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LABORATORY INVESTIGATIONS OF FOOD SELECTION BY THE ASIANSHORE CRAB, HEMIGRAPSUS SANGUINEUS: ALGAL VERSUS ANIMALPREFERENCEAuthor(s): Diane J. Brousseau and Jenny A. BaglivoSource: Journal of Crustacean Biology, 25(1):130-134. 2005.Published By: The Crustacean SocietyDOI: http://dx.doi.org/10.1651/C-2530URL: http://www.bioone.org/doi/full/10.1651/C-2530

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JOURNAL OF CRUSTACEAN BIOLOGY, 25(1): 130–134, 2005

LABORATORY INVESTIGATIONS OF FOOD SELECTION BY THE ASIAN SHORE CRAB,

HEMIGRAPSUS SANGUINEUS: ALGAL VERSUS ANIMAL PREFERENCE

Diane J. Brousseau and Jenny A. Baglivo

(DJB, correspondence) Department of Biology, Fairfield University, Fairfield,

Connecticut 06430 U.S.A. (Brousseau@mail.fairfield.edu);

(JAB) Department of Mathematics, Boston College, Chestnut Hill, Massachusetts 024677 U.S.A.

A B S T R A C T

The Asian shore crab, Hemigrapsus sanguineus (De Haan, 1853), is an opportunistic omnivore with well-developed predatory tendencies

and a strong preference for animal food items over algae. In laboratory experiments, 71 percent (319/448) of the crabs given a choice

between macroalgae (Enteromorpha spp. and/or Chondrus crispus) and benthic invertebrates (Mytilus edulis and/or Semibalanus

balanoides) consumed animals only. There were no significant differences in food preference between sexes or between juvenile and adult

crabs. Relative abundance of food type, either algal or animal, in small food patches did not affect crab food preference. Crabs with

prolonged starvation periods (5-d), however, consumed both food types more often than those that had been starved for 1-d only.

Experiments to determine the effect of conspecifics on food selectivity showed that increased crab density leads to increased diet breadth,

suggesting that competition for food can alter food selection patterns of H. sanguineus. Results reported here and in previous studies

provide strong evidence that predation pressure exerted by H. sanguineus could play an important role in structuring the post-settlement

population dynamics of its invertebrate prey, possibly leading to population declines of commercial shellfish, especially blue mussels.

The introduction of exotic species to marine and coastalsystems throughout the world is recognized as a seriousbiological threat, with potentially harmful consequences forthe ecology of the invaded environments (Carlton, 1989;Carlton and Geller, 1993; Cohen and Carlton, 1998). Thereare cases in which dramatic changes in community structureand function have resulted when invaders have become thenumerically dominant species in the invaded community(Nichols et al., 1990; Lambert et al., 1992; Travis, 1993),but in general, the impacts of invaders on marinecommunities are not well understood. The recent invasionby the Asian shore crab, Hemigrapsus sanguineus, along theeast coast of the United States of America is one suchexample. Despite the rapid dispersal of this invader,relatively little is known about its effect on indigenousspecies populations, and there is still considerable un-certainty concerning the longterm ecological impact of thiscrab in non-native environments (Lohrer et al., 2000).

The Asian shore crab, a highly mobile species (Brousseauet al., 2002) with large reproductive output (McDermott,1998a) and planktonic larvae capable of wide dispersal(Epifanio et al., 1998), has been a remarkably successfulinvader and is now frequently the most abundant crab speciesin habitats into which it has been introduced (Lohrer andWhitlatch, 1997; Celestino and Hales, 1998; Ahl and Moss,1999; Gerard et al., 1999; Lohrer and Whitlatch, 2002). Itsability to feed on a variety of resident species includingmacroalgae, salt marsh grasses, and small invertebrates suchas amphipods, gastropods, bivalves, barnacles, and poly-chaetes (McDermott, 1991, 1999; Lohrer and Whitlatch,1997; Brousseau et al., 2000; Tyrell and Harris, 2000)suggests an important role for this invader in restructuringprey communities in non-native environments.

