Dietary Preference and Digestive Enzyme Activities as Indicators of Trophic Resource Utilization by Six Species of Crab

2005·4.5 MB·

by  Danielle Johnston

Reference: //. Hull. 2(18: 36-46. (February 2005)
2005 Marine Biological Laboratory
Preference and Enzyme Activities as
Dietary Digestive
Indicators of Trophic Resource Utilization by Six
of Crab
Species
DANIELLE JOHNSTON* AND JOEL FREEMAN*
School ofAtjiiaciilti/re, Tasmania!! Ai/naciiltiire and Fisheries Institute, University ofTasmania,
Launceston, Tasmania, Australia 7250
Abstract. The digestive physiology and stomach contents (0.73 0.12 units mg '), to digest the protein in its animal
of six crab species from a variety of habitats were investi- prey. Each species of crab studied had a complex suite of
gated to provide an indication of their digestive capability digestive enzymes, the relative activities ofwhich reflected
and dietary preferences. Stomach contents varied between individual and very different species-specific dietary niches.
species, but the key enzymes present were generally con-
sistent with the types of dietary material being ingested. Introduction
Nectocarcimis intef>rifons (red rock crab) consumed large
quantities of seagrass and had high cellulase activity Crabs live in a variety of habitats with varying distribu-
(0.02 0.004 units mg~') to digest the constituent cellu- tions and abundance of dietary items, so stomach contents
lose. Petrolistheselongatus (porcelain crab) ingested brown typically include a diverse range of prey (Paul, 1981: Wil-
and green phytoplankton and algae and had considerable liams, 1982; Wear and Haddon, 1987). The variation in
laminarinase (0.35 0.08 units mg"1) and /3-glucosidase stomach contents between species from different habitats
(0.025 0.005 units mg~') activities to digest the lamina- may reflect an opportunistic or versatile feeding nature
rin in its diet. Lcptugrapsus vnriegatus (omnivorous swift- where food items are consumed in proportion to their abun-
footed shore crab) had high activities of protease (1.2 dance in the surrounding habitat (Choy, 1986; Wolcott and
0.02 units mg~'), a-glucosidase, and a-amylase and ap- Nancy, 1992), or it may indicate that crabs actively select
peared well equipped to utilize both dietary protein and habitat based on the presence of suitable food. Although
carbohydrate. Stomach contents in Nectocarcimis tnhercit- most studies on the feeding habits of decapod crustaceans
losus (velvet crab) and Carcinus maenas (green crab) also are based on the observation of stomach contents, stomach
suggest that these species are omnivorous. N. tuberculosus contents do not provide any information on dietary prefer-
had high cellulase and chitinase for digesting the cellulose ence orthe suitability ofthe diet for maintaining the animal.
in plants and the chitin in invertebrate shells respectively. C. Similarly, stomach contents cannot help discriminate be-
maenas had intermediate digestive enzyme levels and may tween generalist and targeted feeding strategies. Digestive
employ more of a generalist feeding strategy than other enzymes however, may be a complementary tool useful for
species. Plagusia chabrus (speedy crab) is carnivorous, determining which dietary components are mosteffectively
consuming encrusting bryozoans, hydroids, crustaceans, metabolized (Brethes et al., 1994). By understanding the
and fish. It has high protease activity, particularly trypsin digestion and assimilation of specific dietary components,
we could identify the type of prey that the animals prefer
Received 20 M;,, .1004; accepted 6 October 2004 and those that they are best equipped todigest. Forexample,
*To whom cone.,"indence should be addressed, at WA Marine Re- carnivorous species exhibit a wide range and high activity
search Laboratories. !" < Box 20. North Beach, Western Australia 6920. of proteolytic enzymes to digest their high-protein diet,
Australia. E-mail: djohn 'isliAva.gov.an whereas herbivores and omnivores that ingest large amounts
Sci#enCcuersr,enTtheadAdursetsrsa:liPahnotNoabtiioone.nielrgUentiivcesr,sitRye.seAaurscthralSicahnooClapiotfalBiToelrorgiitcoarly of carbohydrates possess highly active carbohydrases. Pre-
0200. Australia. vious studies on the enzymatic system of decapod crusta-
36
DIGESTIVE ENZYMES IN SIX CRABS 37
ceans have demonstrated this link between dietcomposition stomach contents. Nectocarcinus tuberciilosus was col-
and the presence of digestive enzymes (Kristensen, 1972; lected at night by scuba at Recherche Bay (430'S,
Lee etal., 1984; Johnston and Yellowlees, 1998; Hidalgoet 14724'E) or in baited traps set overnight at Georges Bay
