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Predation and physical disturbance by crabs reduce the relative impacts of nutrients in a tidal mudflat



We evaluated how links between direct and indirect interactions and physical disturbance shaped trophic dynamics in a soft-sediment benthic estuarine community. We crossed presence of burrow-excavating crabs Pachygrapsus crassipes and nutrient enrichment (nitrogen and phosphorus) in cages containing herbivorous surface-feeding snails Cerithidea californica and benthic microalgae in a tidal mudflat and a tidal sandflat in Mugu Lagoon, southern California, USA. P. crassipes consumed up to 85 % of C. californica in enclosures, but there was no evidence of a trophic cascade, as crab reduction of snail density did not increase benthic microalgal biomass. Rather, P. crassipes decreased diatom and cyanobacterial biomass by up to 50 % in the mudflat and 80 % in the sandflat, probably via bioturbation. Subadult C. californica lengths increased 15 to 20 % over 5 wk in treatments without crabs. In the presence of P. crassipes, C. californica lengths increased < 5 %, probably an indirect result of crab reduction of microalgal food availability or increased snail burial. C. californica may have actively burrowed as an escape response from the crabs, or have been passively buried during crab burrowing activities. Nutrient addition did not reduce snail growth, but increased snail mortality at both sites, possibly a result of nutrient-induced shifts towards toxic or poor nutritive quality cyanobacteria. The top-down impacts of P. crassipes reduced the relative bottom-up effects of nutrients in this habitat, illustrating the importance of evaluating both biotic and abiotic interactions simultaneously. Numerous indirect and non-trophic interactions revealed a community structure that was much more complex than suggested by food web structure.
Mar Ecol Prog Ser
Vol. 313: 205213, 2006 Published May 11
Numerous complex direct and indirect interactions
act in concert to shape communities. Predation can be
a strong direct structuring force, altering prey density,
fitness, and species composition (Paine 1974, Blumen-
shine et al. 2000). Nutrient enrichment can have simi-
larly potent impacts by increasing food supply for
upper trophic levels (Widbom & Elmgren 1988), or
facilitating blooms of opportunistic, weedy, or toxic
species (Valiela et al. 1997). Direct impacts of preda-
tors and nutrients are sometimes independent of each
other (Wiltse et al. 1984, Posey et al. 1999, Heck et al.
2000), but the strengths of those top-down and bottom-
up forces are frequently altered through indirect path-
ways (Menge 1995, Thrush 1999). Indirect effects often
involve the impact of one species on another by way of
a direct interaction with a third species (Menge 1995).
Trophic cascades are a well-documented example, in
which predation on a herbivore indirectly increases
primary producer biomass (e.g. Carpenter et al. 1985).
Indirect interactions may also follow non-trophic path-
ways, such as disturbance of other trophic levels
during foraging activities (Palmer 1988, Thrush 1999),
or trait-mediated indirect interactions, including alter-
ations of prey behavior in the presence of predators
© Inter-Research 2006 ·*Email:
Predation and physical disturbance by crabs reduce
the relative impacts of nutrients in a tidal mudflat
Anna R. Armitage
1, 2,
, Peggy Fong
University of California Los Angeles, Department of Ecology and Evolutionary Biology, 621 Charles E. Young Drive South,
Los Angeles, California 90095-1606, USA
Present address: Florida International University, Southeast Environmental Research Center OE 148, 11200 SW 8th Street,
Miami, Florida 33199, USA
ABSTRACT: We evaluated how links between direct and indirect interactions and physical distur-
bance shaped trophic dynamics in a soft-sediment benthic estuarine community. We crossed pres-
ence of burrow-excavating crabs Pachygrapsus crassipes and nutrient enrichment (nitrogen and
phosphorus) in cages containing herbivorous surface-feeding snails Cerithidea californica and ben-
thic microalgae in a tidal mudflat and a tidal sandflat in Mugu Lagoon, southern California, USA. P.
crassipes consumed up to 85% of C. californica in enclosures, but there was no evidence of a trophic
cascade, as crab reduction of snail density did not increase benthic microalgal biomass. Rather, P.
crassipes decreased diatom and cyanobacterial biomass by up to 50% in the mudflat and 80% in the
sandflat, probably via bioturbation. Subadult C. californica lengths increased 15 to 20% over 5 wk in
treatments without crabs. In the presence of P. crassipes, C. californica lengths increased <5%, prob-
ably an indirect result of crab reduction of microalgal food availability or increased snail burial. C.
californica may have actively burrowed as an escape response from the crabs, or have been passively
buried during crab burrowing activities. Nutrient addition did not reduce snail growth, but increased
snail mortality at both sites, possibly a result of nutrient-induced shifts towards toxic or poor nutritive
quality cyanobacteria. The top-down impacts of P. crassipes reduced the relative bottom-up effects of
nutrients in this habitat, illustrating the importance of evaluating both biotic and abiotic interactions
simultaneously. Numerous indirect and non-trophic interactions revealed a community structure that
was much more complex than suggested by food web structure.
KEY WORDS: Bioturbation · Epifauna · Indirect interactions · Microphytobenthos · Nutrients ·
Sediment · Trophic dynamics
Resale or republication not permitted without written consent of the publisher
Mar Ecol Prog Ser 313: 205213, 2006
(Schmitz et al. 1997). The complexity of these types of
direct and indirect interactions within communities is
re-defining food webs and the rules of community
assembly (Menge 1995).
The combined influences of both direct and indirect
interactions on marine communities have been exam-
ined in aquatic, pelagic, and rocky intertidal habitats,
but much less is known about their importance in the
soft-sediment benthic habitats that are important com-
ponents of coastal lagoon and estuarine ecosystems
(Menge 1995, Liess & Hillebrand 2004). Grazers and
nutrients can have indirect impacts on the benthic
microalgal community through alterations of species
composition and diversity (Hagerthey et al. 2002,
Armitage & Fong 2004b), but few studies have investi-
gated the direct and indirect roles of predators in these
systems (Liess & Hillebrand 2004). Direct effects of
predation on soft-sediment infaunal communities are
sometimes weak (Ólafsson et al. 1994), suggesting that
complex indirect interactions may be masking direct
trophic effects in these habitats (Thrush 1999). Mecha-
nisms for predator-mediated indirect interactions
include disturbance of non-prey organisms during
foraging activities (Palmer 1988, Thrush 1999) or
consumption of superior competitors or predators
(Ambrose 1984, Bonsdorff et al. 1995, Hamilton 2000).
The few studies that have experimentally addressed
the relative importance of direct and indirect interac-
tions on soft-sediment communities suggest that indi-
rect predator effects may be as or more important than
direct effects (Ambrose 1984, Palmer 1988). There is
also some evidence that direct effects (predation) and
indirect effects (disturbance) may act synergistically
(Bonsdorff et al. 1995), though most of these studies
focus on predation on infaunal communities. The
importance of epifauna as ecosystem engineers in soft-
sediment systems has been well established (Bertness
1985, Commito et al. 2005), but experimental studies of
the relative importance of both top-down and bottom-
up direct and indirect effects of predators in these
habitats are lacking (Liess & Hillebrand 2004), particu-
larly on the west coast of the USA.
We explored the relative importance of direct and
indirect impacts of a burrow-excavating, predatory
crab and nutrient enrichment on a benthic estuarine
community. Previous work in this system revealed
numerous indirect interactions among herbivorous
mud snails, benthic microalgae, and nutrients, where-
by nutrient addition indirectly impacted the snails
by increasing cyanobacterial abundance, which sub-
sequently increased snail mortality 3-fold (Armitage &
Fong 2004b). The dramatic alterations of community
structure in this system due to nutrient addition sug-
gested that the impacts of the predatory crabs might
vary under different nutrient-enrichment regimes. We
predicted that crabs would directly affect grazing
snails through consumption and that crabs would have
indirect effects on snail survival by reducing micro-
algal abundance through burrowing activities.
Study system. We performed the following studies in
2 tidal flats with sediment ranging from 8% mud
(grains < 62 µm diameter) content (referred to as
the sandflat) to 29% mud content (referred to as the
mudflat) in Mugu Lagoon, southern California, USA
(34.11°N, 119.12°W).
The predator in this system, the shore crab Pachy-
grapsus crassipes Randall (adult carapace width 30 to
40 mm), disturbs surficial sediment through excavation
of shallow burrows and consumes a variety of salt
marsh fauna (Hiatt 1948). The epibenthic gastropod
Cerithidea californica Haldeman (California horn
snail, adult length 20 to 30 mm) is a frequent com-
ponent of its diet (Sousa 1993). C. californica, in turn,
consumes benthic microalgae (Whitlatch & Obrebski
1980), which in this region consists mainly of diatoms
and cyanobacteria (Armitage & Fong 2004b).
Test for cage effects. We performed a study in
August 2001 to test cage design and determine the
extent of cage effects on sediment properties. In both
the mudflat and the sandflat, we installed ten 0.5 ×
0.5 m enclosures constructed from fiberglass window
screening (1.6 mm mesh). Walls of the enclosures
extended 20 cm above and 5 cm below the sediment to
minimize animal immigration and emigration. En-
closure lids were made from window screening and
attached with clothespins. In each site, we also
installed 5 ‘lid only’ structures, consisting of screen lids
held up with bamboo stakes to simulate the shading
effects of the cages. In addition, we established 5 open
plots per site. Crabs Pachygrapsus crassipes were ini-
tially removed from all 10 enclosures; ambient snail
Cerithidea californica densities (~200 m
) remained in
each treatment. Of the 10 complete enclosures, 5 were
randomly designated as ‘–crab’ treatments. Ambient
densities of crabs were added to the other 5 enclosures,
and these were designated as ‘+crab’ treatments. From
tidal creeks in adjacent marsh areas we collected P.
crassipes large enough (carapace width 32 mm) to con-
sume C. californica up to 25 mm in length (Sousa
1993). We haphazardly assigned 4 crabs to each of the
+crab enclosures, approximating local summer crab
densities (A. R. Armitage unpubl. data).
