PREDATORS OF PAGURUS
Robert T. Cok
Hopkins Marine Station
Introduction
The hermit crabs, Pagurus sammuelis and P, granosimanus
are abudant on the rocky shore of California. To date no
special studies of predation on these crabs have been carried
out, although MacGinitie and MacGinitie (1949) note that
Octopus apollyon often feeds on them. Mackay (1942) found
that the primary food of Eancer magister to be Crustacea
under which he lists small crabs as important prey. Pagurids
are well protected by their hard exoskeletons and hard shell
salvaged from the ocean's bottom. In addition the hermit
crabs move about at a fair speed through the water and retract
far into their shells when disturbed. When wedged back into
their shells it is impossible to pull them out without either
damaging the animals or inducing them to release their hold.
Amarine animal which feeds on pagurids needs a special mechanism
and behavior to extirpate these highly protected animals.
The following studies were carried out in an effort to learn
more about predation on hermit crabs in thearea of the Hopkins
Marine Station at Mussel Point, Pacific Grove, California.
Procedure
The hermit crabs used in these studies were large
specimens of Pagurus samuelis (Stimpson, 1859) and P, gran-
osimanus (Stimpson, 1859) occupying shells of Tegula funebralis
and T. brunnea larger than 1.5 cm in diameter. Animals studied
as possible predators on species of Pagurus included the
following: Octopus apollyon (Berry, 1912); the crabs Cancer
productus (Randall, 1839), C. antennarius (Stimpson, 1856),
C. gracilis (Dana, 1852), Pachygrapsus crassipes (Randall,
1839), and Hemigrapsus nudus (Lana, 1857); the starfish
Pycnopodia helianthoides (Brandt, 1835), Patiria miniata
(Brandt, 1835), and Pisaster ochraceous (Erandt, 1835); the
sea anemone Anthopleura elegantissima (Brandt, 1835); and the
hermit crab Pagurus hemphillii (Benedict, 1892). All specimens
of hermit crabs and possible predators were collected in the
intertidal and adjacent subtidal regions near the Hopkins
Marine Station during April and May, 1965. For observations
of feeding rates the animals were kept in the laboratory in
17x11x10 inch tanks provided with running sea water at 1300.
Between 30 and 60 large Pagurus were placed in each of
several tanks and the numbers and weights of each species
were determined. Each tank was then provided with a particular
species of potential predators. Periodically counts were taken
of the numbers of Pagurus remaining and empty shells were
removed. As controls, tanks containing comparable groups
of Pagurus were set up without predators to establish the
mortality rate in the absence of large predators. Details
are shown in Fig. 1. During the experiments, notes and photo-
graghs were made of the feeding mechanisms of the predators.
After termination of the feeding rate experiments the animals
were starved and used for detailed study of feeding behavior.
O
TANK
1.A
2.
3.
4.A
5.A
6.A
7.A
8.A
PREDATOR
NONE
NONE
NONE
P. HEMPHILI
ANTHOPLEURA
ELEGANTISSIN
HEMIGRAGSUS
HUDUS
PACHYGRAPSUS
CRASSIPES
OCTOPUS
APOLLYON 2
OCTOPUS
AFOLLYON
PISASTER
OCHRACEOUS
PYCNOPODIA
HELIATHOIDES
CANCER
PRODUCTUS 54
CANCER
GRACILIS
CANCER
ANTENNARIUS
41
CANCER
ANTENNARIUS
1
CANCER
ANTENNARTUS
NO.
ARIMALS
11 P, sam
28 P, gran
14 P, sam
24 P. gran
42 P, sam
42 P. gran
50 P, hem
28 P. sam
22 1, gran
32 Pagurus
11 P, sam
28 P. gran
1 P, sam
28 P, gran
14 P. sam
24 P. gran
40 Pagurus
7 P. och,
50 Pagurus
2 P, hel.
60 Pagurus
80 Pagurus
25 Pagurus
10 P, hem
28 P. gran
39 Psam
14 P. sam
20 , gran
2 C, ant
27 Pagurus
TOTAL WT
68em
180gm
68gm
196gm
70gm
179gm
69gm
181gm
69gm
191gm
SHELL
LENGTH PAGURUS
DIAMETROF EXP ALIVE LEAI
1.7-3.1
days
cm
1.5-2.9
cm
days
ca.1.5-
14
0
3.Ocm
days
large
days
days
1.6-2.8
cm
days
1.6-2.8
10
cm
days
14
1.5-3.
