C
O
THE INTERTIDAL AND SUBTIDAL DISTRIBUTION
OF FOUR SPECIES OF PAGURUS (Fabricius)
AT MUSSEL POINT, CALIFORNIA
Robert Belknap and John Markham
From the Hopkins Marine Station
of
Stanford University
C
O
C
The hermit crabs Pagurus samuelis (Stimpson, 1857),
Pagurus granosimanus (Stimpson, 1859), Pagurus hemphillii
(Benedict, 1892), and Pagurus hirsutiusculus (Dana, 1651)
occupy a definite distribution by species in the intertidal
area at Mussel Point, California. Prior to making a thorough
ecological study of the four species, it was necessary to
determine their distribution as to the total number of individuals
and the relative predominance of each species under a number
of different ecological conditions.
A survey by Bollay (1964) indicated a definite species
distribution over a 10 meter transect with P. samuelis predom-
inantly in the rocky outcroppings and high pools of the upper
intertidal and P. granosimanus and P. hirsutiusculus gaining
predominance in the deeper channels of the lower intertidal.
The region selected for our survey is larger and encompasses a
greater variety of environmental extremes, including a small
section of the subtidal. Extensive collecticns of the Pagirus
species were made with the objective of determining distribution
of the species present and the factors contributing to their
distribution.
Methods
The area chosen for study (see fig. 1) is 80 meters long and
30 meters wide, extending from the highest to lowest intertidal
areas adjacent to the Hopkins Marine Station. The transect was
laid out by triangulation from two reference points of established
position and height above mean low low water with ai transit
and level rod.
O
A subtidal transect 15 x 3 meters was also selected.
Combined, these transects provide all types of situations avail-
able on a rocky bottom, ranging from an intertidal height of
3 meters to a subtidal depth of -10 meters with raspect to mean
low low water, encompassing all degrees of exposure to air, sun,
currents, and extremes in amount of vegetation.
The sampled areas are selected to be representative of
the different environmental conditions present. Each sample
covers an area of 0.1 meter2, plus or minus 10%, or was corrected
to this value when the results were tabulated. Sampling was
carried out with a shovel and followed by a suction pump until
no Pagurus could be found in the area. A portable submarine
of the type used to tow scuba divers was converted into a suction
pump by restricting the intake to a 4 inch flexible hose and
directing the exhaust into a burlap sack. (fig. 2)
Sampling procedure involved:
(1) Hand collection of the larger or extremely mobile crabs.
(2) Shovel collection of the top 5 centimeters of substrate,
with the exception of rock surfaces.
(3) Suction pump collection of any spilled or
remaining specimens.
(4) Neasurement and description of the sampled area.
Whenever samples were taken above the water line, a cleining
with trowel, brush, or hand was substituted for the (vacuuming)
action of the pump.
Each collected sample was taken to shore and washed through
a series of 6 screens ranging in mesh size from 15.5 x 15.5 mm.
to O.7 X O.7 mm. This separation permitted a study of size
distribution and provided a good correlation with crab size
as expressed by width of hard carapace and length of entire
carapace. (Table I)
Schmitt's key (1921) served as the basis for identification
of large hermit crabs. Identification of the smaller specimens
was done under a dissecting microscope with a key provided by
Markham (1965). The tabulated data from each sample area were
entered on a segmented disc to show percentage composition by
species, and the number of each disc placed at the appropriate
position on a map of the transect. (figs. 3a-d)
Results
The 51 samples taken contained a total of 2,921 individuals.
The results portrayed in figures 4, 5, « 6 show that the species
are differentially distributed with respect to horizontal distance.
Figure 7 illustrates the vertical distribution. Where broad bands
are used in the figures, they represent 90% of the population
given by N, while the narrow lines represent the other 10/.
In figure 8 the adult and juvenile populations of two species
are compared. Ten samples with high population densities were
chosen to represent each species, and the distribution of each
displayed as it was sorted by shell size in the screens. (The
majority of specimens found in screens 5 k 6 were without shells.)
All specimens with a carapace widta of 1.5 mm or greater (screens 1-3
see Table I) are considered mature on the basis f coloration
typical for the species (Markham, 1955), and egg bearing in
the female population (Lamel, 1965).
Discussion
In figure 5 it can be seen that P. samuelis, P. granosimanus,
and P. hemphillii have distinct horizontal distributions.