After 5 wk, we collected cores to determine if the
enclosures altered the physical or biological character-
istics of the sediment. We pooled 8 sediment cores
(2 cm deep, 2.5 cm diameter) from each plot and de-
Armitage & Fong: Indirect impacts of crabs on benthos
termined percentage mud content using the hydrome-
ter fractionation method of Bouyoucos (1962), sediment
organic content as loss on ignition after 10 h at 400°C
and water content as weight loss after drying at 60°C
for 48 h. To estimate total benthic microalgal biomass,
we collected 3 randomly located cores from each plot
(3 mm deep, 1.5 cm diameter), transported them on ice
in a dark cooler, and froze them at –20°C until analysis.
We extracted pigments with 90% acetone and deter-
mined chlorophyll a concentration using the spectro-
photometric method of Lorenzen (1967). Homosced-
asticity of all variances was verified using the F
and transformation of the data was unnecessary. Data
were analyzed with 1-way ANOVA (analysis of vari-
ance) within each site; the factor was cage treatment.
Test of interactive effects. To evaluate the roles of
direct and indirect trophic interactions, non-trophic
interactions, and modifications of habitat characteris-
tics on community structure, we conducted a 2-factor
experiment varying predator presence (plus or minus
crabs) and nutrient supply (plus or minus nutrients) in
May 2002. In both the mudflat and the sandflat, we
installed twenty 0.5 × 0.5 m enclosures as described
above. All Cerithidea californica and Pachygrapsus
crassipes present in the enclosures following installa-
tion were removed.
We augmented development of the microalgal com-
munity by adding nutrients to 10 randomly assigned
enclosures at each site. A window-screen mesh bag
containing 10 g of slow-release Osmocote fertilizer
(18% nitrogen [N] and 12% phosphorus [P] by dry
weight) was secured to the center of all +nutrient
enclosures; empty screen bags were placed in cages
with ambient nutrient (–nutrient) treatments. Biweekly
additions of 2 g of granulated urea fertilizer (46% N by
dry weight) supplemented the Osmocote addition. This
protocol was known to increase microalgal biomass
and induce cyanobacterial growth by the end of the
3 wk pre-experiment period (Armitage & Fong 2004b).
We randomly selected 5 +nutrient and 5 –nutrient plots
to contain ambient crab density (+crab) enclosures at
each site. We collected Pachygrapsus crassipes as
above and haphazardly assigned 4 crabs to each of the
+crab enclosures. No crabs were added to the other 10
cages at each site (–crab).
We collected Cerithidea californica from an adjacent
mudflat and individually numbered them using tags
printed on Nalgene waterproof paper, attached with
Krazy Glue, and lacquered with clear nail polish. Shell
lengths were measured to the nearest 0.05 mm with
calipers and divided into 2 size classes: 15 to 20 mm
(approximately corresponding to juvenile and sub-
adult snails) and 20.05 to 25 mm (adults). Snails outside
these size classes were less common and not included
in this study. Twenty-two C. californica from each size
class were randomly assigned to each experimental
cage to approximate natural densities (Armitage &
Fong 2004a) and were placed in the cages 5 d after
Pachygrapsus crassipes addition, initiating a 5 wk
experimental period.
We determined Cerithidea californica mortality and
burial of live snails to assess the direct and indirect
trophic effects of Pachygrapsus crassipes predation
and nutrient enrichment. C. californica consume ben-
thic microalgae on the sediment surface (Whitlatch &
Obrebski 1980), but may burrow into the sediment as
an escape response (McCarthy & Fisher 2000). Thus, to
assess indirect effects of P. crassipes on C. californica
foraging behavior, we counted all snails visible on the
sediment surface at the conclusion of the 5 wk experi-
mental period. We then collected all snails by sifting
the top 3 cm of sediment from each enclosure through
a 1 mm sieve. We counted the total number of surviv-
ing snails in each plot in both size classes and calcu-
lated snail burial as a percentage of the live snails in
each plot that were not on the sediment surface. In
addition, 2 sources of mortality were assessed. P. cras-
sipes crushes shells into many small pieces to consume
them (Sousa 1993), so recovery of marked broken
shells indicated crab predation. Other, non-predation
sources of mortality were determined by the absence
of an operculum, or by black or white bacterial films on
the mouth of the shell (Byers 2000). Predation mortality
and non-predation mortality are reported as percent-
ages of the total number of snails originally placed in
the plot. Based on the number of live and dead snails
collected, we then calculated the percentage of miss-
ing snails. Complete pulverization of the shells may
have occurred during consumption, suggesting that
many of the missing C. californica were probably
ingested by P. crassipes as well. The homoscedasticity
of all variances was confirmed using the F
test, and
data were log transformed if necessary to conform to
the assumptions of ANOVA. We analyzed percentage
of buried snails with 2-way ANOVA within each site;
the factors were plus or minus nutrients and plus or
minus crabs. We also analyzed percentage mortality
from crab predation with 2-way ANOVA within each
site; the factors were plus or minus nutrients and initial
snail size class (crab treatment was not included as a
factor because no predation by crabs occurred in plots
without crabs). The percentage of non-predation
mortality and the percentage of missing snails were
analyzed with 3-way ANOVA; the additional factor
was plus or minus crabs.
To evaluate direct and indirect effects of crabs and
nutrients on snail growth, we re-measured all intact
snails at the end of the experimental period and used
the average per-plot percentage growth in each initial
snail size class as the response variable. The homo-
Mar Ecol Prog Ser 313: 205213, 2006
scedasticity of variances was confirmed using the F
test, and data were log transformed if necessary to
conform to the assumptions of ANOVA. Small-snail
growth rates were analyzed with 2-way ANOVA
within each site; the factors were plus or minus nutri-
ents and plus or minus crabs. Living large snails were
not recovered from all plots in the sandflat, resulting in
insufficient replication for ANOVA (n 2), so large-
snail growth rates are reported as means (±SE) for that
site. Large-snail growth rates in the mudflat were
analyzed as for small snails.
To quantify the direct effects of nutrient enrichment
and the non-trophic effects of Pachygrapsus crassipes
(e.g. bioturbation) on the benthic microalgal commu-
nity, we estimated the biomass of the 2 dominant micro-
algal groups, diatoms and cyanobacteria, in the experi-
mental cages at the end of the 5 wk study period. From
each plot, we pooled 3 randomly located cores of 1.5 cm
diameter and a depth (3 mm) sufficient to
encompass the bulk of the microphyto-
benthic biomass (Wiltshire 2000). Cores
were transported to the laboratory on ice
in a dark cooler and frozen at –20°C until
analysis. We extracted pigments with 90%
acetone and determined the concentration
of chlorophyll a to estimate total micro-
phytobenthic biomass and the concen-
trations of the pigments characteristic of
the 2 major microalgal groups (fucoxan-
thin [diatom] and zeaxanthin [cyanobacte-
ria]) according to the high-performance
liquid chromatography method of Brotas
& Plante-Cuny (1996). The homoscedas-
ticity of variances was confirmed using the
test, and data were log transformed if
necessary to conform to the assumptions
of ANOVA. Pigment concentrations were
analyzed with 2-way ANOVA within each
site; the factors were plus or minus nutri-
ents and plus or minus crabs.
Test for cage effects
None of the sediment characteristics we
measured were affected by cage treat-
ment (1-way ANOVA, all p > 0.1), sug-
gesting minimal cage effects on the sedi-
ment and microalgae. There were several
differences in sediment characteristics
between sites. Mud content was higher in
the mudflat (mean ± SE: 28.8 ± 2.0%) than
in the sandflat (8.2 ± 0.5%). Sediment
organic content was higher in the mudflat (8.3 ± 0.6%)
than in the sandflat (2.2 ± 0.2%), as was sediment
water content (mudflat: 63.7 ± 2.9%; sandflat: 33.7 ±
1.6%). Benthic chlorophyll a concentration was similar
between sites (mudflat: 482.5 ± 34.9 mg m
; sandflat:
432.4 ± 42.0 mg m
Test of interactive effects
Snail mortality from non-predation sources was
strongly influenced by nutrient addition, as it occurred
almost exclusively in +nutrient treatments in both the
mudflat (df = 1, F = 9.922, p = 0.0035) and the sandflat
(df = 1, F = 20.775, p < 0.0001; Fig. 1a,b). Crabs or ini-
tial size class did not affect this type of mortality, with
no interactions between factors (all p > 0.05). Overall,
non-predation mortality was similar between sites.
