Estuaries and Coasts
Vol. 30, No. 3, p. 451-458
June 2007
Detrital Subsidy to the Supratidal Zone Provides Feeding Habitat
for Intertidal Crabs
TYLER L. LEWIS a, MALTE MEWS ~, DENNIS E. ]ELINSKI s, a n d
~[ARTfN ZIMMER e'*
G;.~m, fi~r Wildti/e Ecology, Simo~ Fraser Universit),, Bun~ab7, British Cd'~cmbia, VBA .1S6, Ca~nada
"~"Zoo[o~isck~;s l'~astitut Lirtmolo~, Christiar>Albrechts-U~iversitdt Kid, OMzmr
40, D-24109,
s Sc]zooZ ofE%zriro'~:~.er~t~z~S~'~.di#s, ()t~.ec~z's U~ziversi~), ~i~zgs~o%, O%mrio, KTL 3N6, Ca'~.ad~z
ABSTRACT: Beach-east wl"ack of tam'hie origin is considered a spafiM subsidy to the maa-hle4en-emn-ial trallsifion zone. We
fotmd lhat lhe x~'ack line on sm~d aald ga'aveI beaches of Vancouver Island was fi-eq~mnted by h~terlidaI purple shore crabs~
Hernig~tpsus nudus (Daua 1851 ) and densely colonized by d e ~ h i v o r o u s talitsrid ampbipods. A m p h i p o d s s p e n d the day bm'ied
in sm~d and forage on beach w*aek ditring the night. H . nudus were f o u n d in supralidal ~'rack patches hnmediately after
nightly high tides in field censuses, but s p e n t m o s t o f the day and ebb tides either s u b m e r g e d subtidally or tfidden u n d e r n e a t h
h~tertidal rocks and boulders. I n feeding trials, intertidal shore crabs were capable o f pre)~ng on talitfid amphipods. We
consider H . nudus an orrmivore feeding on both fresh and d e c a j n g macroalgae as well as a~mnM prey. Although riving
s-upratidally, a m p h i p o d s were significantly p r e f e r r e d over intertidal fitlorine snails by foraging shore mabs. Handling time of
a m p h i p o d s was significantly shorter than for littorine snails. While a m p h i p o d s h a d a r e d u c e d lisk of pl~edation by H. nut/us
w h e n buried in the sand, foraging t m d e m e a t h wrack patches did n o t reduce predation pressm'e on maaphipods by s h o r e crabs.
Rates of a m p h i p o d c o n s u m p t i o n by shore crabs were higher at darkness than da)light. In addition to an apparent day-night
rhylhrn, tidal height and lime elapsed since previous high tide h a d a significan~ influence on s h o r e crab density within the
vcrack. We conclude thai beach-cast vcrack a ~ as a spatial subsidy by virtue o f providing a valuable f o o d source to talitrid
amphipods, which are in turn c o n s u m e d by shore crabs thai ride the nightly high tide into supratidal wrack patches to reduce
the risk o f passing bare s a n d on their way to a feeding habitat rich in valuable prey.
predators, such as birds or small maritime m a m mals. I n v e r t e b r a t e predators relying on contact
p e r c e p t i o n for prey capture, rather than visual cues
(e.g., crabs or beetles), may be effective predators of
a m p h i p o d s in wrack patches, having access to
ainphipods during nocturnal hours, when amphipods are most active (Richards 1983).
In a series of studies on detrital subsidies to
b e a c h e s in g a r k l e y S o u n d (British C o l u m b i a ,
Canada), we observed dense populations of purple
shore crabs (He,migrapszts ,~,udus) sheltered underneath high it~tertidal rocks within close proximity of
wrack det?osizs in the supratidal beach zone, T o
access these supratidal ~wack deposits as a potential
f o o d source, H. ~z~d~s must leave ~heir s h e k e r and
move u p the Beach platfbrm. T h e disadvantage of
u n d e r t a k i n g this potentially dangerous travel may
be counterbalal~ced by the advantages of using
wrack as a vNuable feeding habitat.
The relatively weak claws of the generalist H.
~.udt~s impose restrictions u p o n their ability to feed
on some h~tertidal animal prey (Behrens Yamada
and Boulding 1998), since most animal p r e y available in the intertidal zone, such as Littorina spp., are
hard-shelled and require substantial claw strength
or handling periods. Wrack deposits contain high
densities of calorie-rich talitrid a m p h i p o d s (Gritfiths
Intro duction
Beach-cast deposits of macroalgae a n d seagrass,
t e r m e d wrack, function as i m p o r t a n t spatial subsidies (Polis et al. 1997) of organic material to sand
and cobble beaches, especially in terms os providing
a rich {bod source tor a n u m b e r of consumers
(Pennings et al. 2000; Dugan et al. 2003). Owing to
their high location on the beach plattbrm, these
beach-cast wrack deposits e x p e r i e n c e only very brief
episodes of tidal inundation (cfi, Orr et at. 2005).
