Larval Development of Emerita ratbbunae — Knight 
61 
an even number of setae on the exopodites of 
the maxillipeds. This setation, therefore, ap- 
peared to be a reliable indication of the number 
of molts through which an individual had 
progressed. 
In samples examined, 67% of the planktonic 
larvae of E. rathbunae apparently would have 
molted to megalopa after six zoeal stages, 33% 
after seven zoeal molts. This represents, on the 
average, an abbreviation of the zoeal develop- 
ment observed in the laboratory-reared animals. 
Comparatively few differences were observed 
between individuals in comparable zoeal stages. 
There were no detectable morphological dif- 
ferences between cultured and planktonic larvae 
within stages I, II, and III. In intermediate in- 
stars BV— VI, a detailed comparison of the larval 
cycle of seven zoeal stages, common to both 
cultured and planktonic forms, showed that cul- 
tured larvae were less advanced in growth and 
setation of some appendages than were plank- 
tonic larvae in equivalent stages. The terminal 
zoeas again were similar. The cultured larvae 
were smaller than planktonic larvae at compar- 
able stages of development beyond stage I 
(Fig. A), but the distinctive proportions of 
the carapace and its rostral and lateral spines 
were consistent in both cultured and planktonic 
forms throughout zoeal development. 
The second zoea of E. rathbunae, sorted from 
living plankton, was cultured for 7 of the 11 
days aboard ship under conditions comparable 
with its natural environment. During this period 
it molted three times, following exactly the pat- 
tern of growth and setation observed in plank- 
tonic specimens preserved from the same area. 
The larva was transferred to the laboratory in 
stage V and died in stage VIII, before the molt 
to megalopa (indicated by segmentation of post- 
larval appendages visible beneath the cuticle). 
In stages V— VII, the rate of development of 
appendages was retarded in relation to the 
regular progressive setation (2 setae per molt) 
of the first and second maxillipeds. This larva 
developed more rapidly than did larvae hatched 
and cultured aboard ship which had been sub- 
jected to environmental change at an earlier age, 
but it was less advanced in the late instars than 
were larvae studied from the plankton. Al- 
though the evidence for modification of molt 
cycle midway in zoeal development is based on 
(OT) 
(0) 
X 
0 
XO 
1 I I I I I I I 
I II III IV V VI VII VIII 
ZOEAL STAGES 
Fig. A. Average length of carapace (posterior 
margin to tip of rostral spine) for zoeal stages of 
cultured and planktonic larvae of Emerita rathbunae. 
X, Planktonic larvae; O, cultured larvae; T, terminal 
zoeal stage; (), average based on less than 10 speci- 
mens. 
only a single specimen, it provides a link be- 
tween the cultured and planktonic forms as well 
as an indication of the potential variability in 
the larval development of E. rathbunae. 
Description of Larval Stages 
To facilitate identification of the larvae of 
E. rathbunae in the plankton, the descriptions 
of intermediate stages IV and V are based upon 
planktonic specimens rather than upon the vari- 
able forms obtained in the laboratory. The 
descriptions of stages I— III and the terminal 
zoea are based upon both cultured and plank- 
tonic larvae for structure and development of 
appendages; the setation is based upon zoeal 
stages from the plankton. Measurements of the 
zoeal stages are given in Table 2. Average 
figures are based upon measurements of 10 or 
more specimens, except for zoea VI (not ter- 
minal), for which only 5 specimens were avail- 
able. The length of rostrum and carapace were 
measured from the posterior margin of eyestalk. 
The length of telson excludes telson processes. 
zoea i (Fig. 1); The zoea is colorless and 
translucent, the rounded carapace has a short, 