% Missing snails
–Crabs +Crabs –Crabs +Crabs –Crabs +Crabs –Crabs +Crabs
% Non-predation mortality
Large snails Small snails Large snails Small snails
% Predation mortality
§ § § §
ψ ψψψ ψ
Fig. 1. Pachygrapsus crassipes and Cerithidea californica. Effects of crabs,
nutrients, and initial snail size on (a,b) snail mortality from non-predation
sources, (c,d) snail mortality from crab predation, and (e,f) percentage of
missing snails on a tidal mudflat and on a sandflat. Error bars are SE (§: not
applicable; ψ:no mortality detected)
Armitage & Fong: Indirect impacts of crabs on benthos
Snail mortality from crab predation was significantly
higher for large than for small snails at both sites (mud-
flat: df = 1, F = 5.662, p = 0.0301; sandflat: df = 1, F =
14.838, p = 0.0014; Fig. 1c,d), with no interactions
between factors. In the mudflat, nutrient enrichment
lowered percentage predation mortality (df = 1, F =
9.724, p = 0.0066). There was a trend of decreased crab
predation on large snails in enriched treatments in the
sandflat as well, though the nutrient effect was not
significant due to high variability between nutrient
treatments for small snails. Predation on snails ap-
peared to be more intense in the sandflat than in the
Generally, >90% of the snails were recovered from
plots without crabs, but there were complex treatment
effects on the number of missing snails. In the mudflat,
there was a significant interaction between crab and
nutrient effects on the percentage of missing snails due
to more missing large snails in –nutrient plots when
crabs were present and more missing small snails in
+nutrient plots when crabs were absent (Fig. 1e,
Table 1). In the sandflat, there was a significant inter-
action between crabs and initial snail size class on the
percentage of missing snails (Table 1). This interaction
stemmed from more missing snails in plots with crabs
than without crabs, but this effect was stronger for
large snails (Fig. 1f).
A significantly higher percentage of the surviving
snails was buried in plots with crabs than in plots with-
out crabs at both sites (mudflat: df = 1, F = 17.816, p =
0.0006; sandflat: df = 1, F = 36.761, p < 0.0001; Fig. 2).
Nutrients did not affect snail burial, and there were no
interactions between factors (all p > 0.1). Overall, the
percentage of buried snails was similar between sites.
At both sites, small snail growth was high (15 to 20%
increase in snail length) in plots without crabs, but was
reduced by >50% in plots with crabs (mudflat: df = 1,
F = 30.700, p < 0.0001; sandflat: df = 1, F = 24.475, p =
0.0002; Fig. 3). Nutrients did not affect small snail
growth rates at either site, and there were no inter-
actions between factors (all p > 0.1). In the mudflat,
large snail growth was low (2 to 5% increase in snail
length) in plots without crabs, but was reduced to 0 in
plots with crabs (df = 1, F = 26.072, p = 0.0001). A simi-
lar trend occurred in the sandflat, though statistical
comparisons were not made because live large snails
were not recovered from all +crab plots in the sandflat
(n 2). In the mudflat, nutrients did not affect large
snail growth rates, and there were no interactions
between factors at either site (all p > 0.1).
At the end of the experimental period, the benthic
chlorophyll a concentration was lower in the presence
of crabs at both sites (mudflat: df = 1, F = 4.603, p =
0.0476; sandflat: df = 1, F = 31.201, p < 0.0001), but was
unaffected by nutrients, with no interactions between
factors (all p > 0.05) (Fig. 4a,b). Benthic fucoxanthin
df MS F p
Crabs (C) 1 8.74 54.16 <0.0001
Nutrients (N) 1 0.10 0.62 0.4359
Initial size (S) 1 0.01 0.05 0.8245
C × N 1 1.017 6.30 0.0173
C × S 1 0.52 3.22 0.0820
N × S 1 0.16 1.00 0.3241
C × N × S 1 <0.01 0.01 0.9256
Residual 32 0.16
Crabs (C) 1 10.22 43.26 <0.0001
Nutrients (N) 1 0.36 1.51 0.2282
Initial size (S) 1 0.51 2.17 0.1502
C × N 1 0.06 0.26 0.6150
C × S 1 1.14 4.84 0.0352
N × S 1 0.12 0.49 0.4905
C × N × S 1 0.04 0.17 0.6837
Residual 32 0.24
Table 1. Pachygrapsus crassipes and Cerithidea californica.
Results of 3-way ANOVA of crabs, nutrients, and size class on
the percentage of snails missing from plots on a tidal mudflat
and a tidal sandflat
% Buried snails
–Crabs +Crabs
Fig. 2. Pachygrapsus crassipes and Cerithidea californica. Ef-
fects of crabs and nutrients on the percentage of surviving
snails that were buried at the end of the 5 wk study period on
(a) a tidal mudflat and (b) a tidal sandflat. Error bars are SE
Mar Ecol Prog Ser 313: 205213, 2006
(diatom) concentration was also lower in the presence
of crabs at both sites (mudflat: df = 1, F = 7.666, p =
0.0137; sandflat: df = 1, F = 39.209, p < 0.0001;
Fig. 4c,d), with no nutrient effects or interactions
between factors (all p > 0.05). Benthic zeaxanthin
(cyanobacteria) concentration was lower in the pres-
ence of crabs, but only in the sandflat (df = 1, F =
13.860, p = 0.0019; mudflat: p > 0.1; Fig. 4e,f), with no
nutrient effects or interactions between factors. Over-
all, all pigment concentrations were lower in the mud-
flat than in the sandflat.
The combined forces of direct and indirect interac-
tions are important drivers shaping community struc-
ture in terrestrial and marine ecosystems (Menge 1995,
Hobbs 1996), but they remain poorly understood in the
soft-sediment marine habitats that are widespread in
coastal wetlands and estuaries (Liess & Hillebrand
2004). Our study explored these dynamics in soft-
sediment communities and detected closely coupled
direct and indirect interactions among predators, graz-
ing epifauna, and microalgae. In addition to consump-
tion of Cerithidea californica, Pachygrapsus crassipes
exerted strong indirect impacts on the benthic commu-
nity through sediment and microalgal disturbance and
modifications of snail foraging activities. Evidence for
the prevalence of indirect predator effects has been
documented in a wide variety of habitats, including
tide pool communities (Trussell et al. 2004), grassland
arthropod communities (Schmitz et al. 1997), and tem-
perate pond assemblages (Peacor & Werner 1997). Pre-
vious work in soft-sediment communities has focused
on indirect trophic interactions like trophic cascades
(Liess & Hillebrand 2004) and the roles of habitat mod-
ifications on trophic dynamics (Rhoads & Young 1970).
Ecosystem engineers have important indirect effects
on benthic communities by altering sediment charac-
teristics and structural complexity (Bertness 1985,
Growth (% change from initial)
Large snails Small snails
– Nutrients
+ Nutrients
–Crabs +Crabs –Crabs +Crabs
Large snails Small snails
Fig. 3. Pachygrapsus crassipes and Cerithidea californica.
Percentage change in length of small (15 to 20 mm initial
length) and large (20 to 25 mm) snails over the 5 wk study
period in response to crabs and nutrients on (a) a tidal mudflat
and (b) a tidal sandflat. Error bars are SE
(mg m
Fucoxanthin (mg m
Zeaxanthin (mg m
–Crabs +Crabs
– Nutrients
+ Nutrients
–Crabs +Crabs
Mudflat Sandflat
Fig. 4. Pachygrapsus crassipes. Effects of crabs and nutrients
on the benthic microalgal community from a tidal mudflat and
a tidal sandflat. Effect on (a,b) chlorophyll a (total microalgal
biomass), (c,d) fucoxanthin (diatom biomass), and (e,f) zea-
xanthin (cyanobacterial biomass). Error bars are SE
Armitage & Fong: Indirect impacts of crabs on benthos
Boyer & Fong 2005, Commito et al. 2005), but we illus-
trated a different range of non-trophic, indirect effects
of predators on a soft-sediment marine habitat.
We observed few links between direct top-down
(predation) and bottom-up (nutrient enrichment) tro-
phic forces in this study. The effects of nutrient and
crab addition on the microalgal and snail assemblages
were largely independent of each other, though nutri-
ents may have slightly decreased crab fitness or activ-
ity, as predation-related mortality of snails was lower
in nutrient-addition treatments. Omnivory can decou-
ple top-down and bottom-up forces, as documented in
seagrass beds in the Gulf of Mexico (Heck et al. 2000)
and freshwater wetlands in Florida (Geddes & Trexler
2003). This mechanism may have been important in
our study, as Pachygrapsus crassipes are often omniv-
orous, consuming infauna and macroalgae in addition
to gastropods (Hiatt 1948, Boyer & Fong 2005). Inter-
actions between top-down and bottom-up forces may
also be decoupled if direct predation effects are ex-
tremely strong (Posey et al. 1999). P. crassipes crush
shells into small pieces to consume them (Sousa 1993),
so many of the missing Cerithidea californica were
probably ingested by P. crassipes, suggesting that
crabs may have consumed as many as 70 to 85% of the
large snails. Strong predation impacts have also been
documented in soft-sediment communities on the east
coast of the USA (Posey et al. 1999, Hunt & Mullineaux
2002). Although the predation intensity we observed
may have been somewhat inflated by enclosing the
predators in cages with the prey, non-predation mor-
tality, which was most likely due to nutrient addition,
was very low relative to predation mortality, suggest-
ing that predators decreased the relative importance of
nutrient addition for the snails. This concurs with stud-
ies of the east coast of the United States demonstrating
that predation is often an important, though temporally
variable, structuring force in soft-bottom communities
that may overwhelm bottom-up effects of nutrient
enrichment (Wiltse et al. 1984). Temporal variability in
predation forces may have reduced the effectiveness
of our test for cage effects, which was performed in
August, when compared to the test for interactive
effects, which was performed in May. However, P.
crassipes tend to be abundant and active throughout
the summer months (Quammen 1984), suggesting that
crab impacts on the snails, microalgae, and sediment
were relatively consistent during this period.
Though there were few links between direct top-
down and bottom-up interactions in our study system,
both nutrients and Pachygrapsus crassipes had many
indirect effects on other community components.