P. samuelis is centered around the inner regions of the transect;
P. granosimanus/found primarily in the central portion of tie
transect, an area of pools, rocky bottoms, trenches and crevices
which give some protection against the mild surge found in this
area. P. hemphillii occupies the outermost portions of the
transect, but is found in smaller numbers per 0.1 m'. The
surprising distribution is that of P. hirsutiusculus which can
be found in large numbers throughout the horizontal ixis of the
transect. The next figure should be compared with figs. 1 £ 5
to emphasize the abovementioned points.
Figure 6 represents the species composition of the samples
10 meter areas to show
in fig. 4 averaged over
a pattern of doninance in each horizontal area, Again, the same g-
pattern is present -- P. samuelis occurs primarily in the inner
zone, P. granosimanus in the midile, and P. hemphillii in the outer.
P. hirsutiusculus is found throughout, appearing in the samples
with greater frequency as the distance from shore increases.
The E. hemphillii and P. samuelis populations are completely
separated, with few exceptions, and those occur at the edges of
deep pools, (samples 23 % 27). Some mixing might be exjected at
a natural boundary between the deep water hermit crios and their
higher dwelling neighbors.
Vertical distribution is shown in figure 7, and again E.
hirsutiusculus is found throughout the range, although in
smaller numbers above 0.5 meters.
P. granosimanus, while occupying the whole range above -0.65
meters, occurs primarily in the middle heights. Once again,
P. hemphillii and P. samuelis are separated, the former occurring
entirely at the greatest depths and the latter at the greatest
heights, with only a small overlap between the heights of -O.O1
and 0.15 meters, where few of either species are found. The
occurrence of P. hirsutiusculus in the lower intertidal regions
1910
contrasts with Rickett's/statement that only P. hemphillii is
to be found in the low-tide region from O to -0.75 meters.
Figure 8 is a representation of the striking fact that
P. granosimanus juveniles are seldom found in the areis where
the population density is high for the species. This distribution
contrasts sharply with that of P. samelis where the juvenile
population often outnumbers the mature individuals. When the
population densities (n - given species population per O.1 m*)
are compared, it is evident that the P. granosimanus juveniles
are relatively scarce, in fact, practically absent in contrast
to the large number of juveniles taken in the P. samuelis zone.
Environmental factors affecting distribution are intertidal
height and distance from shore, degree of exposure to sun and
current, and the nature of the substrate. Two samples (lc 5, fig.
taken from protected pools close to shore are very similar in
habitat and population density, both sample areas partially shaded
and covered by 15 cm. of water, both with sandy bottoms covered
by broken mussel shells. Yet one sample is composed entirely of
P. samuelis (102 per 0.1 m2), and the other is almost completely
P. hirsutiusculus (152 out of 159 per 0.1 m2) since the two
areas are located only 2½ meters apart, the segrecation of species
may be explained in part by the 0.42 meter difference in height,
P. hirsutiusculus found in the lower area. The presence of
coralline algae in the lower area constitutes the other major
difference in environment. P. hirsutiusculus is usually found in
areas with algal cover, while P. samuelis is predominant in areas
with rocky or sandy bottoms. We concur with Orians and King (1964)
in this observation.
The segregation by species noted above is not an isolated
example. A glance at the first twenty samples listed in fig. 4
finds seven of them with population densities greater than
100 per 0.1 m2, and in each case, the sample is predominantly
one species or another. P. samuelis and P. hirsitiusculus
will not share the same rock or algal cover if the population
density is high, even when they live in close proximity.
The data suggest the following hypothesis: any sample with
a population density greater than 100 per O.1 m will tend to
be composed predominantly of one species.
Differences in substrate and exposure often account for the
relative predominance of juvenile or adult individuals in a
particular area. Juveniles prefer a sheltered environment and
are usually clustered together in large quantities under rocks
or clumped in sandy beds of coralline algae. Siae alone forces the
adult population into more exposed rocky areas, but similar areas
witn algal cover of protective crevices contain higner population
densities. The lowest population densities are found in swift
surge channels or sunny open areas. Several samples with zero
or less than five individuals per O.im2 were taken from the
graveled bottoms of deep pools with no algil cover or protection
of any sort. However, the population density of both juveniles
and adults is usually high at the water's edge, for either a
ri
O
7.
Further indication of the role of height as a fictor in
distribution is given by the scattered occurrence of P. samuelis
in the outer sections of the transect. All P. simuelis
occurring more than 30 meters from shore were found at a height
of 0.0. maters or greater. Sample 34, which contained the
greatest percentage of P. simuelis in any sample iken past
30 meters, was collected from a height of 0.55 meters, a height
well within tnat containing 90% of the population. Evidently
P. samuelis occupies a definite vert'cal zone vherever it
occurs horizontally in the intertidal.
