Larval Development of Emerita rathbunae — Knight 
75 
able, both in nature and under conditions of 
laboratory culture. In the laboratory, although 
the larvae retained their specific proportions 
and pattern of development, as many as three 
intermediate instars could be added to the larval 
sequences usually observed in specimens from 
the plankton. Variability between individuals 
at comparable stages of development increased 
with an increase in the number of zoeal molts 
in the larval period. 
A variable number of zoeal stages has been 
found in the laboratory culture of two other 
species of Emerita. Dr. Ian Efford (personal 
communication) has noted 9-11 zoeal molts 
among reared larvae of E. analoga, and larvae 
of E. talpoida cultured by Rees (1959) passed 
through 6 or 7 zoeal stages before the molt to 
megalopa. Rees, using setation of the maxilli- 
peds as an indication of the stage of develop- 
ment, compared the zoeal stages observed in 
the laboratory with those described by Smith 
(1877) from the plankton and found that 
larvae from the plankton which molted to meg- 
alopa in the laboratory were apparently only in 
stage V. Rees noted as well that "zoea from 
nature possess features (appearance of thoracic 
limb buds, pleopods, etc.) which show them 
to be farther advanced in development than the 
corresponding laboratory stages.” This relation 
between cultured and ''natural” larvae was also 
observed in the present study. 
Some indication of seasonal variation in num- 
ber of zoeal stages was found in specimens of 
E. rathbunae examined from the plankton. Most 
(94%) of the late-stage larvae taken in August 
and September appeared ready to molt to mega- 
lopa after six zoeal stages; in the December 
samples only 32% would metamorphose after 
six stages and 68% after seven stages. 
Differences in setation and development of 
appendages between individuals in comparable 
intermediate stages were not noted by Rees in 
cultured larvae of E. talpoida and were rare in 
larvae of E. rathbunae from the plankton. In- 
dividual variation became more pronounced 
among reared larvae of E. rathbunae with an 
increase in the number of zoeal stages, and 
might be related to rate of development. Larvae 
of E. rathbunae subjected at an early age to low 
temperatures (and perhaps other variables) of 
the laboratory environment had a larval span of 
81-94 days. E. talpoida completed zoeal develop- 
ment in 23-33 days at 30 °C. The molting fre- 
quency of the planktonic larva of E. rathbunae 
cultured through two instars at 27°-30°C was 
consistent with that found by Rees for E. tal- 
poida, and it appears likely that E. rathbunae 
would have a much shorter larval life at the 
higher temperatures in its natural environment. 
Perhaps culture of Emerita larvae over a range 
of controlled temperatures would show a rela- 
tion between duration and number of zoeal 
stages and degree of individual variability. The 
consistent difference in size found between cul- 
tured and planktonic specimens suggests that 
larvae of this species are restricted in over-all 
size by the conditions of laboratory culture. 
Costlow (1965) has reviewed accounts in 
the literature of variability within larvae of 
Crustacea and has discussed effects of environ- 
mental factors (light, diet, temperature, salinity, 
etc.) on frequency and variability in molting, 
as well as current investigations of endocrine 
mechanisms related to molting in larvae of 
brachyuran decapods. Variation in number of 
zoeal intermolts has been noted in the laboratory 
culture of several anomuran decapods by Pro- 
venzano (1962a, b') for two species of pagurid 
crabs, and by Boyd and Johnson (1963) for 
the galatheid, Pleuroncodes planipes, but ap- 
parently such variation is rare among brach- 
yurans. Gurney (1942) suggested that artificial 
rearing might give misleading results, and 
stated that, while stages I— III in the develop- 
ment of larval decapods seemed to be relatively 
fixed, the natural course of development after 
that might be disturbed with addition of stages 
not found in nature. He noted as well that there 
is no certainty that all stages observed in nature 
are passed through by all individuals of a 
species. The use of only laboratory-reared 
material to investigate the growth patterns of a 
species with such capacity for variability in 
larval development as that shown by E. rath- 
bunae would indeed have been misleading 
unless supplemented by a study of the larvae 
taken from their natural environment. 
ACKNOWLEDGMENTS 
This work was supported by the Marine Life 
Research Program, the Scripps Institution of 