Nutrient addition did not increase cyanobacterial bio-
mass as we expected based on previous tidal flat stud-
ies (Pinckney et al. 1995, Armitage & Fong 2004b).
Grazing can suppress microalgal biomass under vary-
ing levels of productivity (Kaehler & Froneman 2002).
Measurement of microalgal productivity would have
clarified whether the minimal microalgal responses to
nutrient enrichment that we observed were due to
grazer regulation. However, shifts in cyanobacterial
species composition also commonly occur in response
to enrichment (Kuffner & Paul 2001), even if total bio-
mass remains unchanged. A shift towards toxic or
lower nutritive value species (Ferrão-Filho et al. 2000)
provides a likely explanation for increased Cerithidea
californica mortality in enriched plots. This is sup-
ported by previous experimental work in this region
that strongly suggests that diet composition is respon-
sible for nutrient-related C. californica mortality
(Armitage & Fong 2004b). Crabs may have also been
indirectly affected by nutrient addition, as P. crassipes
can consume benthic microalgae (Hiatt 1948). We
observed few qualitative treatment responses by P.
crassipes in this short-term study, but negative effects
of nutrient addition on P. crassipes are suggested by
lower snail mortality from crab predation in the
enriched treatments.
Despite Pachygrapsus crassipes predation on Ceri-
thidea californica, there was no evidence of a trophic
cascade, where benthic algae proliferate following a
release from herbivore pressure. Rather, benthic
microalgal biomass (as indicated by chlorophyll a con-
centration) was markedly lower in plots with crabs. P.
crassipes may have disrupted other trophic relation-
ships in this community by disturbing or ingesting the
macroinfauna or meiofauna that consume a large por-
tion of benthic microalgal productivity (Hiatt 1948,
Buffan-Dubau & Carman 2000), although disturbance
of infauna should reduce grazing pressure and result
in increased benthic microalgal biomass. P. crassipes
may have reduced benthic biomass directly by con-
suming microalgal mats (Hiatt 1948). In addition, bio-
turbation from P. crassipes burrowing activities likely
physically disturbed the microalgal mats. Similar
responses of microphytobenthos to bioturbation have
been documented in a range of soft-bottom habitats,
including estuarine marshes (Boyer & Fong 2005) and
streams (Usio & Townsend 2002). The reduction of food
availability below a critical threshold level necessary
for C. californica growth may partially explain the
decrease in snail growth rates in the presence of P.
crassipes. Bioturbation can also alter physical charac-
teristics and epifaunal and infaunal fitness (Rhoads &
Young 1970, Palmer 1988), but further studies are
needed to explore the trophic implications of such
modifications, particularly on epifaunal communities
on the west coast of the USA. We did not detect any
caging artifacts in our study design, nor qualitatively
observe marked erosion or sediment deposition
Mar Ecol Prog Ser 313: 205213, 2006
around our enclosures, but cage alterations of flow-
mediated effects that may be impacted by bioturba-
tion, including nutrient flux from the sediments and
biofilm erosion, should also be considered in future
Pachygrapsus crassipes may have further lowered
snail growth rates by increasing snail burial, ef-
fectively reducing the amount of time available for
Cerithidea californica to forage. C. californica typically
grazes on the sediment surface (Whitlatch & Obrebski
1980), but may have actively burrowed below the sed-
iment surface as a behavioral response to the threat of
predation (McCarthy & Fisher 2000). Prey behavioral
modifications by predators have been documented in a
wide range of habitats, including marine (Trussell et al.
2004), aquatic (Peacor & Werner 1997), and terrestrial
(Schmitz et al. 1997) ecosystems and often result in a
decrease in prey fitness. Alternatively, C. californica
may have been passively buried as an indirect conse-
quence of crab burrowing activities. Either mechanism
of burial, whether active or passive, resulted in a
reduction of the amount of time snails could graze on
the sediment surface and likely contributed to lower C.
californica growth rates in the presence of crabs. It is
likely that a combination of these active and passive
burial mechanisms reduced snail grazing activities,
illustrating that predators can have a range of indirect
impacts on prey assemblages.
Though crabs and nutrients exerted several discrete,
measurable impacts on snails and microalgae in this
ecosystem, each effect acted in concert with others to
shape community structure. Previous studies sug-
gested that nutrients can markedly alter microalgal
community composition and grazer survival in this and
other soft-sediment systems (Pinckney et al. 1995,
Armitage & Fong 2004b). However, Pachygrapsus
crassipes introduced a complex of direct and indirect
interactions that reduced the relative importance of
nutrients, illustrating the importance of evaluating
biotic and abiotic interactions simultaneously. The
community-level impacts of the biotic and abiotic
interactions in this system revealed a level of complex-
ity not readily apparent from a trophic food web.
Rather, a web of direct and indirect, trophic and non-
trophic interactions shaped this community. Consider-
ation of this more complete range of interactions will
facilitate efforts to understand the suites of forces that
structure communities.
Acknowledgements. We are indebted to T. Keeney and the
US Navy for providing access to the research site, to R. A.
Cohen, L. Green, B. Huntington, R. L. Kennison, V. Minnich,
D. Reineman, and S. Wang for their tireless assistance in the
laboratory and the field, to C. Janousek and M. Vernet for use
of their HPLC equipment and expertise, and to R. R. Vance
and R. F. Ambrose for advice on experimental design and
comments on the manuscript. This project was funded in part
by a UC Coastal Environmental Quality Initiative Graduate
Fellowship to A.R.A. and a grant from the EPA (No. R827637)
to P.F.
Ambrose WG Jr (1984) Role of predatory infauna in structur-
ing marine soft-bottom communities. Mar Ecol Prog Ser
Armitage AR, Fong P (2004a) Gastropod colonization of a
created coastal wetland: potential influences of habitat
suitability and dispersal ability. Restor Ecol 12:391400
Armitage AR, Fong P (2004b) Upward cascading effects of
nutrients: shifts in a benthic microalgal community and a
negative herbivore response. Oecologia 139:560567
Bertness MD (1985) Fiddler crab regulation of Spartina alter-
niflora production on a New England salt marsh. Ecology
Blumenshine SC, Lodge DM, Hodgson JR (2000) Gradient of
fish predation alters body size distributions of lake ben-
thos. Ecology 81:374386
Bonsdorff E, Norkko A, Sandberg E (1995) Structuring
zoobenthos: the importance of predation, siphon cropping
and physical disturbance. J Exp Mar Biol Ecol 192:
Bouyoucos GJ (1962) Hydrometer method improved for mak-
ing particle size analyses of soils. Agron J 54:464465
Boyer KE, Fong P (2005) Co-occurrence of habitat-modifying
invertebrates: effects on structural and functional proper-
ties of a created salt marsh. Oecologia 143:619628
Brotas V, Plante-Cuny MR (1996) Identification and quantifi-
cation of chlorophyll and carotenoid pigments in marine
sediments: a protocol for HPLC analysis. Oceanol Acta 19:
Buffan-Dubau E, Carman KR (2000) Diel feeding behavior of
meiofauna and their relationships with microalgal re-
sources. Limnol Oceanogr 45:381395
Byers JE (2000) Differential susceptibility to hypoxia aids
estuarine invasion. Mar Ecol Prog Ser 203:123132
Carpenter SR, Kitchell JF, Hodgson JR (1985) Cascading
trophic interactions and lake productivity. BioScience 35:
Commito JA, Celano EA, Celico HJ, Como S, Johnson CP
(2005) Mussels matter: postlarval dispersal dynamics
altered by a spatially complex ecosystem engineer. J Exp
Mar Biol Ecol 316:133147
Ferrão-Filho AS, Azevedo SMFO, DeMott WR (2000) Effects
of toxic and non-toxic cyanobacteria on the life history of
tropical and temperate cladocerans. Freshw Biol 45:119
Geddes P, Trexler JC (2003) Uncoupling of omnivore-medi-
ated positive and negative effects on periphyton mats.
Oecologia 136:585595
Hagerthey SE, Defew EC, Paterson DM (2002) Influence of
Corophium volutator and Hydrobia ulvae on intertidal
benthic diatom assemblages under different nutrient and
temperature regimes. Mar Ecol Prog Ser 245:4759
Hamilton DJ (2000) Direct and indirect effects of predation by
common eiders and abiotic disturbance in an intertidal
community. Ecol Monogr 70:2143
Heck KL Jr, Pennock JR, Valentine JF, Coen LD, Sklenar SA
(2000) Effects of nutrient enrichment and small predator
density on seagrass ecosystems: an experimental assess-
ment. Limnol Oceanogr 45:10411057
Hiatt RW (1948) The biology of the lined shore crab, Pachy-
grapsus crassipes Randall. Pac Sci 2:134213
Armitage & Fong: Indirect impacts of crabs on benthos
Hobbs NT (1996) Modification of ecosystems by ungulates.
J Wildl Manag 60:695713
Hunt HL, Mullineaux LS (2002) The roles of predation and
postlarval transport in recruitment of the soft shell clam
(Mya arenaria). Limnol Oceanogr 47:151164
Kaehler S, Froneman PW (2002) Herbivore-mediated increase
in the photosynthetic capacity of marine biofilms: indirect
effects of changing microalgal assemblage composition.