Semiterrestrial a m p h i p o d s (Talitridae) are corfffrton
detritivores in d e c o m p o s i n g wrack deposits, often
occurring at high densities (Grif~iths et al. 1983;
McLachlan 1985).
While it is well k n o ~ l that talitrid amtphipods use
wrack as a f o o d source (Grif~lt.hs et. al. 1983;
Pennings et al. 2000), it remains an o p e n question
whether ~,rack patches effectively serve as shelter
f i o m potential predators (cs Buck et al. 2003), as is
known to be the case for aquatic a m p h i p o d s
(Norkko 1998; Vetter 1998; C o r o n a et al. 2000).
Aanphipods resting a n d tbraging u n d e r n e a t h wrack
patches would be h i d d e n f r o m visually-oriented
* Corresponding author; tele: (+49) 431 880 4153; fax: (+49)
431 880 4747; e-mail: mzimmer@zoologie.uni-kieLde
9 2007 Estuarine Research Federation
451
452
T. L, Lewis et al.
et al. 1983; Creswell 1994), which can be easilycrushed by the weak claws of H. ~udu.s, owing to
their soft exoskeletons.
Intertidal crabs are highly motile predators that
exploit patches of high prey density; an attribute
that permits exploitation of e p h e m e r a l p r e y patches
(Behrens Yamada a n d Boulding 1996). Many crab
species restrict feeding activity- to certain intertidal
zones or periods of tidal inundation (Robles et al.
1989). Crabs residing within tile high intertidal
zone experience reduced daih/ submersion time,
significantly restricting the a m o u n t of tidally inu n d a t e d foraging time. Foraging activity of intertidal crabs m m also be highly d e p e n d e n t u p o n light
levels (Dare arid Edwards 1981; Robles et at. 1989).
Many crab species exhibit diel cycles of activity- with
higher foraging activity- during night (Marsden a n d
Dewa 1994), Increased activity during night may
d a m p e n the risk of p r e d a t i o n by visual-based,
diurnal predators such as birds, fish, and maritime
m a m m a l s . This c o m b i n a t i o n of diel a n d tidal
patterns may act in concert to regulate the daily
foraging and activity- patterns of intertidal crabs.
Despite the tbtct that H. ~udus are a m o n g the
most a b u n d a n t intertidal crabs in coastal British
Columbia, very limited information exists regarding
their dietary habits a n d preferences, T h e y are
described as generalist herbivores mainly feeding
on diatoms, desmids, a n d green algae (Behrens
Y a m a d a and Boulding 1998), b u t evidence of
carnivory also exists (Dalziel and Boulding 2005).
To shed m o r e light on the feeding behavior a n d
t r o p h i c status of such an a b u n d a n t generalist
feeder, we examine the m o v e m e n t of H. nudus to
wrack patches, as well as their role as potential
predators in these patches. We aim to prove that H.
~v~d,~s are omnivorous ~eeders and elucidate potential animal prey items, determine if H. nudus access
supratidal ~Tack deposits, a n d if so, u n d e r which
tidal and diel circumstances, a n d test the hypothesis
that snpratidal talitrid a m p h i p o d s can be used as
a f o o d source by- H. nudus, and are even p r e f e r r e d
over other potential t b o d items of the intertidal
area.
nudes reside within the rock fields. During each
tidal cycle, a supratidal b a n d of wrack is deposited
a m o n g the rocks mad along the entire length of the
beach. A rich c o m m u n i t y of talitrid a m p h i p o d s is
t 0 u n d a m o n g the x~ack, with Megalorchestia pugettensis (Dana 1853) a n d Traskorchestia traskiana
(Stimpson 1857) being tile most c o m m o n amphip o d species.
Night Trar~sects
Beach-cast uTack was sampled [br the presence ol
H. nudus on the nights of August 8 (4 d alter n e a p
tide), 1B, a n d 22 (5 d before spring tide), 2003.
T h r e e transects were sampled p e r night, the survey
of each being separated by a 2-h interval. T h e first
transect p e r night coincided with the time of either
the slack high or slack low tide after sunset and the
overall sampling time (6 h) covered the entire
interval of flood or ebb. Sample transects were
established along the course of the wrack line.