Reports of laboratory observations by numerous inves-tigators confirm that H. sanguineus readily consumes both

plant and animal food (McDermott, 1999; Gerard et al., 1999)and gut content analyses of wild-caught crabs suggest thatmacroalgae is its primary food (Lohrer and Whitlatch, 1997;McDermott, 1999; Tyrell and Harris, 2000). Food-preferencestudies have shown that H. sanguineus selects among variousspecies of macroalgae as well as among various bivalvespecies when given a choice (Brousseau et al., 2001). None ofthese studies, however, critically examines food preferencewhen both plant and animal food items are available, makingit impossible to assess with certainty the importance of eitherin the overall diet of this invader.

In this study we tested the following null hypotheses inthe laboratory:

Ho: When given a choice between macroalgae and animalfood items H. sanguineus shows no preference;

Ho: Local availability/abundance of food type does notinfluence food selection by H. sanguineus; and

Ho: Crab density does not influence food selection byH. sanguineus.

We used the macroalgae Chondrus crispus and Enter-omorpha spp. and the invertebrates Mytilus edulis andSemibalanus balanoides. All are abundant species in rockyintertidal habitats of Long Island Sound and known foods ofHemigrapsus sanguineus (McDermott, 1999; Brousseauet al., 2000; Bourdeau and O’Connor 2003). Informationabout the feeding preferences of H. sanguineus undercontrolled conditions provides a clearer understanding of thedynamics of Asian shore crab feeding and could provideinsights into the potential impact of this invader on nativeintertidal biota of the northeast coast.

MATERIALS AND METHODS

Hemigrapsus sanguineus used in experiments were collected by hand froman interidal area at the mouth of Black Rock Harbor, Bridgeport,

130

Connecticut, U.S.A. (41808.89N lat.; 73813.59W long.). Macroalgae,Chondrus crispus and Enteromorpha spp., were harvested from rocks atLighthouse Point, New Haven, Connecticut, U.S.A. (41815.09N lat.;72854.39W long.), and small mussels, Mytilus edulis (� 10 mm SL), werecollected at Cape Hedge Beach, Rockport, Massachusetts, U.S.A.(42837.99N lat.; 70836.79W long.). Barnacle larvae (Semibalanus bal-anoides) were set on plexiglass ‘‘tiles’’ covered with 3M� nonskid tape inan outdoor seawater table at the National Marine Fisheries ServiceLaboratory (NMFS) in Milford, Connecticut, in late March 2002. Smallpost-settlement barnacles (1.0–1.5 mm maximum diameter) were used inthe preference experiments. Macroalgal (Chondrus crispus and Enter-omorpha spp.) and mussel (Mytilus edulis) ‘‘tiles’’ were prepared byepoxying the macroalgae and mussels to 4 cm 3 9 cm pieces of plexiglass,so that the entire surface of each ‘‘tile’’ was covered by one food type.These single-species ‘‘tiles’’ were attached to the four sides of a concretepaver (14 cm 3 14 cm 3 5.5 cm) creating small food patches which werethen placed in the middle of perforated plastic microcosms (30.5 cm 325.4 cm 3 7.6 cm) containing the experimental crab(s).

In the food preference experiments, crab(s) were given either two (oneanimal, one alga) or four (two animal, two algae) food choices. Parallelexperiments in which crabs were given a single food choice were also run.All other experimental conditions (age-group, crab density) in the one-foodchoice trials were the same. Crab response to differences in local foodabundance was tested by varying the relative number of algal and animaltiles in the two-food choice experiments (1:3, 2:2, 3:1). To test the effect ofcrab density on food selection patterns, four-food choice experiments with1, 4, 8, or 16 crabs per trial were used.

At the end of a 24-h trial, stomachs were removed from live crabs, andtheir contents were removed and examined under a dissecting microscope todetermine food type. Partially digested mussels and small shell fragmentsmade up the animal portion of the gut contents, and pieces of macroalgaemade up the plant portion. Indirect evidence of feeding, such as algaegrazed from ‘‘tiles,’’ the presence of shell fragments, and algal fecal pelletsin the containers was also noted. Crabs were placed in one of fourselectivity categories based on the results of the stomach-content analyses:algae only, animal only, both algae and animal, or neither (empty stomach).At the end of each trial, all food types were still available; therefore, foodlimitation did not influence crab food selection.