Mar Ecol Prog Ser 234:1522
Kuffner IB, Paul VJ (2001) Effects of nitrate, phosphate and
iron on the growth of macroalgae and benthic cyanobacte-
ria from Cocos Lagoon, Guam. Mar Ecol Prog Ser 222:
Liess A, Hillebrand H (2004) Direct and indirect effects in her-
bivoreperiphyton interactions. Arch Hydrobiol 159:
Lorenzen CJ (1967) Determination of chlorophyll and
pheopigments: spectrophotometric equations. Limnol
Oceanogr 12:343346
McCarthy TM, Fisher WA (2000) Multiple predator-avoid-
ance behaviours of the freshwater snail Physella hetero-
stropha pomila: responses vary with risk. Freshw Biol 44:
Menge BA (1995) Indirect effects in marine rocky intertidal
interaction webs: patterns and importance. Ecol Monogr
Ólafsson EB, Peterson CH, Ambrose WGJ (1994) Does
recruitment limitation structure populations and commu-
nities of macroinvertebrates in marine soft sediments? The
relative significance of pre- and post-settlement pro-
cesses. Oceanogr Mar Biol Annu Rev 32:65109
Paine RT (1974) Intertidal community structure: experimental
studies on the relationship between a dominant competi-
tor and its principal predator. Oecologia 15:93120
Palmer MA (1988) Epibenthic predators and marine meio-
fauna: separating predation, disturbance, and hydro-
dynamic effects. Ecology 69:12511259
Peacor SD, Werner EE (1997) Trait-mediated indirect inter-
actions in a simple aquatic food web. Ecology 78:
Pinckney J, Paerl HW, Fitzpatrick M (1995) Impacts of season-
ality and nutrients on microbial mat community structure
and function. Mar Ecol Prog Ser 123:207216
Posey MH, Alphin TD, Cahoon L, Lindquist D, Becker ME
(1999) Interactive effects of nutrient additions and preda-
tion on infaunal communities. Estuaries 22:785792
Quammen ML (1984) Predation by shorebirds, fish, and crabs
on invertebrates in intertidal mudflats: an experimental
test. Ecology 65:529537
Rhoads DC, Young DK (1970) The influence of deposit feed-
ing organisms on sediment stability and community
trophic structure. J Mar Res 28:150178
Schmitz OJ, Beckerman AP, O’Brien KM (1997) Behaviorally
mediated trophic cascades: effects of predation risk on
food web interactions. Ecology 78:13881399
Sousa WP (1993) Size-dependent predation on the salt-marsh
snail Cerithidea californica Haldeman. J Exp Mar Biol
Ecol 166:1937
Thrush SE (1999) Complex role of predators in structuring
soft-sediment macrobenthic communities: implications of
changes in spatial scale for experimental studies. Aust J
Ecol 24:344354
Trussell GC, Ewanchuk PJ, Bertness MD, Silliman BR (2004)
Trophic cascades in rocky shore tide pools: distinguishing
lethal and nonlethal effects. Oecologia 139:427432
Usio N, Townsend CR (2002) Functional significance of cray-
fish in stream food webs: roles of omnivory, substrate
heterogeneity and sex. Oikos 98:512522
Valiela I, McClelland J, Hauxwell J, Behr PJ, Hersh D, Fore-
man K (1997) Macroalgal blooms in shallow estuaries:
controls and ecophysiological and ecosystem conse-
quences. Limnol Oceanogr 42:11051118
Whitlatch RB, Obrebski S (1980) Feeding selectivity and
coexistence in two deposit feeding gastropods. Mar Biol
Widbom B, Elmgren R (1988) Response of benthic meiofauna
to nutrient enrichment of experimental marine ecosys-
tems. Mar Ecol Prog Ser 42:257268
Wiltse WI, Foreman KH, Teal JT, Valiela I (1984) Effects of
predators and food resources on the macrobenthos of salt
marsh creeks. J Mar Res 42:923942
Wiltshire KH (2000) Algae and associated pigments of inter-
tidal sediments, new observations and methods. Limno-
logica 30:205214
Editorial responsibility: Kenneth L. Heck (Contributing
Editor), Dauphin Island, Alabama, USA
Submitted: March 4, 2005; Accepted: October 4, 2005
Proofs received from author(s): March 31, 2006
... In North America, the majority of marsh studies to date have been short-term experiments (1-2 yr) on grass-dominated (e.g., Spartina spp.) systems Tyrrell 2012, Moore 2019). On the west coast of the United States, there have been many studies on bottom-up processes in tidal marshes (Thorne et al. 2014, 2018, Janousek et al. 2017, Osland et al. 2019) and comparatively few on consumer effects (Boyer and Fong 2005, Armitage and Fong 2006, Wasson et al. 2021. The role of consumers in woody perennial marsh systems, like those found along the Pacific Northwest Coast of the United States, Europe, Australia, South Africa, and South Asia (Kadereit et al. 2007), is likely to operate at considerably longer timescales than herbaceous grass-dominated systems, thus providing a novel perspective on consumer effects. ...
... In our estuarine salt marsh system, Elkhorn Slough, P. crassipes is the numerically dominant crab species, followed by the less common and smaller grapsid crab, Hemigrapsus oregonensis, and invasive green crab, Carcinus maenas; both are rarely observed or trapped in our focal marshes and are almost exclusively subtidal. Generally considered an omnivorous crab because it feeds opportunistically on algae, diatoms (Armitage and Fong 2006), mud snails (Sousa 1993, Wasson et al. 2020, and detritus (Quammen 1980), feeding assays conducted in this study are the first to directly test whether P. crassipes also consumes vascular marsh plants, including the spatially dominant woody perennial, Salicornia pacifica. We observed crabs grazing plant roots in both the field and the laboratory and patterns of sparser marsh cover in areas with high crab and crab burrow densities. ...
... Similarly, we expected burrows to fill with fewer crabs resulting in a delayed positive response in belowground biomass and bulk density associated with crab reduction treatments. We expected that reducing crab densities would result in a related increase in epiphytic and benthic algal cover, as crabs are known consumers of benthic macroalgae and diatoms (Armitage and Fong 2006). With the assumption that burrows would fill, and marsh plant production would be higher in crab reduction compared to ambient treatments, we also expected salt marsh vertical accretion, measured using feldspar marker horizons, to be higher. ...
Full-text available
Consumers can structure plant communities and may function as keystone species or ecosystem engineers. In salt marshes, the prevailing paradigm has shifted in recent decades from nearly complete focus on bottom‐up processes to inclusion of top‐down effects. Although the number of studies investigating top‐down control continues to climb, few experiments span multiple years, so temporal variability in or long‐term impacts of consumers have not been well characterized. In addition, while top‐down control has been found to be common in Western and Eastern Atlantic and Western Pacific salt marshes, our study is one of the first to experimentally consider top‐down control of salt marsh plants in the Eastern Pacific. We conducted a five‐year field experiment along eroding creekbank edges of a California salt marsh in which we manipulated densities of the shore crab, Pachygrapsus crassipes, and tracked marsh responses over time. Our results demonstrate that, through both consumption and engineering activities, this superabundant crab is regulating marsh vegetation and soil structure. Experimentally reducing crab abundance enhanced vegetation biomass and sediment bulk density. Moreover, root biomass and bulk density—factors known to increase marsh resilience to erosion and sea‐level rise—decreased linearly with increasing burrow density. Our long‐term study uniquely revealed that burrows can persist for years after crab abundances are reduced and that plant responses from grazer exclusions gradually strengthen over time, likely due to the relatively slow growth of woody perennial foundational plants. Since shore crabs are abundant throughout the marsh in most major estuaries within the range of the species (from Baja California, Mexico to Oregon, USA), we hypothesize that this species is exerting significant, yet underappreciated top‐down control and modifying the sediment properties of many West Coast salt marshes.
... High rates of early juvenile mortality can affect not only adult abundance and population size, but also population characteristics such as reproductive output and age at maturity (Gosselin & Qian 1997). One small crab species that has been implicated in causing juvenile mortality in snails and urchins is the lined shore crab Pachygrapsus crassipes (Armitage & Fong 2006, Clemente et al. 2013. ...
... Although it has been described as ''pugnacious'' by Ricketts et al. (1985) and is highly abundant in exposed and sheltered rocky habitats of the northeast Pacific (Hui 1992, Cassone & Boulding 2006, some studies describe Pachygrapsus crassipes as an herbivore or scavenger, consuming primarily diatom films, algae (e.g., Ulva spp.), and detritus (Hiatt 1948, Barry & Ehret 1993. More recent studies recognized the predation potential of this species and examined its feeding on juvenile urchins (Strongylocentrotus purpuratus) (Clemente et al. 2013) and snails (Cerithidea californica) (Armitage & Fong 2006). ...
... This mussel is a common invasive species and is the most abundant mussel in the bays and estuaries of southern and central California, where P. crassipes is also present (Wonham 2004, Braby & Somero 2005. Because P. crassipes can consume juvenile snails and urchins (Armitage & Fong 2006, Clemente et al. 2013), a combination of laboratory and field experiments was used to test the hypothesis that P. crassipes was a predator of a range of juvenile invertebrates including abalone, mussels, and whelks. ...
