They swim by trailing their front chelae and 
flapping their abdomens, thus propelling them- 
selves backward through the water. 
FEEDING HABITS: More than 20 specimens of 
P. eriomerus were examined microscopically to 
determine the nature of the stomach content. 
As all of the contents seemed quite similar, 
detailed notes were kept only for 10 individuals. 
The majority of stomachs contained almost pure 
masses of diatoms. Only one contained sessile 
diatoms found growing on the rocks within the 
Petrolisthes habitat. The others contained dia- 
toms of a pelagic form. Two or three had ex- 
tremely large fragments (up to 1 mm long) 
of some species of green algae. From the gut 
content alone it seems that this animal is pre- 
dominantly a filter feeder, though evidence 
shows that some cropping of sessile algae is 
carried out also. Observations in the field and 
in the aquarium show that this species spends 
most of its time with its ventral surface held 
against the bottom side of a rock and with the 
face showing at the edge of the rock where 
water currents are continuously being sampled. 
The movement of the mouth parts suggests that 
this animal filters its food from the water. Lab- 
oratory animals, for the most part, refused meat 
fragments offered to them. This species, then, 
is thought to stay within the confines of the 
rock under which it is hiding, and to feed at 
the periphery of this rock by filtering or by 
cropping algae growing within chelae reach. 
SYNOPSIS OF REPRODUCTIVE ACTIVITY: Fig- 
ure 7 shows that oogenesis probably begins in 
the month of October and continues until the 
time of ovulation, fertilization, and egg deposi- 
tion. We assume that copulation probably takes 
place in January. Egg deposition begins in the 
early part of February and increases rapidly until 
the early weeks of April, when the majority of 
females have produced their first brood of eggs. 
Hatching probably begins around the first of 
May for the first brood and continues into the 
first week of August. Egg deposition for brood 
number two begins about the middle of May 
and reaches its peak about the early part of 
August. At this time hatching of brood number 
two begins and continues into early October. 
Referring to the physical conditions represented 
on Figure 7, one could make the suppositions 
that copulation begins shortly after the shift 
Fig. 7. The reproductive cycle of Petrolisthes erio- 
merus. 