Quadrats of 0.5 x 0.5 m 2 were placed on the wrack
line a n d all H. nudus within the quadrat were
counted. Transect distance of 0 m was defined as
the p o i n t where the beach wrack line m e t the
b o r d e r i n g rock field. T h e first quadrat (-- 1 m) was
randomly placed between 0.B and 1.5 m distance
f~om ~he rock field; all s~bsequent quadrats were
placed evmT 3 m along the wrack 1inc. Since an
u n d i s t u r b e d wrack line was essential for additional
studies in a n o t h e r context, we were unable to
d e t e r m i n e the wrack by means of mass p e r area;
p e r c e n t cover of ~-ack within qnadrat a n d minim u m distance of rock shelter was r e c o r d e d for each
quadrat. T h e wrack within the rock field, where it is
deposited more patchily a n d in larger amounts, was
also sampled by means o f 5 quadrats that were
r a n d o m l y placed within the area of wrack deposition in the rock field. Percent cover of x.~ack and
n u m b e r of crabs were r e c o r d e d for each qnadrat.
M i n i m u m distance to potential shelter within the
rock field was defined as 0 m.
Da), Trar~sects
Materials and Methods
FIELD SAMPLING
Study Site
Field studies were c o n d u c t e d on a small, protected sand ~ d gravel b e a c h on the northeast
c o r n e r of Edward King Island (48~
125017 , 18.86"W), Barkley Sound, off ~he west coast
of Vancouver Island (British Columbia, Canada).
T h e study beach is approximately 75 m long and
f}inged o n each side by large rock fields c o m p o s e d
of cobbles mad boulders. Dense populations of H.
Daytime sampling ~bllowed the m e t h o d s used tbr
nocturnal sampling, A total of 4 diurnal transects
were sampled o n August 13 (9 d after n e a p tide)
a n d 19 (8 d before spring tide), 2003. Because
shore crabs are typically m o r e active during diurnal
high tides (Williams 1991; Marsdon a n d Dewa
1994), diurnal sampling was p e r f o r m e d at high
tides to maximize likelihood of finding H. ,mtd~s
within wrack. This p r o c e d u r e biased our results
towards higher crab densities during da}r
so
that lower diurnal crab densities, relative to night,
can be considered conservative.
Intertidal Crabs in Supratidal Subsidy
LABORATORY EXPERIMENTS
Predators
Laborato~ T experiments were c o n d u c t e d at the
nearby Bamfield Marine Sciences Centre. H. nudus
of various size classes were collected by hand from
u n d e r n e a t h cobbles near the study site on Edward
King Island. Crabs were held in seawater tanks with
a constant flow of fresh seawater and a b u n d a n t
macroalgae ( F u ~ s spp). Only crabs that had spent
at least 24 h in the laboratory were used in
experiments. Randomly chosen crabs were sexed,
and the carapace width and the height of the
p r o p o d i t e os the right chela were measured to the
nearest millimeter (cf., Buck et al. 2003) betore they
were used in the experiments. Since claw size is
related to fbod choice, we aimed at c o m p a r i n g H.
~udus, which we c o n s i d e r an omnivore, with
a n o t h e r grapsid crab, A r~ases cinereum, which has
recently been shown to exhibit omnivorous feeding
beha~4or (Buck et al. 2003).
Predation Trials
Predation trials with H. nudus were c o n d u c t e d in
dry plastic tanks (30 • 17 cm~). One randomly
selected crab was placed in the tank along with 10
talitrid amphipods and 10 individuals of one of
three littorine snails: Littorina sitka~'~a, L. scutulata, or
L, subrotundata. Each Littorb~a sp. was used in
separate experiments, and a total of five trials were
pe~Ibrmed per species; no sheker was offered to
prey organisms. Crabs and prey were lett in darkness
for 12 h per predation trial, and r e m N n i n g prey
items were c o u n t e d at the conclusion of each trial.
Dry- tanks mimicked emersion at low tide and
prevented Littorina spp. ti-om climbing out of reach
of crabs. Littorina spp. differ in shell thickness, with
L. sitkana and L. subrotundata being lighter shelled
(Dalziel personal c o m m u n i c a t i o n ) . Only small
Littorina (< 7 ram) were used to f~acilitate handling
and crushing.
Previous feeding eN)eriments have d o c u m e n t e d
H. nudus predation on L. sitkana (Behrens Yamada
and Boulding 1998). Additional teeding experiments were c o n d u c t e d with L. sitkana as the only
prey item. O n e randomly selected crab was placed
in a tank with 10 L. sitkana. Five trials were
c o n d u c t e d for this experiment, using the same
m e t h o d as for the prey preference experiments.