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Major rocky intertidal predators in the northeast Pacific such as sea stars, whelks, and birds can consume foundation species such as mussels and thereby affect zonation patterns and diversity in these habitats. Predation specifically on juvenile intertidal invertebrates can also substantially impact population dynamics and influence community structure. The lined shore crab Pachygrapsus crassipes (Randall, 1840) is an abundant denizen of sheltered and exposed intertidal habitats in the northeast Pacific from Canada to Mexico. This study examined potential P. crassipes predation on juvenile mussels (Mytilus galloprovincialis), whelks [Nucella ostrina (Gould, 1852)], and abalone [Haliotis rufescens (Swainson, 1822)] due to conflicting reports on the diet of this species. Crabs consumed more juvenile mussels and abalone than seaweed (Ulva lactuca) and fed preferentially on the smallest mussels (6- to 10-mm size class). Further experiments showed that predation on mussels by P. crassipes was highly size dependent, with the largest crabs consuming over twenty-five 15-mm mussels per day. Field outplant experiments revealed that P. crassipes consumed high numbers of juvenile mussels in a natural setting, meaning that it could substantially affect mussel recruitment. This crab species appears to be an opportunistic predator that could have significant impacts on the recruitment and early life history of several invertebrate prey species. Major rocky intertidal predators in the northeast Pacific such as sea stars, whelks, and birds can consume foundation species such as mussels and thereby affect zonation patterns and diversity in these habitats. Predation specifically on juvenile intertidal invertebrates can also substantially impact population dynamics and influence community structure. The lined shore crab Pachygrapsus crassipes (Randall, 1840) is an abundant denizen of sheltered and exposed intertidal habitats in the northeast Pacific from Canada to Mexico. This study examined potential P. crassipes predation on juvenile mussels (Mytilus galloprovincialis), whelks [Nucella ostrina (Gould, 1852)], and abalone [Haliotis rufescens (Swainson, 1822)] due to conflicting reports on the diet of this species. Crabs consumed more juvenile mussels and abalone than seaweed (Ulva lactuca) and fed preferentially on the smallest mussels (6- to 10-mm size class). Further experiments showed that predation on mussels by P. crassipes was highly size dependent, with the largest crabs consuming over twenty-five 15-mm mussels per day. Field outplant experiments revealed that P. crassipes consumed high numbers of juvenile mussels in a natural setting, meaning that it could substantially affect mussel recruitment. This crab species appears to be an opportunistic predator that could have significant impacts on the recruitment and early life history of several invertebrate prey species.
... Macrofauna, through the process of bioturbation, can modify the environment in different ways, depending on the involved bioturbation mode, its intensity and abiotic characteristics of each environment (Kristensen et al., 2012). Through bioturbation, macrofauna indirectly impacts MPB given that it generates changes on sediment water and organic matter (OM) content, destabilization of chemical gradients and modification of inorganic nutrient fluxes (e.g., Armitage and Fong, 2006;Needham et al., 2011). Moreover, macrofauna directly affects MPB, since many of these organisms are herbivores (e.g., the snail Hydrobia ulvae, Austen et al., 1999 and the urchin Echinochardium sp., Lohrer et al., 2004). ...
In shallow soft-bottom intertidal systems, microphytobenthic assemblages (MPB) and macrofauna interact and play important roles in ecosystem functioning. Macrofauna can affect MPB through bioturbation or/and herbivory. Most bioturbators also exert herbivory pressure on MPB, hence the results of the interaction between benthic macrofauna and MPB is not easy to predict. In this study we performed a small scale field experiment in order to discriminate the effect of bioturbation and herbivory by the intertidal burrowing crab Neohelice granulata on MPB structure, and primary production in SW Atlantic mudflats. The implemented treatments allowed us to separate the effects of bioturbation from those of herbivory on MPB. Results showed that N. granulata, through bioturbation + herbivory did not affect MPB structure (i.e., biomass, abundance and composition); and crab bioturbation without herbivory also did not modify MPB structure. Nevertheless, crab bioturbation without herbivory almost doubled MPB primary production, compared to the values registered in the treatment without bioturbation and herbivory and the treatment with bioturbation + herbivory. Sediment properties were equally affected by bioturbation in the four treatments. Thus, our results shows that the only significant effect on MPB structure is due to crab bioturbation.
... The effect of nutrient addition can also be mediated by other organisms, for example the crab Pachygrapsus crassipes, via processes such as bioturbation, reduced the relative importance of nutrients to soft-sediment benthic assemblages (Armitage and Fong, 2006). It is possible that the crab species found in our sites, such as the burrowing Heloecius cordiformis, have a similar effect on nutrients. ...
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Proliferation of urban structures and mangrove forests in estuaries are altering the shading of intertidal sediments. Urbanization also tends to increase nutrient loads in estuaries, which can have numerous direct and indirect effects on estuarine flora and fauna. Mangrove canopy shades the sediment and provides nutrients to the ecosystem via leaf litter. Microphytobenthos, macrofauna, sediment erodibility, and various biogeochemical properties of sediments have been shown to differ significantly between unshaded intertidal sediment and nearby sediment under a mangrove canopy. This study tested the effects of experimental manipulation of shading and addition of nutrients on the microphytobenthos, macrofauna, sediment erodibility, and selected biogeochemical properties of exposed intertidal flat next to the seaward edge of a mangrove forest. In the first of two experiments, plots were shaded with roofs to give lightly shaded plots and heavily shaded plots, for comparison with unshaded control plots; nutrients were added in an orthogonal design. Sediment and benthos were sampled after 2 weeks. Nutrients were omitted in the second experiment, with plots sampled after 2 weeks or approximately 3 months. The only effect of nutrients was a small negative effect on chlorophyll a and colloidal carbohydrate. Light shading (clear roofs) generally increased measures of microphytobenthos biomass (e.g., Fo and chlorophyll a) and biogeochemical properties associated with microphytobenthos such as colloidal carbohydrate. Heavy shading (black roofs) generally decreased measures of microphytobenthos biomass and microphytobenthos-associated biogeochemical properties. Effects on the fauna were much smaller and inconsistent with previous studies, after 3 months, assemblages were different under heavy shading compared to light shading and unshaded control plots, with differences primarily driven by changes in the oligochaetes. Natural or anthropogenic changes in shading at larger spatial scales are likely therefore to directly and indirectly change microphytobenthos, sediment properties, macrofauna and hence ecosystem functions; but any flow-on effects to the fauna are difficult to predict without further experiments to understand the indirect and direct responses of fauna to changing microphytobenthos and properties of intertidal sediment.
... Laboratory trials revealed that Pachygrapsus can readily attack and consume Batillaria. Grapsids have relatively small claws with limited biomechanical strength for cracking large snail shells (Yamada and Boulding 1998), however, multiple studies have demonstrated grapsid crab predation on Batillaria's native analogue, the California horn snail (Cerithideopsis californica) (Sousa 1993;Armitage and Fong 2006;Lorda et al. 2016). Furthermore, other studies have documented crab predation on congeneric Batillaria sp. in the native range (Ö rstan 2006;Miura et al. 2012), and even Batillaria attramentaria itself in Washington (Grason et al. 2018). ...
Full-text available
Boom-bust dynamics of invasive species have long intrigued scientists and managers alike, but quantification of such dynamics, let alone their causes, is rare. We documented the decline of a previously prolific invasive mudsnail, Batillaria attramentaria, at Elkhorn Slough estuary in central California, USA. The mudsnail was the most abundant epibenthic invertebrate in the estuary, maintaining very high densities for many decades before declining heterogeneously throughout the estuary over the past decade, decreasing in density by three orders of magnitude at some sites. We used field and laboratory experiments to test several possible mechanisms for its demise. We show that the crab Pachygrapsus crassipes can prey heavily on Batillaria. We detected high dissolution rates of Batillaria shells, and we measured greater predation rates on tethered snails with dissolved versus intact shells. Warm water temperatures and high water levels coincided with the period of most dramatic Batillaria declines (2013–2015). Localized water impoundments appear to buffer environmental drivers of the decline because Batillaria remained abundant at sites with artificial tidal restriction, while the population crashed at one site after full tidal exchange was restored. We also investigated trematode parasite prevalence and molluscicide applications to the surrounding watershed as possible causes of mudsnail declines, but they had little explanatory power. Our findings illustrate the potential for population crashes even for long-established introduced species at pest levels of abundance, and demonstrate that such declines can exhibit spatial heterogeneity. Both of these results highlight the value of investigating population dynamics of invaders across multiple temporal and spatial scales.
... Fanjul et al. 2007, 2008, Alberti et al. 2011). This kind of box design does not significantly affect flow dynamics or light incidence (Hillebrand & Kahlert 2002, Armitage & Fong 2006, Cheverie et al. 2014 and thus these boxes do not affect MPB dynamics (Alvarez et al. 2013). Once installed, the experiment ran for 60 d (summer 2015), and the experimental plots were periodically inspected to guarantee the absence of crabs in crab exclusions; small crabs were manually removed when necessary. ...
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In coastal systems, benthic microalgae are important primary producers that significantly contribute to global primary production. Microphytobenthic (MPB) community structure and the ecosystem functions that it mediates are modulated by biotic and abiotic factors. Through burrowing activity, different types of bioturbators can impact MPB in different ways. Through directed sampling and field experiments performed at the Mar Chiquita coastal lagoon (Argentina), we evaluated the effect of regenerative bioturbation (continuous digging and maintenance of burrows, with sediment transfer from depth to surface) by the intertidal burrowing crab Neohelice (Chasmagnatus) granulata on MPB. We compared sediment properties and MPB attributes between natural burrowed and non-burrowed areas. Moreover, we experimentally manipulated regenerative bioturbation to evaluate if MPB composition and primary production are altered by crab burrowing activity. Field sampling showed that MPB and sediment properties differed between natural burrowed and non-burrowed areas. Experimental results indicated that regenerative bioturbation by N. granulata increased sediment oxygen concentration at each measured depth and also changed MPB composition (due to the reduced total abundance of cyanobacteria), but nearly doubled primary production in comparison with plots without bioturbation. Thus, the net effects of regenerative bioturbation were positive for MPB productivity, reinforcing the idea that this kind of bioturbation is an important biological force that enhances primary production in intertidal systems.