5grap
om
days
3sam
large
12
days
large
daya
foot
26
large
days
large
15
62
days
large
15
days
hen
14
large
gre
days 2 sam
8 sam
.7-2.3
cm
days 9 gra
24". 24
5-1.8cm
days
Quantitative Results
The mortality rate was zero among the control populations
elegantissima,
of Pagurus. Patiria miniata, Anthopleura
Pachygrapsus crassipes, Hemigrapsus nudus and Pagurus hemphillii
Of the other animals
showed no predation over a one week span.
Results are shown
studied all were found devouting Pagurus.
in Fig. 1. Feeding rates, calculated from the data in Fig. 1.
are shown in Fig. 2. Cancer antennarius and C, productus were
the major predators upon Pagurus within the confines of the
lab. The feeding rate of Pycnopodia is based on the average
of two one foot specimens. The feeding rate of Pisaster is
based on seven specimens feeding for two weeks.
Fig. 2. Rates of predation upon Pagurus.
Key: Cl - controls, P.o. - Pisaster ochraceous,
P.h. - Pycnopodia helianthoides, O.a. - Octopus
apollyon (2'), O.a.' - Octopus apollyon (1t),
C.a. - Cancer antennarius (24"424"), C.g. -
Cancer gracilis, C.a. - C, antennarius (3."),
C.p. - C. productus, C.a. - C. antennarius (44").
PO.
Ph.
O.0.
O.o.
C.O.
C.9.
C.o.
C.p.
C.O
15
10
2 5
20
MORTALITY
(No. of Pogurus enten per week)
30
3
Qualatative Observations of Predation
Pycnopodia helianthoides
In the lab the only animal to which Pagurus showed a
marked escape response prior to capture was Pycnopodia helian
thoides. Pagurus moved quickly away from any P, helianthoides
which crawled along the bottom of the tank. Inthe eighteen times
I saw starfiches' arms encircle Pagurus' shells I never saw
one escape. Within 15-30 seconds the shell was transported to
the long tube feet near the oral opening always with the aperture
of the shell toward the mouth of the starfish. It usually
took between 10 minutes and 2 hours for P, helianthoides to
extract the Pagurus from its shell. If at the end of two or
three hours the starfish had not extracted the Pagurus it
either dropped the shell containing the live Pagurus, or inverted
its stomach and digested the Pagurus in the shell.
Pisaster ochraceous
Although capture by P, ochraceous was never observed,
Pagurus after captured showed several escape responses to
P, ochraceous. When Pagurus was placed under a starfish arm,
Pagurus usually retracted and remained still. After a few
minutes the hermit crab usually came out of its shell and
struggled actively. Typically it pushed against the glass of
the tank and raised the P, ochraceous' arm and then tried to
struggle free. Ifthe aperture of the Pagurus' shell was not
facing the glass the Pagurus came out, grasped the arm of
P. ochraceous, and oscillated the shell back and forth until
the tube feet released their grip and the Pagurus could climb
free. (Fig. 3
ig. J. 4 Pacumnaten
Sek its shell fre
from the tube feet of the arm of P, ochraceous.
Once a shell was placed under one of the starfish's
arms it took at least two hours to transport the shell to
the mouth. In the only three complete observations of moving
the shells to the mouth, the shell en route to the center
was always flipped over so the aperture faced the starfish
before it finally reached the mouth. In all cases, P, ochraceous
fed by inserting its everted stomach into the shell of the
Pagurus
In the intertidal region field observations of two
P, ochraceous feeding on Pagurus granosimanus were made.
(Figs, 4, & 5.) These findings are in contradiction to the
survey of Feder (1959) who studied the feeding habits of
P. ochraceous on the Monterey Peninsula and never found an
hermit crabs.
