12 
PACIFIC SCIENCE, Vol. XIV, January I960 
Fig. 4. Locomotion of the megalops stage. See text. 
The rate of directional swimming is remark- 
able. Zoeae placed in a piece of glass tubing 1 
inch in diameter and one meter long swam the 
meter distance in a horizontal plane in 45 to 60 
seconds. For this experiment a light was used to 
maintain the direction of swimming and a wrist 
watch for timing. These times compare exactly 
with those recorded by Foxon (1934: 842) for 
Galathea, Eupagurus, Pandelus, and P or ce liana 
larvae. Xanthid larvae swam up a 1 -meter 
column in 50 to 60 seconds, while swimming 
down the column required only 12 to 15 seconds. 
Xanthid megalops were only timed on one occa- 
sion, at shipside while at anchor. The average 
approximated rate was 2 meters per minute, or 
about twice the speed of zoeae. Foxon (1934: 
843) suggests that the larval swimming rate 
is sufficient for diurnal migration, which actually 
does occur (Russell, 1927). 
Orientation 
Mast (1938: 215) suggests that a portion of 
the eye functions as a receptor which detects the 
direction of light. When a light source is present 
the larvae show no response to gravity as seen 
above, and as reported by Weldon 1889: 169), 
Spooner (1933: 395), and Foxon (1934: 838). 
However, Foxon demonstrates that when light is 
absent a normal dorsal-up orientation is main- 
tained. Gurney (1902: 475) suggests that the 
carapace spines play some role in orientation, 
since statocysts are not present in the zoea. 
Foxon demonstrated that drugged zoeae, or those 
with the maxilliped exopodites excised, sank un- 
controllably, suggesting awareness and func- 
tional exopodites are required also. However, 
more testing of these theories is necessary. 
Pood and Feeding Habits 
Lebour (1928: 500) states that the natural 
food of zoeae consists of planktonic organisms, 
such as diatoms and other microscopic algae, 
larval and small adult animals. In the laboratory, 
mixtures' of protozoans, turbellarians, diatoms, 
and other aquarium dwellers were fed to the 
zoeae. However, the nauplius larva of brine 
shrimp, Anemia , proved to be an excellent 
source of food for the zoeae. Megalops fed upon 
Artemia, zoeae, and small sessile algae. 
Zoeae were observed under the microscope 
while feeding on nauplius larvae. A zoea catches 
a nauplius by swimming just a little above it, 
tilting backward, and dropping down upon it 
from above. The nauplius is caught between the 
maxillipeds (which are used for feeding as well 
as for locomotion ) and held by the endopodites. 
The rostral spine and antennae prevent the 
nauplius from escaping anteriorly. The abdo- 
men and telson are curled beneath the nauplius, 
preventing escape posteriorly and ventrally. The 
mandibles are then in position to feed upon the 
nauplius. During the feeding process the ex- 
opodites of the maxillipeds are used for the 
locomotion or balance of the zoea. 
Ecdysis 
In the laboratory the periods of larval ecdysis 
are the times of the greatest mortality. The molt 
from the first to the second zoeal stage is the 
most critical, while the molt and transition from 
the fourth zoea to the megalops stage is next. 
During the act of molting a great deal of energy 
is spent in an attempt to get free of the old 
"skin. 5 ’ Zoeae which pause to rest generally ex- 
pand to a point where withdrawal becomes im- 
possible. Larvae in this stage live for a while 
with the old skin attached to them, but feeding 
and future molting is generally impossible. Other 
larvae are so weakened after ecdysis they are 