Hemigrapsus sanguineus used in the experiments were held in an indoorflow-through seawater table and starved for either one or five days beforetesting. Only crabs whose chelae and other appendages were intact wereused. Recently molted (soft) crabs were not included in experimentsbecause they do not feed. The carapace width (CW) and sex of each crabwas determined. Crabs with CW , 12.0 mm were considered juveniles(McDermott, 1998a). A total of 604 crabs (293 males; 236 females; 75juveniles), CW ¼ 16.4 6 4.1 mm, were used in this study. Each crab wasused in one feeding trial only. Experiments were conducted from Maythrough October within a temperature range of 15–248C and at a salinity of19.5–23.5 psu, conditions typical of the ambient environment during thetime when Asian shore crabs are active in Long Island Sound.

All statistical tests use Pearson’s v2 statistic and the 5% significancelevel. P-values were computed using permutation methods.

RESULTS

Seventy-nine percent (475/604) of the crabs tested con-sumed food during the course of these experiments; the resthad empty stomachs when dissected. Of those crabs offereda choice of food items, 71 percent (319/448) ate animal food

only. In experiments involving adult crabs, food selectionpatterns did not vary with sex. Sixty-two percent (49/79) ofthe crabs offered animal food only consumed some foodcompared to 38 percent (29/77) of those offered algae only(Table 1). The only food items found in crab stomachs werethe ones they were fed (Chondrus crispus, Enteromorphaspp., Mytilus edulis or Semibalanus balanoides). At the endof each trial, small fecal pellets were found in containersof all crabs that had consumed macroalgae, and crushedshells were present in containers of barnacle- and mussel-eating crabs.

Food Selection and Prey Abundance

Results of food-selection experiments pairing Entermorphaspp. with mussels under varying conditions of local foodabundance using single adult or juvenile crabs starved for1 d prior to testing are shown in Table 2, Experiment A.Initial permutation tests revealed that food-selection dis-tributions did not vary by age group (adult, juvenile) or byrelative patch size of the two food types offered (1:3, 2:2,3:1). In all cases crabs preferentially consumed mussels.Ninety percent of crabs in these choice experiments atesome food, compared to 93 percent (26/28) of crabs offeredone type of food only. In trials using adult crabs with5-d starvation periods, crabs again overwhelmingly selectedmussels over Enteromorpha spp., regardless of the relativeabundance of food type (Table 2, Experiment B). All crabsin these trials consumed some food, compared to 91 percentof crabs in the one-food choice experiments. Using per-mutation analysis, the food-choice distributions for 1-d and5-d starvation are significantly different (chi-squarestatistic ¼ 10.06, permutation P value ¼ 0.014). Thedifferences were in the ‘‘neither’’ category because all crabsthat were starved for five days consumed some food.

Food Selection and Predator Density

Results of experiments using varying crab density and fourfood choices (Enteromorpha spp., Mytilus edulis, Chondruscrispus, Semibalanus balanoides) are given in Table 3.Using permutation analysis, the four-food selection distri-butions are significantly different (chi square statistic ¼55.95, permutation P value¼0.0002). At crab densities 1, 4,and 8, crabs selected animal food (either mussels orbarnacles) over algae. Of those crabs preferring animalfood, 90 percent (160/178) ate mussels only, whereas nonewere found with only barnacle shells in their stomachs. Atcrab density 16, more crabs consumed both or neither thanconsumed animal food only, suggesting that increased

Table 1. Numbers of Hemigrapsus sanguineus stomachs found with andwithout food in all experimental trials.

ExperimentCrab stomachs

with foodEmpty crabstomachs Total

Choice (2-food or 4-food) 397 51 448No Choice (animal) 49 30 79No Choice (algae) 29 48 77

Total 475 129 604

Table 2. Numbers of Hemigrapsus sanguineus found in each foodselectivity category in experimental trials with two food choices(Enteromorpha spp. and Mytilus edulis) under varying conditions ofrelative food abundance (1:3, 2:2, 3:1): A) adult and juvenile crabs, 1-dstarvation, crab density 1; B) adult crabs, 5-d starvation, crab density 1.Numbers in parentheses are proportions.

