707
EGG BEARING POTENTIAL AND EGG DEVELOPMENT IN PAGURUS
GRANOSIMANUS (STIMPSON) AND PAGURUS SAMUELIS (STIMPSON)
David Lamel
Hopkins Marine Station of Stanford University
Pacific Grove, California
June 1, 1965.
Introduction
Information concerning the egg bearing potential
of an animal is often useful to know, but sometimes
difficult to obtain. Very little information is avail-
able for the hermit crabs of the west coast of the
United States. H. G. Coffin (1960) gives brief data
on Pagurus samuelis (Stimpson, 1859), but none on
Pagurus granosimanus (Stimpson, 1859). Also, no infor-
mation is available concerning egg development in these
two species. This paper endeavors to fill these
vacancies.
Methods and Procedures
The crabs used in these experiments were collected
randomly from the intertidal region of China Point in
Pacific Grove, California. The work was carried out
during the month of May, 1965.
The collected crabs were removed from their shells
by cracking the shells in a vise. The eggs were removed
from the female by cutting off the three pleopods holding
the egg mass. The eggs were dried on a paper tissue
for a brief time to remove excess water, and then
weighed on a Mettler balance (accuracy + O.1 milligram).
A small correction was made for the weight of the three
pleopods, amounting to 0.0025 gram or less depending on
the size of the individual crab. A small number of these
eggs (approximately 50) were counted and then weighed.
These data yielded an average weight for a single egg
and the total number of eggs carried.
The total length of the carapace, from the median
tooth to the posterior notch, was chosen as the index
of size. This seemed more accurate than carapace width,
Brie8
fes this distance lies in one plane.) The measurement was
taken using an ocular micrometer (accuracy + 0.1 millimeter).
The age of the eggs was determined by use of a table
reproduced in Coffin's article. (See Table I) By inter-
polation of this table, ages were determined to f l day.
Finally, egg size was measured using an ocular
micrometer (accuracy + 0.01 millimeter). Young eggs are
very nearly spherical in shape, but older eggs assume
the shape of an elongate spheroid. In this case the
longest dimension was measured.
Results
The number of eggs carried by a female is plotted
as a function of carapace length in Figure 1.
The gravid P. granosimanus specimens from the inter-
tidal region have a large range of egg numbers with a
moderate range in size. There is some evidence of the
larger crabs carrying larger numbers of eggs, but not
as much as might be expected. However, for a crab of
C
a given size, there is a considerable variation in the
number of eggs carried. In the intertidal region there
Were no gravid P. granosimanus found larger than 11.5 mi
or smaller than 7.0 mm, although non-gravid females and
males of these sizes were present.
However, one sample of 56 P. granosimanus was
brought up from the subtidal region, approximately 20
feet below mean sea level. Of this sample, 54 were
female and 48 were gravid. A sampling of these dis-
closed a very narrow range of size and egg number. This
size range does not overlap with the intertidal size
range and may imply that the smaller females occur in
a subtidal environment and then migrate to the intertidal
region as they grow larger.
The P. sanuelis have a size distribution very sim-
ilar to that of the intertidal P. granosimanus, ithough
it is shifted slightly towards smaller sizes. The num-
berof eggs carried is generally only a little greater
than half of the number carried by P. granosimanus. There
is also a group of very small individuals carrying fewer
than 50 eggs each. In between these two groups there is
a gap in the size range. This gap could possibly cor-
respond to u subtidal group analogous to the P. gran
imanus subtidal group.
The above results are summarized in Table II.
The results of egg weight as a function of egg
77
O
age, for the two species, is plotted in Figure 2. The
lines drawn are the most probable ones as determined by
the method of least squares. These show that the weight
SfighHy
of a P. granosimanus egg increases more rapidly with
elis egg.
than that of a P. sam
Figure 3 illustrates the relationship of egg size
and egg age. The fitted lines indicate that P. sanuelis
eggs increase in size more rapidly; while at the same
osimanus eggs increase more rapidly in
time, P. gra
weight. This is due to the fact that P. samuelis esce
become slightly more elongated with age than P. grun¬
simanus eggs, and the size measured was the largest
dimension of theegg.
Summary
The size distributions of gravid females in the
intertidal region for the two species are almost identical,
manus carries about twice as many eggs is
but P. grano:
samdelis.
2. For an animal of a given size, there is a largt
variation in the number of eggs carried.
osimanug smaller then ony in the
3. Gravi . gra
intertidal region are found in the subtidal regions.
4. P. granosimanus eggs increase more rapilly in
weight than do P. Samueli
segs.
seggs increase slightly more rapidly
anuel
5. T.
in size, because they become more elongated.
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TABLE I
GROSS EMBRYOLOGICAL DEVELOPMENT OF PAGURUS EGGS
Days Incubation
Gross Description
Eggs uniformly dark brown or purple-brown.
Apparently all eggs are in the zygote stage
with no segmentation.
Eggs uniformly dark brown except for germinal
disc which is translucent white; early gas-
trula stage; yolk broken up into cell-like
granules.
Embryo displacing about one-fourth of egg
capsule; little differentiation.
Color of eggs now fading towards reddish;
differentiation into cephalic and abdominal
regions; limb buds present.
Embryo in rough zoeal form; eyes, limbs, and
abdomen clearly discernible; dark brown yolk
occupying about one-half the egg.
Eggs red in color; heart beating slowly;
ommatidia with rust pigment; some spasmodic
movement.
Eggs orange in color; zoeae appear fully
developed in both structure and pigmentation;
heart beating strongly; little yolk remaining.
Eggs light yellow; some zoeae beginning to
13
hatch.
14
Hatching completed; empty egg capsules on
pleopods on abdomen.
1. From Coffin(1960).
C
Intertidal
P. gran.
Subtidal
P. gran.
Intertidal
- sand.
No.
33
14
20
TABLE II
SIZE AND EGG NUMBER
Carapace Length
7.2 - 11.4 mm
Av. 9.1 mm
4.8 - 6.0 mm
Av. 3.1 mm
6.6 - 10.8. mm
Av. 8.7 mm
2.8 - 3.7 mm
Av. 3.3 mm
Egg Number
645 - 6820
Av. 2440
428 - 1310
Av. 725
446 - 3080
Av. 1450
6 - 44
Av. 23
NO.
O
6000
2000
2000
SUB. P GRAN.
• INTER. P GRAN.
+ INTER. P SAMU.

8
880 +
o 4
C
MM
WT
AG
00
50
— P GRAN.
----- P SAMU.

5 +6
1
g01
8
AGE
DAYS
SIZE
MM
60
— P GRAN.
----- P SAMO.
40
6ooo
+ 9 +
100
8
AGE
DAYS
0
Figure Legends
Figure 1. Number of eggs as a function of carapace
length. Subtidal P. granosimanus, open circles.
Intertidal P. granosimanus, filled circles. Inter-
tidal P. samuelis, crosses.
Figure 2. Egg weight as a function of egg age. Most
probable lines determined by method of least squares.
P. granosimanus, solid line. P. samuelis, dashed line.
Figure 3. Egg size as a function of egg age. Most
probable lines determined by method of least squares.
Bibliography
Coffin, H. G., 1960. The Ovulation, Embryology and
Developmental Stages of the Hermit Crab Pagurus
samuelis (Stimpson). Walla Walla College Pub.
Dept. Biol. Sci., No. 25, 1-30.
173