Tound devouitee
on a rock at low tide.
antennarius
Approximately 45 complete feeding observations were
made on three adult C. antennarius ranging from 38" to 5'
across the widest part of the carcpace. Following is a detailed
composite description of its behavior in feeding upon Pagurus spp.
Capturing Pagurus
As long as C. antennarius did not move violently
Pagurus did not react to its presence. During the crabs
sedentary periods Pagurus were found crawling over, under,
and around its body. Depending on thelocation of the prey,
C, antennarius was observed to use three different methods
to capture it. If a Pagurus touched the sensitive hairs on the
walking legs of C, antennarius, the legs touched were raised
and extended over the hermit crab then lowered and flexed to
draw the hermit crab below the crab's body. (Fig. 6.) In
capturing prey behind its body, C. antennarius extended its
last pair of walking legs, encircled the hermit crab posterierly,
and swept it forward under its body. C, antennarius was seen
to gather as many as eight hermit crabs under its body at one
time in;

walking legs in
Fig.
A second means by which C. antennarius captures Pagurus
is by reaching out with a cheliped and clamping its pincers
around a Pagurus' shell. I have observed t is procedure only
about six times, mainly when a Pagurus was located in a corner
or was olimbi
ig. 7.)

t chela
Fi
aroune
and
ina oe o
The third common method of capture was observed over
thirty times. It occurred when a Pagurus moved rapidly in
front of C, antennarius. After sighting the moving animal
some distance away the crab raised itself on its walking legs
so its body cleared the floor by about two inches. The chelae
were raised high and the pincers were opened. The crab next
tiptoed to the prey by walking on the distal part of the
dactyls of the walking legs. (Figs. 8. & 9.) Once the prey
was beneath its abdomen the crab lowered its body quickly to
the floor while sliding all the walking legs in under the
abdomen. At the same time the chelipeds were moved down to
2
Fig. 8.
Crab tiptoing over a pagurid in a light
colored shel
Fig.
C. antennarius tiptoing directly over a Pagurus.
In the next few seconds the body was lowered and the
Pagurus caged.
Fig. 10. 4 capture.
a under
the abdomen of C, antennarius
Rackay (1943 described a claw raising behavior in
a tank-adapted Cancer magister at the approach of food.
He, however, attributed this behavior possibly to a conditioned
response resulting from feeding a crab eliciting a defensive
behavior. My observations support this hypothesis as I never
witnessed any such behavior in any of the Cancer spp. when
feeding undisturbed on hermit crabs.
Positioning the Shell
When shells containig pagurids were located beneath
its body, C. antennarius scrapæd a shell forward with either
of its two front pairs of walking legs and raised it upward.
The chelipeds, first walking legs, and third maxillapeds
were used to rotate and position the shell. (Fig. 11.) Usually


Fig. 11. This coab gathered six Pagurus under his body
and then proceeded to examine one.
the aperture of the shell was then examined thoroughly by all
three pairs of maxillapeds. Next, the shell was manuevered
around while the pincers closed and opened on the shell at
various angles. The crab occassionly attempted to crack a
shell during this period as shown by the scratches and nicks
in the shell. About three out of four times the drab discarded
the shell it Whad been examining and brought another shell
forward from the storehouse of
leleneath its abdomen.
(Fig. 12.)


Fig. 12. Discarding a shell after examining it.
When beginning to feed for the day, C, antennarius
examined and tested the first captured shells for five to
thirty minutes before it actually applied enough leverage
to chip off a portion of a shell. If the crab had already
been cracking shells and feeding on Pagurus immediately before
he began on another shell, the examination period was very
short and never lasted over ten minutes.
Cracking the Shell
With small or partially broken shells, C. antennarius
usually held the Pagurus' shell with one cheliped and crushed
it by using the other chela much like a nutcracker. The
shell was placed at the extreme proximal end between the
pincers, where the greatest leverage exists, and broken.
(Fig. 13.)
Fig. 13. C. antenarius syrrting off the upper
part of a Thais shell with its left pincer.