Ten samples in the subtidal transect yielded 13 P. hemphillii,
1 P. granosimanus, and ten small specimens which have not yet
been identified. The bottom area is covered with large gravel
and rippled by surge, only a few boulders forming protected
pockets for the Pagurus population. Although the sampled area
covered only 2 m2, it was determined by visual inspection and
selected sampling that no hermit crabs are present in the large
gravel surge channels which cover most of this subtidal region.
Sumnary
A survey of the intertidal and subtidal area off Mussel Point
has shoun:
(1) The Pagurus spp. found intertidally occupy definite
horizontal and vertical zones. The inper section of
the middle
the transect contains primarily P.
section P. granosimanus, and the outer regions P. hemphillii,
with P. hirsutiusculus extending over most of the transect.
(2) Juveniles of P. granosimanus are not clustered together
in large quantities under rocks and algal cover, whereas
the other three species of Pagurus exhibit high population
densities under these conditions.
(3) The data on population in fig. I suggest the following
hypothesis -- any sample with a population iensity
greater than 100 per 0.1 m' will tend to be composed
predominantly of one species.
Pagurus Spp. may be located in terms of i referred
(4)
environment. P. samuelis can oe found in rocky exposed
areas, where P. hirsutiusculus prefers regions with
algal cover. P. granosimanus is found in the pools and
mild surge channels of the midile ind lower intertidil,
where P. hemphillii prefers the deeper witers and
constant surge of the lower intertidal und sibtiiil regions.
The number of Pagurus spp. is reduced in the sibtidal
(5)
region. P. hemphillii is the dominant srecies,
P. granosimanus is found occasionally, with P. sinielis
and P. hirsutiusculus absent entirely in the smill
region studied here
c
LEGEND
Figure 1: Photograph of the transect at Mussel Point, calif.
Looking northward, the Bird Rocks appear in the upper righthand
corner. Reference point Bis indicated in the lower righthand
corner. Subtidal transect is 30 meters East of Bird Rock.
Figure 2: Photograph of the suction purp used in sampling, with
the collecting bag, shovel, and series of graded gcregns in the
background. The inverted bucket covers a waterrurp to recirculate
sea water used to waeh specimens and substrate isto the profer
screens.
Figure 3: Sketch of intertidal transect. Sample nurbers on wap
corresrond with those below dieks (3e). Inset (7c) is of ertire
transect 1/4 X scale of main sketch to show relationskigs of
figs. 3a-c. In this figure and thoee which follow, the abbreviations
used are: P. sam, Pagurus samuelis; P. hirs, Pagurus hirsutlusculus;
11111.
drus granosimanus; P. hewp, Pagurus he
P. gran, Pa
Scale at right incicates Clstance from refererce point B in meters.
Liscs ahow a composition of each sample by species.
Figure 4: Species comrosition of intertidal samples. N indicates
total number in each samrle per 0.1 metere, D indicates Cistances
from shore groupe within 10 meter bands. S indicates sample nümber,
corresponding to the same number on map and disc, fig. Ja-d. Species
designations same as fig. 30.
Figure 5: Horizontal distribution by species in the intertical,
Percentages are of total crabs taken, 2921 indivicuals. M, meters
from shore (ref. pt. B). N, number of given species collected.
Eroad bands represent 90 of the given population, narrow lines,
the remaining 10%.
Figure 6: Species comrosition of intertidal samples averaged over
10 meter horizontal bands. Solid lines with croeses, Pagyryg samuelis
dotted lines with circles, Fagurus granosimanus; dashed lines with
squares, Pagurue hirsutiusculus; dots and dacheg with triangles,
1111. Peints plotted above are obtained from fig. 4
Pagurus herph
by averaging the percentages recorded in each 10 meter tand.
M. meters from ref. pt. B; % are of total crats collecte in the
given 10 meter band.
Figure 7: Vertical Cistribution by species. M, reters above mean
low low water; broad bands, percentages, & Nas Gesignated in fig. 5.
Figure 8: Adult and juvenile populations in samples of two épecies.
Grav incicates adults found in screens 1-3, white indicates juvenilee
found in screens 5 & 6, and stripes indica'e mixed pogulations fouge
in sereen 4. N, number of P. Samuelis or P. grancgigarus per 0.1 mo.
