Larval Development of Emerita rathbunae — Knight 
59 
glass wool, and fed newly hatched Artemia 
nauplii. The laboratory cultures were main- 
tained at room temperature which, because of 
seasonal cooling, decreased gradually from 22° 
to 19° C, with daily fluctuations of 1° C or less. 
The salinity range during the culture period was 
33-5-33.8% 0 . The larvae were kept under 
natural illumination but away from direct sun- 
light. 
Both aboard ship and in the laboratory, all 
exuviae and some specimens of each develop- 
mental stage were removed and preserved in 
5% formaldehyde buffered with hexamethylene 
tetramine. The casts were transferred to glyc- 
erine for study. The cultures, the preserved 
specimens, and the exuviae were maintained in 
such a way that the individual history of each 
larva could be followed. In the course of the 
study, 328 exuviae and 50 specimens of reared 
larvae were examined and dissected. 
In order to compare zoeal stages occurring 
naturally in the plankton with those obtained 
in the laboratory, 143 zooplankton samples 
(taken in August-September and November- 
December in the area between Cape San Lucas 
and Cape Corrientes, north into the Gulf of 
California to Tiburon Island, and along the 
west coast of Baja California north to Magda- 
lena Bay) were examined. The majority of 
samples were taken with a 1 -meter net towed 
obliquely from 140 m to the surface, filtering 
approximately 500 m 3 , during cruises 5612 
(SIO Ref. 61-22, 1961), 6108 (SIO Ref. 62- 
16, 1962), 6208-9, Azul II and El Golfo 
(Snyder and Fleminger, 1965). A total of 150 
specimens were obtained for comparison with 
cultured larvae. In addition, 70 larvae of 
Emerita analoga were sorted from zooplankton 
samples taken off Point Conception, on cruises 
32 (SIO Ref. 52-1, 1952^) and 33 (SIO Ref. 
52-7, 1952^) and off the Coronado Islands, 
for detailed comparison of the two species. 
Larvae were dissected in glycerine. Drawings 
of whole specimens and appendages were pre- 
pared with the aid of a camera lucida. Young 
stages were stained with lignin pink to facilitate 
dissection and study. 
RESULTS 
The cultured larvae molted 7, 8, or 9 times 
before metamorphosis to megalopa, with the 
majority passing through 8 zoeal stages. The 
stages became progressively longer and the dura- 
tion of the last stage was twice that of the pre- 
ceding one (Table 1). Of the larvae cultured 
42% completed zoeal development and molted 
successfully to megalopa. The highest mortality 
(20%) occurred in the terminal stage. 
Within stages I, II, and III morphological 
development was similar for all individuals. In 
subsequent stages, slight variation was found 
among individuals that completed development 
in seven zoeal molts, but larvae passing 
through eight or nine stages showed consider- 
able individual variation in relative growth and 
setation of appendages, so that the number of 
stages through which an individual had pro- 
gressed could be positively ascertained, beyond 
stage IV, only by a study of its molting history. 
Despite this variability, the order of develop- 
mental events was similar for all larvae. The 
degree of growth attained with each molt be- 
yond stage III decreased with an increase in the 
number of zoeal stages in the larval period, 
but the terminal zoeas were alike in develop- 
ment of appendages (addition of pleopods, 
growth of thoracic appendages and flagellum 
of second antenna, etc.) and differed mainly in 
details of setation and size. No larvae inter- 
mediate between the terminal zoea and mega- 
lopa were observed. 
The variation among the cultured larvae in 
number of zoeal stages, and in morphology of 
individuals with similar molting histories, 
prompted detailed examination of specimens 
from preserved plankton samples. The ratios of 
carapace length to rostrum length (Table 2), 
together with study of appendages, were used 
for identification of planktonic specimens. The 
stage of development was determined by exami- 
nation of the natatory setation of the first and 
second maxillipeds and of relative change in 
other appendages. The cultured larvae, hatched 
with four natatory setae on the exopodites of 
the first and second maxillipeds, added two 
setae with each molt through stage III. Either 
one or two setae were added with each succes- 
sive molt. Rees (1959:368) and Sankolli 
(1965:39) found similar patterns of progres- 
sive increase in setation in cultured larvae of 
E. talpoida and E. holthuisi. Planktonic larvae 
of E. rathbunae, with rare exceptions, had only 