With larger shells, especially Tegula shells 2 cm. or more
in diameter, the crab commonly began by inserting the dactyls
of the two chelipeds into the aperture of the shell. While one
pincer held the shell the other crimped the outer lip until
a piece broke off from the whorl. (Fig. 14.) If the pincer
did not break off part of the whorl the walking legs many times
/02
Fig. . ra loken from a shell.
raised one side of thecrab's body into a tilted position
while its chelipeds were extended forward, while squeezing
the shell harder and harder. In this tilted position the
C. antennarius appeared able to apply a greater force with its
pincers. By tilting its body and extending its chelipeds
the crab usually formed a 90 degree angle or less between
its two chelipeds clamped to the shell. Normally the
crab's chelipeds function at very obtuse angles to each
e chelipeds in Fig.
other. (Contrast the
13. & 14, with Fig. 15.) The chela holding the shell always
remained in a low position during tilting while the crimping
chela moved into a near verticle position. The tougher the
shell the more the C. antennarius tilted its body and extended
its chelipeds. A tilt of 45 degrees was observed once inan
unsuccessful attempt at cracking a very hard shell containing
a Pagurus.
The average time it took the 44" C. antennarius to
extract a Pagurus from a Tegula shell from the time the
first chip was made was fifteen minutes (based on 12 timings).
The times ranged from 3 minutes to thirty minutes depending
pf the shell. (Fig. 16.)
mainly on size, ty
broken by C. antennarius
Fig. 16. A Te
graghed innediately
which was retrievede
after the Pagurus was extracted. If such pieces
are left in the tank, the C, antennarius crushes
all the large sections to fragments. Note how the
crab ehipped away at the body whorls leaving the
columella still intact. This shell was examined
by the 44" C, antennarius for six minutes and
broken down in eleven minutes.
Devouring the Pagurus
When a shell was broken down so that the Pagurus
was exposed, the chelipeds thrust the remaining section of
the shell up to the mouth region. With the chelipeds and
sometimes the first pair of walking legs the shell was held
firmly while the maxillapeds pulled, turned, and pushed
the struggling Pagurus. The prey usually disappeared into
ompletely exposed.
the mouth 1-3 minute

the maxillapeds
Fig. 17. A sh
for extract fon of tne erposed Pagurus.
Other Predators
Due to its secretive habits Octopus apollyon was
only observed feeding seven times in the lab. The octopus
grabbed a shell, examined the inside of the shell with the
distal portion of an arm, enclosed the shell completlely
with its body and arms, and extracted the Pagurus by some
unobserved means.
Evasterias troschelii was found feeding on a Pagurus
hemphillii in the subtidal region in about 20 feet of water.
No detailed study was made of the predatory behavior
of Cancer productus and C, gracilis. Through casual observation,
however, their behavior seemed to resemble that of C. antennarius.
Summary
1. Under lab conditions hermit crabs were preyed
upon by Cancer antennarius, C, productus, C, gracilis, Pycnopodia
helianthoides, Pisaster orchraceous, and Octopus apollyon.
In the field fisaster orchraceous and Evasterias trochelii
were found preying on Pagurus.
2. Cancer antennarius in the lab was observed to have
a specific behavior for capturing and retaining Pagurus.
After which the shell was examined and later cracked by varied
techniques using both of the crab's pincers. Once the fagurus
was exposed from the cracked shell the maxillapeds helped
extract the hermit crab and push it toward the mouth.
3. Pisaster orchraceous and oftentimes Pycnopodia
helianthoides when unable to extract the Pagurus whole will
insert their stomach into the shell and digest the Pagurus.
4. Pagurus in lab tanks reacted to the mere presence
of a moving fycnopodia helianthoides and showed an escape
response to Pisaster ochraceous after capture.
References
Feder, Howard M. 1959 The food of the starfish, Pisaster
ochraceous, along the California Coast. Ecology 40.
No. 4, Oct. 1959.
MacGinitie, George E. and Nettie MacGinitie 1949 Natural
History of Marine Animals. MoGraw-Hill Book Co., New
York, 1949.
Mackay, Donald C. G. 1942 The Pacific Edible Crab, Cancer
magister. Bull. LXII, Fish. Res. Brd. Can. Ottawa,
Mackay, Donald C. G. 1943 The behavior of the Pacific Edible
Crab, Cancer magister Dana. J. of Comparative Psych. 36,
No. 3, Aug. 1943.