... Our experiments focused on organisms from rocky intertidal habitats in the northeast Pacific: crabs, whelks, abalone, mussels, and seaweed. We used the lined shore crab Pachygrapsus crassipes Randall, 1840, because it is a common 1 , major predator of small invertebrates, so shifts in its populations with climate change could have substantial effects on rocky shore communities (Armitage & Fong 2006, Cassone & Boulding 2006, Clemente et al. 2013, Lord & Barry 2017. We targeted 2 sets of interactions: (1) crabs (P. ...
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Recent marine climate change re - search has largely focused on the response of individual species to environmental changes in - cluding warming and acidification. The response of communities, driven by the direct effects of ocean change on individual species as well the cascade of indirect effects, has received far less study. We used several rocky intertidal species including crabs, whelks, juvenile abalone, and mussels to determine how feeding, growth, and interactions between species could be shifted by changing ocean conditions. Our 10 wk experiment revealed many complex outcomes which highlight the unpredictability of community-level responses. Contrary to our predictions, the largest impact of elevated CO2 was reduced crab feeding and survival, with a pH drop of 0.3 units. Surprisingly, whelks showed no response to higher temperatures or CO2 levels, while abalone shells grew 40 % less under high CO2 conditions. Massive non-consumptive effects of crabs on whelks showed how important indirect effects can be in deter mining climate change responses. Predictions of species outcomes that account solely for physiological responses to climate change do not consider the potentially large role of indirect effects due to species interactions. For strongly linked species (e.g. predator-prey or competitor relationships), the indirect effects of climate change are much less known than direct effects, but may be far more powerful in reshaping future marine com munities. © The authors and Monterey Bay Aquarium Research Institute 2017.
... In the northeast Pacific, there are three common crabs that occupy this habitat including two congeners of H. sanguineus: Hemigrapsus nudus Dana, Hemigrapsus oregonensis Dana, and Pachygrapsus crassipes Randall. P. crassipes is aggressive and opportunistic, consuming seaweed as well as snails, mussels, abalone, and urchins (Barry and Ehret 1993;Armitage and Fong 2006;Clemente et al. 2013;Lord and Barry, in review). This species outcompetes H. oregonensis (Willason 1981), which lives slightly lower in the intertidal zone and largely consumes algae but will also eat mud crabs (Knudsen 1964;Jordan 1989) and can outcompete juvenile green and Dungeness crabs for shelter (Jensen et al. 2002;Visser et al. 2004). ...
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Asian shore crabs (Hemigrapsus sanguineus De Haan) are a dominant invasive species in the northwest Atlantic, where recent evidence suggests that they have been introduced multiple times from their native range of Japan and Korea. Despite favorable environmental conditions and a high level of shipping traffic across the Pacific, this crab has not become established in the northeast Pacific. This may be due to interactions with the native shore crab assemblage, which previous studies have found to outcompete juvenile H. sanguineus for shelter. The present study used a combination of feeding and behavioral experiments to quantify the feeding behavior of native crabs (Pachygrapsus crassipes, Hemigrapsus nudus, Hemigrapsus oregonensis) with and without the presence of H. sanguineus. H. sanguineus ate more than the native crabs combined at both 7 and 13 °C. Crabs in the mixed treatment which included all four species ate less than expected, indicating that at least one species reduced its feeding rate. However, time-lapse photography revealed similar amounts of time feeding in both treatments for all 4 species, suggesting that there were changes in feeding intensity when all crabs were together. The high feeding rates but lack of a competitive advantage for H. sanguineus make it an unlikely but potentially impactful invader in the northeast Pacific.
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The California horn snail, Cerithideopsis californica, and the shore crabs, Pachygrapsus crassipes and Hemigrapsus oregonensis, compete for epibenthic microalgae, but the crabs also eat snails. Such intraguild predation is common in nature, despite models predicting instability. Using a series of manipulations and field surveys, we examined intraguild predation from several angles, including the effects of stage-dependent predation along with direct consumptive and nonconsumptive predator effects on intraguild prey. In the laboratory, we found that crabs fed on macroalgae, snail eggs, and snails, and the size of consumed snails increased with predator crab size. In field experiments, snails grew less in the presence of crabs partially because snails behaved differently and were buried in the sediment (nonconsumptive effects). Consistent with these results, crab and snail abundances were negatively correlated in three field surveys conducted at three different spatial scales in estuaries of California, Baja California, and Baja California Sur: (1) among 61 sites spanning multiple habitat types in three estuaries, (2) among the habitats of 13 estuaries, and (3) among 34 tidal creek sites in one estuary. These results indicate that shore crabs are intraguild predators on California horn snails that affect snail populations via predation and by influencing snail behavior and performance.
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Uca pugnax excavates burrows in Spartina alterniflora-dominated salt marshes. In tall-form S. alterniflora stands at intermediate tidal heights, experimental reduction of fiddler crab density for a single growing season decreased aboveground production by 47% and increased root mat density by 35%. On the lower intertidal soft edge of the marsh and in the high intertidal, dense root mat, short-form S. alterniflora zone, reduction of crab density did not have as marked an effect on cordgrass production. Crab burrows increase soil drainage, soil oxidation-reduction potential, and in situ decomposition of belowground plant debris. The relationship between fiddler crabs and S. alterniflora represents a facultative mutualism. On the seaward edge of marshes in soft sediment that would not otherwise support burrow structures, the roots, rhizomes, and debris of S. alterniflora provide structural support and consequently facilitate burrowing. At intermediate tidal heights, intensive burrowing activity increases aboveground grass production and prevents the establishment of a dense root mat, while crabs are largely precluded from burrowing in the dense root mat, short-form S. alterniflora stands at higher tidal heights. -from Author
Predation and disturbance have been well studied in intertidal communities. However, the impact of vertebrate predators, particularly waterfowl, has been only infrequently determined in long-term intertidal studies. Using predator exclusion cages and simulated abiotic disturbance, I studied the direct and indirect effects of predation by Common Eiders (Somateria mollissima) on an intertidal mussel bed community in Passa-maquoddy Bay, New Brunswick, Canada. Eiders fed heavily on blue mussels (Mytilus edulis), the dominant invertebrate present, reducing their abundance in "+duck" (exposed to predation) relative to "-duck" (exclosure) plots by nearly 50% within eight months of initiation of the experiment. Based on counts and estimated consumption rates, eiders appeared to be responsible for most predation observed in this system. Although ducks fed heavily on the dominant intertidal invertebrate, they had little effect on species diversity or richness in the community. However, relative abundance of the most common invertebrates did vary. In particular, exclusion of eiders led to an increase in abundance of dogwhelks (Nucella lapillus) one year into the experiment. Whelks, in turn, fed heavily on mussels under cages and obscured the longer-term effect of ducks in the system. This indirect effect appears to be an example of asymmetric exploitation competition, with ducks influencing the food supply of whelks, but whelks having little effect on ducks. When ducks were excluded, whelks were released from this competition and acted as compensating predators. No other indirect effects developed following duck exclusion, presumably due to the increased whelk effect. Disturbance, in contrast, did lead to an increase in species diversity, which later returned to predisturbance levels as the community recovered. Predation delayed the recovery of disturbed sites, because ducks began feeding in these plots before mussel abundance had completely rebounded. Disturbance, while initially deterring predation somewhat, ultimately allowed the effects of predators to persist longer. This interaction of predation and disturbance resulted from compensatory growth of mussels under exclosures in disturbed sites. Mussels protected from eider predation grew quickly after disturbance and rapidly became larger than the preferred prey for whelks. As a consequence, whelks did not feed as heavily on mussels in disturbed sites as in undisturbed sites, where more mussels were of the preferred size, and the observed effect of ducks on mussel biomass persisted. This experiment demonstrates that eiders were significant, and, probably, keystone predators in this system. Eiders directly reduced abundance of mussels, thereby indirectly increasing whelk density in plots where ducks were excluded. Compensatory predation by whelks following duck exclusion blocked other potential indirect effects, thus preventing eiders from having more widespread effects in the system. Such compensatory predation may act to stabilize communities and evidently can occur in relatively simple systems as well as the more species-rich communities with which it is usually associated.