Experiment

Food Selectivity Category

Animal only Algae only Animal & algae Neither Total

A 85 (0.77) 2 (0.02) 12 (0.11) 11 (0.10) 110 (1.00)B 36 (0.75) 0 (0.00) 12 (0.25) 0 (0.00) 48 (1.00)

131BROUSSEAU AND BAGLIVO: FOOD SELECTION BY THE ASIAN SHORE CRAB

competition for food resources can alter food-selectionpatterns of H. sanguineus.

DISCUSSION

Many crabs rely on plant material for a significant part oftheir nutrition, yet it appears that few crabs are strictherbivores (for review see, Wolcott and O’Connor, 1992).

Macroalgae is a readily available food resource alongrocky shores of temperate and tropical regions but oftenpresents nutritional challenges for consumers. It is difficultto harvest and digest and can even be toxic (Duffy and Hay,1990; Targett et al., 1992). Macroalgae is also known to beof poor nutritional quality (Mattson, 1980), making it aninadequate food source for many herbivorous animals.Eating a varied diet that includes animal material as well asplants may ensure that crabs obtain the nutrients they need(nitrogen, vitamins, and fatty acids) but are not availablefrom plant material alone. Ultimately, food choice/prefer-ence among most brachyuran crabs is influenced bya complex array of species-specific factors including theabundance and availability of the resource in the environ-ment (Kennish et al., 1996), prey profitability (Mascaro andSeed, 2000a; 2000b), food palatability and digestability(Coen, 1988), the time available for foraging (Choy, 1986;Depledge, 1989), the need for camouflage (Kilar and Lou,1984), and predator avoidance behaviors (Laughlin, 1982;Alexander, 1986).

Studies of Hemigrapsus sanguineus in introducedenvironments suggest that, like most crabs, it is omnivorous,and stomach-content analyses done on wild-caught crabsshow that algae makes up at least 50% of its natural diet(Lohrer and Whitlatch, 1997; McDermott, 1999; Tyrell andHarris, 2000). Gut-content analyses, however, can bemisleading. Animal food in an advanced stage of digestionis often unidentifiable, and there is always the possibilitythat some food items may have been incidentally ingested.Shelled prey such as bivalve molluscs are often removedfrom the shell by edge chipping or umbo attack beforeingestion (Brousseau et al., 2000), making prey identifica-tion very difficult. Also, the amounts of algae in the diet ofH. sanguineus may simply reflect opportunistic consump-tion of plant material at a time when animal alternatives arenot available.

Food-preference studies conducted in the laboratory haveshown that H. sanguineus will consume both algae andanimal food (Brousseau et al., 2000, 2001), but showspreferences for particular species of algae (Brousseau et al.,2000) and will select among different species and sizes ofbivalve prey (Brousseau et al., 2001). In the present study,Asian crabs overwhelmingly preferred animal prey tomacroalgae (71 percent of the test crabs ate animal foodonly), providing evidence that despite the diversity in theirdiet and feeding habits, Asian shore crabs are opportunisticomnivores with well-developed predatory tendencies anda strong preference for animal food.

Diet differences between sexes and in different size groupshave been reported for several species of crabs (Ropes, 1968;Paul, 1981; Jewett and Feder, 1982; Choy, 1986; Rangeleyand Thomas, 1987; Mascaro and Seed, 2001). In previousstudies, we found differences in consumption rates and size

preferences between male and female Asian crabs feeding onbivalves (Brousseau et al., 2001). We attributed this to sexualdifferences in chela size and shape (McDermott, 1998b),because studies have shown that brachyuran crabs possessingstronger and larger chelae are able to exploit a wider sizerange of hard-bodied prey (Lee and Seed, 1992). That findingalso led us to hypothesize that male crabs would exhibita greater preference for shelled animal prey, especiallybivalves, than either female or juvenile crabs. In this study,however, no differences in food selection (based on thepresence/absence of food types in crab stomachs) eitherbetween sexes or between adult and juvenile crabs werefound, suggesting that little if any partitioning of theseresources occurs among individuals within the population.

