California Xanthidae— Knudsen 
11 
feed on smaller animals or plants. Each zoeal 
stage resembles the previous one, but shows a 
marked increase in size. The megalops more 
closely resembles a crab, having chelae and four 
pairs of walking legs. This stage settles and 
molts, thus becoming the first crab stage which 
resembles the adult. In the laboratory an average 
of 5 weeks was required to complete the larval 
stages of the four species. The first zoeal stage 
lasted from 4 to 6 days. The megalops stage 
lasted from 7 to 10 days. The first crab stages 
lasted 13 days, but each stage beyond this re- 
quired an increase in time for completion. 
LARVAL HABITS AND ECOLOGY 
Locomotion 
The zoea uses two forms of locomotion. The 
first is a form of directional swimming in which 
the larva moves in a straight line. Swimming is 
always carried out in the direction of the dorsal 
spine so that the body is either in a normal or 
inverted position (Fig. 3, A, B). The first and 
second maxillipeds are used as paddles and 
rapidly move up and down as the larva swims. 
Fig. 3 . Methods of zoeal locomotion: A, Normal 
maxilliped locomotion; B, inverted maxilliped loco- 
motion; C1-C2, maxilliped-telson or hop-up-and-sink 
locomotion. 
On the distal end of the exopodite of the maxil- 
lipeds are four or more plumed natatory "hairs.” 
These "hairs” fan out as the blade of an oar and 
give the zoea added thrust. After each ecdysis 
several new natatory "hairs” are added to help 
propel the additional mass of the zoea. 
Weldon (1889: 169) states that steering is 
effected by means of the telson and abdomen. 
This may be the case where positive photo- 
taxis is not involved. Mast (1938: 209-216) 
has shown that photopositive animals with bi- 
laterally located photoreceptors and locomotor 
organs are affected by the amount of light that 
each photoreceptor receives. Thus when more 
light strikes the right eye the left bank of loco- 
motor organs become more active until the body 
is turned to a point where both eyes are equally 
illuminated. To test this for the Xanthidae, many 
zoeae were placed in an 8-inch cylindrical jar 
in a darkened room. When a light was turned 
on the larvae swam towards it, but when one 
light was replaced by a second light 90 degrees 
away, the larvae turned and swam in that direc- 
tion. When a light was rotated around the jar 
the larvae swam in circles in a horizontal plane. 
When the light was slowly passed over, around 
and under the jar the larvae swam in vertical 
circles. 
The second form of zoeal locomotion is 
a random hop-up-and-sink type of movement 
which follows no definite direction. As a zoea 
sinks, it draws its abdomen up against its thorax 
and raises its maxillipeds (Fig. 3Ci). Simulta- 
neously the abdomen and maxillipeds are thrust 
downward, resulting in an upward propulsion of 
the larva (Fig. 3C 2 ) . The process is repeated so 
that the zoea remains at approximately the same 
level in the water column. 
Megalops do not use their mouth parts for 
locomotion but swim by means of pleopods and 
walk by means of pereiopods. Newly molted 
megalops are photopositive while older individ- 
uals become somewhat photonegative. When 
swimming, the pereiopods (Fig. 4A) are folded 
against the body and the abdomen is fully ex- 
tended. When the megalops turns upward ( Fig. 
4B) or downward (Fig. 4C) the abdomen is 
bent up or down, as the elevator of an airplane, 
to effect the change in direction. 