C
LITERATURE CITED
Bollay, Melodv, 1964. Distribution and utilization of Gastropod
shells by the hermit crabs Pagurus samuelis, Pagurus
granosimanus, and Pagurus hirsutiusculus at Pacific Crove,
California. The Veliger (Supplement): 71-76.
Lamel
David A, 1965. Egg bearing potential and egg development in
Pagurus granosimanus and Pagurus samuelis. Unpublished paper,
Stanford university.
Orians, Gordon H. & Charles E. King, 1964. Shell selection and
invasion rates of some Pacific hermit crabs. Pacific Science
18: 297-306.
Putnam, John D. & John C. Markham, 1965. Keys to larval and post-
larval stages for selected species of Pagurus. Unpublished
paper, Stanford University.
Ricketts, Edward F & Jack Calvin, 1948. Between Pacific Tices,
Revised Ecition, Stanford, California, Stanford University Press:81.
Schmitt, W.L., 1921. The Marine Decapod Crustacea of California,
Univ. Calif. Publ. Z0ol., 23: 128-143.
TABLE I
Correlation of Screen Size with Carapace Size -
Grid dimensions
Screen 41 -- 4.0 mm. and larger in width
15.0 mmX 15.0 mm
7.0 mm. and larger in length
Screen 42 -- 2.1 mm. to 3.9 mm. in width
8.5 mmX 8.5 mm
4.0 mm. to 6.9 mm. in length
Screen 43 -- 1.5 mm. to 2.1 mm. in width
6.0 mmX 6.0 mm
3.5 mm. to 5.0 mm. in length
Screen 44 -- 1.4 mm. to 2.0 mm. in width
3.O mm X 3.0 mm
2.6 mm. to 3.6 mm. in length
Screen 45 -- 1.0 mm. to 1.5 mm. in width
1.4 mmX 1.4 mm
2.0 mm. to 2.6 mm. in length
Screen 46 -- 0.7 mm. to 0.9 mm in width
O.7 mmX 0.7 mm
1.1 mm. to 1.8 mm. in length
About fifteen samples were measured to determine the mean
size range in each case.
Table II
Vertical height of intertidal samples (in meters)
Sample f
Vertical ht.
Sample
0.83
0.96
.96
0.92
0.50
0.99
0.70
0.3
0.36
0.19
1.27
0.53
10
1.20
1
0.43
12
o.4
0.47
44
0.00
0.52
16
19
.51
0.51
19
0.15
.05
-0.14
-0.18
0.05
Vertical ht.
0.03
-0.03
O.03
-0.23
0.07
0.26
0.55
-0.51
0.38
0.05
O.13
-0.01
-0.65
-0.41
-0.39
-0.39
-0.65
-0.1
-0.58
-1.19
-0.15
-0.77
0.02
-0.08
C
Figure 1: Photograph of the transect at Mussel Point, Calif,
Looking northward, the bird Rocks appear in the upper righthanc
corner. Reference point E is indicated in the lower righthand
Subt'dal transect is 30 meters ast of sird oc.
corner.
Fig.!
Fig. 2
Figure 2: Photograph of the suctien pump used in earpling, with
the collecting bag, shovel, and series of graced screens in the
The inverted bucket covers a waterpurp to recirculate
backrround.
oer
the
sea water used to wash specimens and
betrate t
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Fig. 5
Figure 5: Horizontal distribution by species in the intertical.
Percentages are of total crabs taken, 2921 individuals. M, meters
from shore (ref. pt. B). N, number of given species collected,
Broad bands represent 90% of the given population, narrow lines,
the remaining 10.
EMes
a103
N1390
E son
1269
E. pran
224
P. Nemp
76
Fig. 7
Figur
Vertic
10 1 10 10 u incn
D INCNE
reurest e aoeen e
1
P samuelis
P. granosimanus
Fig. 8
Figure 8: Adult anc juvenile populations in samples of two species.
Gray indicates adults found in screens 1-3, white indicates juveniles
found in screens 5 & 6, and stripes indicate mixec populations fougé
in screen 4. N, number of P, samuelis or P, granosiranus per O.1 me
N-
Acknowledgments
We would like to gratefully acknowledge the advice and
assistance of Professors Laurence R. Blinks and Donald P.
Abbott. Thanks to Bill Austin for advice and materials,
to Robert Cook and Jim Landers for photographs and help with
the subtidal region, and to Jack Putnam for uncomplaining
help with the disagreeable task of screening.