Simultaneously, teeding trials (n - 10) in waterfilled, lid-sealed plastic tanks (30 • 17 cm c) with
a continuous flow of fiesh seawater yielded comparative data o n overnight c o n s u m p t i o n of different
subtidal and intertidal tbod items that enabled us to
decide whether 14. nudus acts as a detritivore, an
herbivore, a carnivore, or an o m n i v o r e when
submerged. For this, cobbles with kno+t~+, numbers
453
of barnacles (mixed genera), 10 snails (L. sitkana),
and knowa amounts of fresh and detrital Fucus cf,
gardneri were placed in the tank betore a randomly
chosen crab was added. Aficer 12 h, the numbers of
barnacles m)d snails were counted, and algae were
oven dried (24 h, 60~ and weighed. The dry mass
of the initial a m o u n t of algae was determined on
the basis o f a fl-esh mass:dry mass ratio fbr fi-esh and
detrital Fucus (n - 15, each),
~Nhen several types of t o o d are offered silr, ultaneously, c o n s u m p t i o n of one cannot be assumed to
be i n d e p e n d e n t of c o n s u m p t i o n of the others. To
avoid f~dlacies in statistical analyses of preference
tests (cf,, Peterson and Renaud 1989; Roa 1992;
Manly 1993), all preference tests were statistically
analyzed t h r o u g h randomization of data by using
resampling statistics with 9999 iterations as implem e n t e d in the sottware poptools ( h t t p : / / w ~ . c s e .
c s i r o . a u / p o p t o o l s / ) , since this approach incorporates such interdependencies. Fisher (1951) introd u c e d permutation tests as a theoretical a r g u m e n t
supporting Student's t-test, but this approach is also
valuable fbr statistical tests in its o ~ l right (Bfirlocher 1999). The repeated resampling of experimental data gen.erates sets of artificial data. Instead of
listing all possible arrangements, r a n d o m arrangements of the original data are generated. By
increasing the n u m b e r of iterations, it can be
decided whether the original experimental data
deviate f)om a r a n d o m distribution that would t?e
expected if no preference was observed.
Protection Jko~n Predatio~
Eftects of potential shelter on supratidal predatorprey interactions were tested in trials with H. nudus
and a m p h i p o d prey overnight (n - 10, each). A
single randomly chosen crab was placed in a plastic
tank (30 • 17 cm 2) that was either bare, filled with
4 cm of moist sand, or filled with sand and a small
patch of wrack that covered approximately 50% of
the sand surfkce area. Prior to introducing the crab,
10 m e d i u m sized taiitrid amphipods were given time
to acclimatize in the tank for 1 h. At the end of the
experiment, crabs were removed, and the remaining amphipods were counted.
To compare nocturnal versus diurnal supratidal
predation, plastic tanks (30 X 17 cm ~) were filled
with 4 cm of moist sand. A circular bowl (10 cm
diameter) containing seawater was sunk into the
sand and a small rock shelter was constructed within
the water bowl. Crabs were aisle to move fleely
between the water bowl and sand. A small pile of
wrack was placed in the tank opposite the water
bowl. Ten mediuln-sized talitrid amphipods were
placed into the wrack pile. O n e randomly selected
H. nudus (n - 5) was placed in the water bowl and
either studied during daytime at 12 h of light or
454
T.L. Lewis et al.
14
100 -
140
120
80-
r
10~
oo
60-
8
,.,.,
!~
40
"ID
20-
0
100
females
8O
~2r
e'~
6
c~
4
,~
2
B
._~
"10 13 "16 "19 22 25 28 31 34 37 40 43 46
D i s t a n c e f r o m r o c k field (m)
Fig. 1. Wrack cover (bars) and density of He~nig~apsus~*udizs
(dots) underneath wrack along transects with increasing distance
from shelter in an adjacent rock field. Crab density is not
correlated with percent cover of wrack, but with distance from the
rock shelter. Data are median -- median absolute deviation (n 9
3 transects at 3 dates, each).
o v e r n i g h t at 12 h o f dark. A m p h i p o d s r e m a i n i n g in
t h e t a n k a f t e r 12 h w e r e c o u n t e d .
6O
E
4O
20
.__.
o_
m
0 3
E
120
t-
T o c o m p a r e t h e t i m e n e e d e d to h a n d l e a caugh~
a m p h i p o d with t h e t i m e n e e d e d for a h a r d - s h e l l e d
snail ( B e h r e n s 'l<~mada a n d B o u l d i n g 1998), we
measured handling time of medium-sized amphip o d s f o r H. nudus. O n e r a n d o m l y s e l e c t e d c r a b (n
- 10) was p l a c e d in a dry, plastic tank with 10
a m p h i p o d s . Crabs were w a t c h e d by an o b s e r v e r
u n d e r r e d l i g h t to m i n i m i z e o b s e r v e r d i s t u r b a n c e
o n c r a b b e h a v i o r . H a n d l i n g t i m e was m e a s u r e d f o r
o n l y tire first a m p h i p o d c a p t u r e d , b e g i n n i n g w h e n
the crab caught the amphipod and ending when the
entire amphipod had been consumed.