Despite the importance of food availability and abun-dance in influencing the diets of marine brachyurans, severalfield studies have shown that food availability, measuredboth as prey numbers and biomass, is not solely responsiblefor diet composition. Selective feeding behavior is importantas well (Paul, 1981; Choy, 1986; Wieczorek and Hooper,1995; Kennish et al., 1996). In the laboratory, the Asiancrab was very selective in its feeding, most often choosinganimal food items even when the likelihood of encounteringmacroalgae in small food patches was three times as great asthat of encountering animal prey. This finding supports theargument that preference for particular food items may bea more important factor than food abundance in determiningcrab diets, at least in situations where the food patches areclose together and small in scale.

Asian shore crabs exhibit less dietary selectivity,however, as the density of conspecifics feeding on thesmall food patch increases. Also, crab feeding diminishes ascrab density increases. These results are consistent with thestudy of Clark et al. (2000), which reports lowered preyconsumption rates among blue crabs (Callinectes sapidus) athigh densities because of increased interactions amongindividuals competing for limited food supplies. Asian shorecrabs seek shelter under intertidal rocks and debris duringlow tide, often occurring at densities as high as 150individuals per m2 (Brousseau, unpubl.). With the incomingtide, they leave the cover of shelter to forage for food. Whenforaging, they often travel long distances from the sheltersite, rarely returning to the same shelter on the next low tide(Brousseau et al., 2002). Such high mobility may be anadaptive behavior enhancing dispersal by allowing them tomove among prey patches, thereby reducing intraspecificinteractions that could interfere with foraging success.

Table 3. Numbers of Hemigrapsus sanguineus adults starved for 1d found in each food selectivity category in experimental trials with fourfood choices (Enteromorpha spp., Mytilus edulis, Chondrus crispus, andSemibalanus balanoides) using varying crab density. Numbers inparentheses are proportions.

Crabdensity

Food Selectivity Category

Animal only Algae only Animal & algae Neither Total

1 37 (0.80) 0 (0.00) 8 (0.17) 1 (0.02) 46 (1.00)4 43 (0.68) 1 (0.02) 2 (0.03) 17 (0.27) 63 (1.00)8 94 (0.78) 2 (0.02) 17 (0.14) 7 (0.06) 120 (1.00)

16 24 (0.39) 0 (0.00) 22 (0.36) 15 (0.25) 61 (1.00)

Total 198 (0.68) 3 (0.01) 49 (0.17) 40 (0.14) 290 (1.00)

132 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 25, NO. 1, 2005

Although to what extent feeding observations made in thelaboratory are representative of field events is alwaysuncertain, they are a critical step in understanding detailsof the complex interactions that take place between predatorand prey. As such, they can serve as a basis for proposing,designing, and interpreting results of field investigations andquickly provide important data to ecologists, resourcemanagers, and policy makers concerned with the effects ofnonindigenous species. The argument that in Long IslandSound H. sanguineus may be an even more importantbivalve predator than the voracious green crab Carcinusmaenas because of its exceptional densities (Lohrer andWhitlatch, 2002) is reinforced by our evidence of strongmussel preference in this invader. The Asian shore crab islikely to play an important role in structuring the post-settlement population dynamics of its invertebrate prey. Forspecies of commercial bivalves, such as blue mussels, thiscould mean unforeseen economic impact to shellfishaquaculturists who rely on supplies of natural seed forgrowout (Brousseau and Filipowicz, 2001).

ACKNOWLEDGEMENTS

We thank the staff of the National Marine Fisheries Laboratory in Milford,Connecticut, especially A. Calabrese and R. Goldberg, for providingsupport and laboratory space for this study. This study was supported inpart by a small grant from the Connecticut Seagrant-CP development fundand regional/multiprogram initiative and funds from the DeCamp Chair inHealth Sciences from Fairfield University.

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RECEIVED: 3 July 2004.ACCEPTED: 29 September 2004.

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