A HPLC protocol is described which permits analysis of the pigment composition of coastal and intertidal sediments. The HPLC technique has been extensively applied to phytoplankton pigments for fifteen years. Papers dealing with living benthic microphytic populations have been more rare, due to complexity existing in the sediment surface, where pigments (chlorophylls and carotenoids) from several taxonomic groups of microalgae are mixed with degraded pigments from senescent phytoplankton cells and macrophytic material. The present paper describes in detail a protocol adapted from Kraay et al. (1992), which allows a good discrimination of photosynthetic pigments of microphytobenthos communities. The following procedure is recommended. Sediment samples are collected using hand-held cores, during emersion of sites in intertidal zones or by scuba-diving in submerged zones. Cores are sectioned in slices of 5 mm, weighed and lyophilized. Subsamples of 0.3-3 g are extracted in 95% methanol buffered with ammonium acetate, during 10-20 min in the dark, at 5 °C; the extract is then filtered with the Sweenex system with GF/F Whatman filters. Fluorescence excitation is performed at 430 nm and emission is detected beyond 580 nm. Absorbance is detected simultaneously at 436 and 450 nm. A binary solvent system is employed to separate the pigments. Standards for chlorophyll a, chlorophyll b and β- carotene were obtained from Sigma; chlorophyllide a was prepared from thallus of the Phaeophyta Halopteris; pheophytins a and b and a major pheophorbide a were prepared by acidification (HCl 0.4 N, final molarity 3 10-3 M); chlorophylls c1 + c2 and several carotenoids were also obtained from Halopteris. The remaining carotenoids were identified by comparison with published chromatograms. Calibration of standards was done spectrophotometrically in acetone or other solvents, subject to the existence of published extinction coefficients. Pigments were dried and redissolved in methanol in order to calculate the HPLC factor. A table of the coefficients used is given. Five chromatograms of three different intertidal stations from the Tagus estuary are presented as examples; 29 pigments were identified. Chromatograms obtained in fluorescence and absorbance at 436 nm and 450 nm from the same sample are shown to permit comparison between performances of the different types of detection. All chlorophylls, active or degraded, are distinguished in fluorescence. Some of them appear clearly in absorbance at 436 nm, while 450 nm is the best wavelength for identifying carotenoids. Pigment composition of sediment samples is related to microscopic species identification and cell abundance. Chromatograms obtained with absorbance at 450 nm from a mud sample in a salt-marsh showed high levels of violaxanthin, lutein (due to the abundance of plant detritus), zeaxanthin (corresponding to a population of 20% cyanobacteria), and neoxanthin, diadinoxanthin and chlorophyll b (matching the abundance of euglenophytes: 10%). A sample from a sandy station presented high levels of chlorophylls c1 and c2, neofucoxanthin and diatoxanthin; cell countings confirmed a major abundance of diatoms (99%) with a dominance of Cylindrotheca closterium. A mud sample from a low-tide station exhibited a great variety of pheophorbides, which is related to the high grazing pressure at this site. Although no method exists that is perfect for all types of pigments, the one described showed a good capability for analysing the taxonomic composition of microphytobenthos communities.
In estuarine and coastal soft-sediment systems, the role that predators play in structuring communities appears to be variable. Attributes of a particular predator that influence its role in structuring the community include: the rate of prey consumption; the behaviour, morphology and mobility of the predator; and, in soft-sediment communities, sediment disturbances associated with feeding. Reviews of field experiments designed to assess the role of predators in influencing the structure and function of soft-sediment communities have concluded that many of the predators are generalists and there is usually a lack of competitive exclusion. Thus predation structures communities by many complex and indirect interactions that are often difficult to predict and generalize. Variations in the apparent strength and role of predation in structuring benthic communities may depend on a variety of ecosystem characteristics and/or aspects of study design. In this paper, I consider whether we have been conducting our experiments at the appropriate scales. Five case studies from Manukau Harbour (New Zealand) illustrate how small changes in the spatial scale can affect results, due to predator perceptions and prey mobility. The results of these studies demonstrate the need to identify scales at which predator effects are likely to be important and to fit experiments within the dynamics and heterogeneity of the system being studied. To do this, we need basic information on the natural history, behaviour and spatial and temporal variability of both predators and prey communities. We also need to be specific about scales of measurement when matching theoretical predictions to field observations/experiments. Finally, to enhance our ability to generalize from specific studies, we need to gather data that will enable us to both predict and test the importance of predation over a range of spatial and temporal scales.
A review of the literature relating population variability to reproductive mode in soft-sediment marine invertebrates does not provide compelling support for the hypothesis that planktotrophic species have more variable adult population sizes. Abundant evidence exists to show that post-settlement mortality can operate to regulate density of invertebrates in soft sediments. Predation by large epibenthic consumers often appears to control infaunal abundances in shallow-water, unvegetated habitats. Density-dependent inhibition of recruitment by adult deposit-feeders and infaunal predators also has been commonly documented, in most cases operating after settlement. Even where inhibitory adult-juvenile interactions regulate settlement, the existence of this process is inconsistent with recruitment limitation in its strongest form because larval availability is not limiting. Physical disturbance of the sea floor kills small infaunal invertebrates, although the full implications of post-settlement sedimentary dynamics to pattern generation in soft sediments have not been determined. Although food supply typically regulates benthic secondary production by affecting individual growth and fecundity of adult invertebrates, some evidence also implies density-dependent starvation of recent settlers. Recruitment limitation is not the dominant determinant of spatial and temporal pattern in this system. -from Authors
In many marine soft-bottom systems, natant predators disturb the bottom while foraging for infaunal prey. This study was designed to determine if the predatory fish Leiostomus xanthurus exerts its effect on meiofaunal prey primarily via predation (consumption) or via disturbance and if flow modifies this effect. Experiments were conducted in a laboratory flume that allowed for both the addition and removal of fish and the mimicking of predator-induced disturbance, while controlling flow. Prey species differed with respect to the relative effects of predation and predator-induced disturbance. Disturbance accounted for 30-55% of the predator-associated decreases in copepods and foraminiferans, while disturbance accounted for virtually all of the nematode mortality. The latter was significant only when flow was absent. For other prey taxa, mortality was not influenced by flow; however, predator-induced drift (down-stream transport) of all meiofauna was significantly increased when flow was present. If experimental sites are not in equilibrium between upstream and downstream transport, then ignoring this drift could result in an overestimation of fish consumption on meiofauna by as much as 50%. Thus, the mechanisms by which fish predators influence meiofauna prey included direct consumption, disturbance-induced mortality, and enhanced water-column transport.
Ecosystem ecologists traditionally have focused their attention on direct interactions among species, particularly those interactions that control flows of energy and materials among trophic levels. Emerging evidence suggests that indirect interactions may be more important than direct ones in determining ecosystem patterns and processes. Here I review indirect effects of ungulates on nutrient cycling, net primary production, and disturbance regimes in terrestrial ecosystems. Ungulates influence the nitrogen (N) cycle by changing litter quality, thereby affecting conditions for N mineralization, and by adding readily available N to upper levels of the soil in urine and feces. As a result of these additions, natural heterogeneity in the spatial distribution of N within landscapes is amplified by ungulate selection of habitats and patches. The magnitude of returns of plant N to the soil in urine and feces is a function of animal body mass and characteristics of the diet, particularly N content and levels of tannin. Effects on N cycling can cascade throughout the ecosystem, and can stabilize or destabilize the composition of plant communities. Net primary production can increase or decline in response to ungulate grazing. The direction of this response depends on the intensity of grazing or browsing, the evolutionary history of the ecosystem, and the opportunity for regrowth. Opportunity for regrowth is determined by physiological and morphological characteristics of the plant as well as environmental conditions, particularly the extent and timing of moisture availability. Ungulates influence fire regimes by altering the quality and quantity of fuels available for combustion. In grasslands, ungulates often reduce the extent, frequency, and intensity of fires, while in shrublands and forests, their effects can increase the likelihood of crown fires, while reducing the likelihood of surface fires. I develop the case that the way that ungulates influence ecosystem process is contingent on historical context, in particular the long-term context provided by plant-animal coevolution and soil development and the short-term context created by climate and weather. I show that ungulates are important agents of change in ecosystems, acting to create spatial heterogeneity, modulate successional processes, and control the switching of ecosystems between alternative states.
Fish appeared to be of minor importance. The effects of shorebird predation were very different among the 3 California mudflats and depended upon subtle differences in sediment composition among sites. Shorebirds were most abundant at the muddiest site, and infaunal densities at this site were lowest in the winter when birds were present. Shorebirds were least abundant at the sandiest mudflat, and infaunal densities were highest in the winter when birds were present. Excluding birds had no demonstrable effect on infaunal densities. However, lined shorecrab Pachygrapsus crassipes, was present only at the sandiest mudflat and only in the summer. Exclusion of crabs at this site demonstrated that the crabs reduced infaunal densities during the summer when they are active on the mudflats.-from Author
Rates of mortality and transport of newly settled benthic invertebrates can be very high and potentially have profound effects on patterns of recruitment. We used the soft shell clam Mya arenaria as a model organism to compare the magnitude of rates of postlarval transport and predation on juveniles. We hypothesized that there is a critical size below which spatial variation in abundance of juvenile Mya is hydrodynamically controlled and above which abundance is mainly influenced by mortality, particularly due to epibenthic predators. To test this hypothesis, we examined transport and predation of early juvenile Mya in Barnstable Harbor, Massachusetts, USA. A caging experiment demonstrated that exclusion of epibenthic predators had a large impact on the density and size distri- bution of juvenile Mya within a few weeks of settlement and disproportionately affected juveniles of .2-mm shell length. Enclosure of the green crab Carcinus maenas changed the size distribution of Mya but did not significantly affect abundance. We attributed the effect of predator exclusion primarily to juvenile green crabs and fish. Current meter measurements suggested that tidal currents were strong enough to cause bedload transport of sediment and Mya. Measurement of gross and net rates of transport of Mya, using bedload traps and pans of defaunated sediment, confirmed that juveniles up to 5-mm shell length were routinely redistributed by tidal currents, particularly during spring tides. The number of postlarval Mya transported varied among sites and dates and was significantly related to ambient density of Mya and sediment flux. The loss of Mya during the caging experiment was compared to the rate of net flux of Mya due to transport during an intermediate tide. This comparison suggested that the loss due to predation was considerably larger than the flux due to transport for individuals .2 mm, but that rates of the two processes were more similar for individuals ,2 mm. These experiments indicate that, while predation is likely the ultimate factor controlling abundance and size of juvenile Mya at this site, transport often redistributes individuals, at least at the scale of a tidal flat (10s to 100s of m). Our understanding of recruitment variability of benthic marine invertebrates and its role in population and commu- nity dynamics has increased considerably over the last 20 years. Studies often have assumed that recruitment patterns reflect patterns of larval supply or settlement, although there has been increasing recognition of the importance of the ear- ly postsettlement period. Patterns set up at the time of set- tlement may have little effect on the distribution and abun- dance of adults because newly settled invertebrates usually 1