Results
Daytime transects revealed an almost complete
a b s e n c e o f H. nudus f r o m b e a c h wrack; in 80
d i u r n a l q u a d r a t s s a m p l e d , o n l y f o u r indix4duals
were f o u n d (0.05 c r a b s p e r q u a d r a t ) versus 97
indit4duals ~ b u n d at n i g h t ( n - 189 q u a d r a t s : 0.5
crabs p e r q u a d r a t ) . T h e d i s t r i b u t i o n o f H- ~z~d'~t,sat
n i g h t l y h i g h tides was o n l y mm-ginalIy i n f l u e n c e d by
t h e p e r c e n t c o v e r o f u a a c k (Fig. 1; S p e a r m a n rs 0.409, n - 17, p > 0.06), b u t was s t r o n g l y
i n f l u e n c e d b y b o t h t h e d i s t a n c e iS'ore t h e r o c k
s h e l t e r (rs - 0.803, n - 17, p < 0.001) a n d tidal
h e i g h t ( l i n e a r r e g r e s s i o n : r 2 - 0.47, n - 9, p <
0.05).
O b s e r v a t i o n s in t h e field, b o t h p r i o r to a n d
b e t w e e n t r a n s e c t s a m p l i n g s , i n d i c a t e d t h a t initial
m o v e m e n t o f H. nz~dus i n t o t h e w r a c k line c o i n c i d e d
with t h e t i m e s o f h i g h t i d e m a x i m a . A t h i g h tide, t h e
water comes into close contact or even resnspends
a~
males
100
t~
r
8O
4O
20
0
Seaweed
Prey Handling Time
.
r
0
Wrack
Barnacles
Snails
Fig. 2. Feeding preferences of submerged H,'mi~z*psus ,~tudus
when
9
offered different food items simuhaneousl), indicating Jmt
shor'e crabs may act as ornnivores. Females and males differ in
fl:eh" rates of barnacle consmnption. Data are median + median
absoktte deviation (n 9),
p r e v i o u s l y d e p o s i t e d xwack ( O r r et al, 2005). T h e
density- o f H. nudus within t h e v a a c k d e c r e a s e d
s i g n i f i c a n t l y with t i m e since p r e v i o u s h i g h t i d e
m a x i m a (i.e., t i m e since tide last t r a n s p o r t e d crabs
to w r a c k d e p o s i t s ; l i n e a r r e g r e s s i o n : r 2 - 0.71, n 9, p < 0.005).
D u r i n g l a b o r a t o r y p r e d a t i o n trials, s u b m e r g e d H.
nudus f e d o n s i m i l a r a m o u n t s o f f r e s h a n d b e a c h t o s s e d (wrack) Fucussp. (Fig. 9; Utest: p > 0.8), a n d
p r e t e r r e d p r e y i n g o n b a r n a c l e s (mLxed g e n e r a ) o v e r
p r e y i n g o n snails (L. sitkana; U test: p < 0.001). T h e
absence of a preference between flesh and wracked
s e a w e e d suggests t h a t t h e p r e s e n c e o f d e c o m p o s i n g
s e a w e e d a l o n e d o e s n o t e x p l a i n H. ,,t;~dus p r e s e n c e
in b e a c h - c a s t wrack; b o t h f i e s h a n d w r a c k e d
m a c r o a l g a e are a b u n d a n t h~ t h e i n t e r t i d a l zone.
The consumption of both seaweed and barnacles
c o n f i r m s a n o m n i v o r o u s diet {0r H. ~udus, W h e n
g i v e n a c h o i c e b e t w e e n a m p h i p o d s a n d snails
(Littorina s p p . ) , e m e r g e d H. nudus always s t r o n g l y
p r e f e r r e d a m p h i p o d s , i r r e s p e c t i v e o f Littorina species (Fig. 3; Utest: p < 0.001). I n f e e d i n g trials with
L. sitkana as t h e sole p r e y c h o i c e , no snails were
c o n s u m e d at all ( d a t a n o t s h o w n ) .
I n b o t h sexes o f H. ~udus; claw size i n c r e a s e d
l i n e a r l y with c a r a p a c e w i d t h (males: r 2 - 0.749,
s l o p e - 0.37 + 0.04, n - 24, p < 0.0001; females: r 2
- 0.664, s l o p e - 0.25 -+ 0.04, n - 25, p < 0.0001),
Intertidal Crabs in Supratidal Subsidy
455
100
10
80
02
Q)
4,-*
9
o~
6
03
"13
O
60
C1.
>.,
n
40
4
E
<
20
2
0
0
,T,
M p
L. sit.
I~ p L scut.
M p. L, sub.
Fig. 3. Feeditag p r e f e r e n c e o f e m e r g e d Hemig'~ap~u~ ~ d ~ s
w h e n offered talitrid a m p h i p o d s ([vL p.
lgieg~o';~,/~esti~ p~g~te~sis) a n d different species o f Iittorine snails (L. sit = 5it~o~r*{z
sit!{ana, L. scut - L. scutugata, L. sub - L. su&otundata)
simultaneously, indicating that s o f t b o d i e d a m p h i p o d s are preferred over hard-shelled snails. Data are m e d i a n • m e d i a n
absol~tte deviatiol-~ (~s - 9).
but males had bigger claws than females of the same
size (ANCO\:%: p < 0.00I). T h e difference increased ~dth increasing body size, i.e., the regression
coefficients were signilicantly different (p < 0.01},
indicating that with increasing size tomes develop
disproportionately larger claws than do females.
Males differed froln females in that thev consnmed
fewer barnacles than females, perhaps because the
smaller female claws facilitate pulling out the
barnacle soft body from the hard carapace (Mews
and Zimmer personal obselwation). Females (9 -+ 1)
and males (9 _+ 1) did not differ from each other in
how many amphipods (M. p~,~r
were consumed within the time provided during feeding
tests (U test: p > 0.%.
100
q-
t'-
80
o~
60
O
Q..
'-
40
E
20
0
w/o shelter
w/sand
w/sand 9 wrack
Fig. 4. Effect of s a n d ( w / s a n d ) a n d additional wrack (~v..~ sated
+ wrack) o n c o n s u m p t i o n of a m p h i p o d s (Megalor~hes~a p'ug~t~e*zs/s) by Hemigrapsus mzdus in c o m p a r i s o n to shelter free trials (w/'o
shelter), indicating that sand, b u t n o t wrack, provides protection
f r o m p r e d a t i o n by f o r a g i n g crabs. Data are m e d i a n -- mediat~
absolute deviation (n - 9).
Fig~ 5.
I
I
Day
Night
Daytime a n d o v e r n i g h t c o n s u m p t i o n of a m p h i p o d s
( 3,'Iegzzl~rches*ia pzzg'eih'~is) by Hemig'mps.us ,~zv,d,us, indicating Iligher
consumptiolz rates d u r i n g night. Data a,'e m e d i a n + ntedian
absolute de~{atiola (n - 9).
Con:pared wifl~ sheher-free conn-ols, s.'.md significantly (,~NO\,q~k: p < 0.001) reduced predation on
Meg~go~chestia by H. ~z~,d.~s (Fig. 4). Adding wrack
only marginally resulted in a ch.'.~nged 5reqneilc.y of
predation (ANOVA: p > 0.07), suggesting tl:at.
wTack does not provide an effective sheher ii'om
predation. H. nudus consumed significantly more
talitrid amphipods at night than during day (Fig. 5;
U test: p < 0.05), consistent with the observation of
higher H. ~zudus abundance in the nocturnal field
transects.
From time of a m p h i p o d capture m complete
consumption, H. nudus required 118 + 8 s. This
handling time tbr amphipods is approximately five
times faster than values reported for Littorina snails
(Behrens Yamada and Boulding 1998}.
Discussion
T h e significant nunlbers of H. nudes within
beach-cast wrack deposits during nightly high tides
support our hypotheses that this intertidal crab may
act as an omnivore and exits the water and travels
up the beach gradient platform to forage underneath supratidal wrack patches rich in prey.
Clear diel patterns of H. nudus accessing wrack
were observed. Observations at night revealed
obvious crab movements out o f the water and up
tile exposed beach towards tile wrack line, while no
crabs were observed moving or foraging out of the
,~vtter during the day. O f the four diurnal transects,
only four H. nudus were f o u n d in tile wrack,
indicating a 10-times lower density of e m e r g e d H.
n.udus during the day than during night. These few
H..nudus were buried u n d e r a deep pile o f wrack
(ca. 25 cm) a m o n g tile rock field, quite possibly
using the wrack primarily as shelter. A complete
diurnal data set with regards m tidal level would be
45G
T.L. Lewis et al.
necessary- to draw m o r e firm conclusions, but o u r
present data conservatively confirm the essential
absence of H. n u d u s fl-om the supratidal wrack line
during the day. Day and night simulated laboratory
e x p e r i m e n t s p r o v i d e d g o o d evidence regarding diel
activity patterns. H. ~ u d u s c o n s u m e d significantly
m o r e a m p h i p o d s during simulated night than day-,
a h h o u g h they were still active and foraging during
simulated day, Although these e x p e r i m e n t s may
indicate increased n o c t u r n a l activity, laboratory
conditions are admittedly limited. Foraging crabs
only had t o move short distances (ca, 15 cm) f r o m
water to ~.~-ack and had no tid~ or crep~scwlar
signals to cue movement. N o c t m n a l activity in
b e a c h x~ack confers potential major advantages to
H. ~ud~s, Nocturnally foraging crabs are hidden in
darkness ~}om visually oriented terrestrial predators,
and nighttime foraging coincides with maximal
activity of talitrid arnphipods that s p e n d days mostly
buried in sand (Richards 1983).
W h e n given a choice, H. mtd,ts foraging o n land
clearly p r e t e r r e d a m p h i p o d s over littorine snails,
which are highly a b u n d a n t at the study site. Igittori,~a
spp, occupy the intertidal z o n e a n d could b e
e~t5ciently p r e y e d u p o n by crabs during times of
tidal submersion, eliminating potentially dangerous
foraging trips onto exposed flats. T h e handling time
for an individual a I n p h i p o d (ca. 2 Inin) was m o r e
t h a n 6 rain lower t h a n the h a n d l i n g time t o t
littorine snails (ca. 500 s) as r e p o r t e d by Behrens
Yamada a n d Boulding (1998). This time-energetic
benefit may b e a major driving torce p r o m p t i n g H.
~ a d u s m o v e m e n t s to wrack deposits. W h e n submerged, ILl. ~zud*ts p r e f e r r e d feeding on barnacles or
F u ~ s wrack, and to some extent on t}esh Fucus,
rather than on littorine snails.
:Mmther factor Mtiecting teeding behavior of crabs
may be claw size (Behrens Yamada a n d Boulding
1998). Males of the grapsid crab, A. d,~ereum, have
bi,,,,er claws than females (Buck et al. 2003). Male
A. cb~er~,.u.m eat more snails than temales do, likely
o~fing to their improved ability to crush shelled prey
items. Both A. ~d~e~ez~zsexes .prefer amphi.pods and
isopods over snails (Buck et al. 2003). Although
male H. '~v~,.d,~,.shave biooer claws than females of the
same size, feeding on talitrid a m p h i p o d s did not
differ between sexes; preying ~ p o n barnacles even
appears to be fi~cilitated by small claws. Allomet.ric
differences between sexes likely have other reasons
than sex-specific prey selection in ILl. *~udus and may
simply be an evolutionary heritage rather than an
adaptation to teeding habits.
In addition to an a p p a r e n t day-night rhythm, tidal
height and time elapsed since previous slack high
tide a p p e a r to have the most significant it~luences
o n crab density- within the wrack. T h e strong
relationship between H. ~ u d u s density- a n d time
elapsed since previous high tide suggests that H.
nud,~s ride the high tide into the wrack line. We
p r o p o s e that high tides contacting the wrack line
may carry aqueous chemical signals ti-om decomposing wrack to s u b m e r g e d H. nudus, providing
a chemical cue that initiates travel to the wrack, b u t
this hypothesis awaits e x p e r i m e n t a l testing. Waiting
for the slack high tide to access wrack would entirelyminimize the travel distance between wrack a n d
water. Concerning which portion of the wrack line
will be used, the distance t-ore rock shelter strongly
dictates H. nud.~ts distribution. T h e propensity of H,
~x~.dus to remain close to s h e h e r in rock fields
adjacent to the Beach could preclude t h e m f r o m
wrack access at larger beaches or areas lacking
p r o p e r rock shelter. This observauon could be an
artit~ct of the study site, since a b u n d a n t wrack
deposits abutted rock fields at the study site,
potentially eliminating the n e e d for ILl. ~*tdus to
stray tar li-om their daytime habitat.
It is unlikely that the m o v e m e n t of ILl. ~zudus to
wrack patches is fi)r shelter purposes alone, since
adequate shelter is available u n d e r rocks in the
intertidal zone, and reaching the wrack line might
require m o v e m e n t over exposed areas of the beach.
T h e presence of H, ~ u d u s within the wrack does not
necessarily m e a n that a m p h i p o d s are being used as
a t b o d source. The wrack itself may b e fed u p o n
(Griffiths et at. 1983), possibly being the main
attractant for ~oraging crabs, while the occasional
capture a n d ingestion of a m p h i p o d s within wrack
may- b e a secondary reason for m o v e m e n t into the
wrack. We tilled this out, since H. n u d u s did not
show any p r e f e r e n c e for f e e d i n g o n wrack in
laboratory trials, but did exhibit a cleat- p r e f e r e n c e
for feeding on a m p h i p o d s rather than other available prey. Owing to substantial a m p h i p o d densities
(Griftiths et at. 198a; McLachlan 1985), shore crabs
are likely e n c o u n t e r i n g n u m e r o u s a m p h i p o d s when
in the wrack, These e n c o u n t e r s are potentially
profitable feeding opportunities that may influence
the decision of an indi~r
H. ,~z~.~.&~.sto access the
wrack, and we hold that high densities of amphipods in supratidal ~.~.~ack patches render t h e m an
attractive prey fbr H. ~xt~d-u.s.T h e feeding habitat of
talitrid a m p h i p o d s is also a feeding hN~itat for their
potential predators such as shore crabs (this study),
rove beetles (Neame, Zimmer, Jelinksi, and Mews
u n p u b l i s h e d data), frogs (Orr personal c o m m u n i cation), m iJ:tks, and bears (Mews, Zimmer, a n d
Jelinski personal observations),
The predators' eftect o n wrack d e c o m p o s i t i o n by
a m p h i p o d s (Mews et al. 2006) may be substantial,
b u t n u m e r o u s studies indicate that increasing
environmeJ~tal structure r e d u c e s the ability of
predators to capture prey (Gotceitas a n d Colgan
1989; Lewis m~d Eby 2002; Buck et al. 2003). While
Interlidal Crabs i~] Supratidal Subsidy
bmying in the sand obviously provides some prot e r t i a n ticom p r e d a t i o n
b y H. n u d u s t o t a l i t r i d
amphipods, foraging underneath
wrack does not.
W r a c k d i d n o t s i g n i f i c a n t l y attiect c a p t u r e t i m e o f
amphipods
by the beach-dwelling
rove beetle,
Thinopinus .pictus ( N e a m e , Z i m m e r , J e l i n s k i , a n d
Mews unpublished
data). Spending a significant
a m o u n t o~ t i m e w i t h i n ~ a c k
patches appears to
i n c r e a s e ~he risk o f p r e d a t i o n
b y p r e d a t o r s ~hat
actively seek those wrack patches for foraging. We
conclude
that freshly deposited
wrack, which
becomes densely colonized by talitrid amphipods
w i t h i n less t h a n o n e h o u r ( P e l l e t i e r , M e w s , J e l i n s k i ,
a n d Z i m m e r p e r s o n a l o b s e r v a t i o n ) , is m o r e i m p o r t a n t t o a m p h i p o d s as a t b o d s o u r c e w i t h s u i t a b l e
temperature
and moisture
conditions
t h a n as
a s h e l t e r fi'orn p r e d a t o r s .
C o n s i d e r i n g t h e a v a i l a b l e e v i d e n c e , we d e d u c e
o l n n i v o r o u s f e e d i n g b e h a v i o r o f H. n u d u s t h a t u s e s
t a l i t r i d m n p h i p o d s as a v a l u a b l e f b o d s o u r c e . T h e
abundant availability of these detritivores in wrack
lines renders this supratidal subsidy a habitat worthy
o f i n t r u s i o l ~ f o r f o r a g i n g . T h e risk o f h a v i n g t o l e a v e
s h e l t e r , b e it s u b t i d a l l y o r u n d e r n e a t h
intertidal
r o c k s a n d b o u l d e r s , is m i n i m i z e d
by nocturnal
f b r a g f n g at h i g h t i d e s . B e c a u s e w r a c k l i n e s a r e
deposited at the previous high water mark, delaying
f o r a g i n g u n t i l h i g h t i d e a l l o w s H. n u d u s t o r i d e t h e
tide into the wrack lines without having to pass
e x p o s e d b e a c h a r e a s . As a n e x t s t e p , r e m o v a l a n d
addition of natural and artificial wrack in field
e x p e r i m e n t s , as well as g u t a n a l y s e s o f f i e l d c o l l e c t e d
c r a b s , m a y r e v e a l d i r e c t e v i d e n c e 1o c o r r o b o r a t e o r
question our conchtsions based on tield observations and laboratory experiments~
ACK.NOWLEDGME N'I-S
We are grateful to the former Director, Andy Spencer, and staff
of the Bamtie/d Manne Sciences Centre, especially Jean Paul
Danko, Andrea Hunter, and James Mortimer, for their support.
This is contribution SIRG/03-04 of the Supratidal Interface
Research Group. The experiments herein comply with the laws of
Canada and British Columbia.
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Received, lane 30, 2006
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Accepted, March 22, 2007