nl., 1999; Figueiredo et al., 2001 ). (4119'S, 14815'E). Petrolitheselongaius)andLeptograp-
The measurement ofdigestive enzyme synthesis is a tool sus variegatus) were collected by hand from the intertidal
commonly used to study trophic relationships in many in- zone at Little Beach (4131'S. 14816'E) and in the Der-
vertebrate groups (McClintock et al.. 1991; Brethes et al., went River (4253'S, 14719'E) respectively. Carcinus
1994). However, thesestudieshave typically been limited to maenas) and Nectocarcinus integrifons) were collected in
afew enzymes within one species (McClintock ettil., 1991; baited traps set overnight at Georges Bay (4119'S,
Brethes et al.. 1994; Johnston and Yellowlees, 1998; 14815'E). Plagusia chabrns) was collected at night from
Figueiredo et al., 2001) orjust one enzyme in a number of the research vessel RV Challenger, using baited traps at
species (Galgani et al., 1984). Our knowledge of crab di- Great Oyster Bay (4215'S, 14816'E). Following collec-
gestive enzyme physiology is also limited. The few en- tion,eachcrab wasplacedon icefor 10-20min, afterwhich
zymes that have been documented include trypsin and car- its carapace, digestive gland, and stomach were removed.
boxypeptidases A and B in Callinectes sapidus (blue crab) Digestiveglands were frozen in liquid nitrogen and stored at
(Dendinger, 1987; DendingerandO'Connor, 1990); a-amy- 20 C; stomachs were fixed in 10% formalin in 35 ppt
lase in Carcinus maenas (green crab) (Blandamer and seawater.
Beechy, 1964); and a-glucosidase in Cancer borealis
(jonah crab). Cancer irroratus (rock crab) (Brun and Woj- Stomach content verification
at(ho1,w9i9c01z)9,91u)1s.9e7d6F)ra,cotmaivniadtydCi.oeftsalarapymiipdneuarsrsip(enbcaltsuieeveac,sraNabno)ri(mnMadcinCclaatinonrdtooJcfoknteheset ethmopEstaeyc,hwi2tsh=toam2a5scc%ho,rew3ao=sf 53vi0st%uo,al54lyw=ears7es5ed%si,ssesd5ec=ftoerd1.0fuT0lh%lenefsuclsoln),t(1eannt=ds
autbiilliiztey tohfeLbiroocawrncianlugsaepufbreeqrue(nvtellyveftousnwdimimniintgs sctroamba)cht.o were examined usingdissectingandcompound microscopes
Brethes et al. (1994) used laminarinase and other enzymes and identified to the lowest possible taxonomic grouping by
as an index of trophic resource utilization by Chionectes using appropriate keys. The stomachs of specimens of L.
variegatus were not collected in this study, preventing a
opilio (snow crab).
The present study investigates stomach contents and di- stomach content analysis.
gestive enzyme activities in six species ofcrab that inhabit
a variety of habitats and specialized dietary niches. We Enzyme analysis
examined the following crabs: Nectocarcinus integrifons Individual digestive glands were thawed and homoge-
mM mM
(Latreille) (red rock crab), Petrolisthes elongatits (Milne nizedfor5 mininchilled 100 Tris, 20 NaCl buffer,
Edwards) (porcelain crab), Leptograpsus rariegatus pH 7.0, using an UltraTurrax homogenizer. The homoge-
(Fabricius) (swift-footed shore crab), Carcinus maenas nate was centrifuged at 968 g, and the supernatant contain-
(Linnaeus) (green crab), Plagusia chabrns (Linnaeus) ing digestive gland extract was transferred into microfuge
(speedy crab), and Nectocarcinus tuberculosus (Milne Ed- tubes and stored at -20 C.
wards) (velvet crab). Theobjectivesofthis study were (1) to Detailed procedures for enzyme assays are discussed
determine dietary preferences for each of the six crab spe- elsewhere (Johnston, 2003). Briefly, total protease activity
cies, using stomach content analysis, (2) to quantify the was measured using the casein hydrolysis method (Kunitz,
activities of a range of digestive proteases and carbohy- 1947) as modified by Walter (1984) using tyrosine as the
drases in each crab species to determine how various food standard. Trypsin activity was measured using/V-a-benzoyl-
sources available to the crabs are utilized, and (3) to use arginine-p-nitroanalide (BAPNA) as substrate using the
dietary preferences and substrate utilization to help identify molar absorption coefficient, e, of 9300 M~' cm"1 for
the position ofthecrabs inthetrophic networkofthecoastal p-nitroanaline (Stone et al., 1991). a-Amylase activity was
environment. determined using the method of Biesiot and Capuzzo
(1990), modified after Bernfeld (1955). a-Glucosidase.
Materials and Methods j3-glucosidase, and chitinase activities were measured using
Collection the substrates p-nitrophenyl a-D-glucopyranoside, p-nitro-
phenyl /3-o-glucopyranoside. and p-nitrophenol A'-acetyl
Crabs were collected from a number ofsites in Tasmania /3-D-glucosaminide, respectively (Erlanger et al., 1961).
by hand or by trapping. Collection was standardized to Cellulase activity was measured using the substrate sodium
adults ofeach species during theirperiods ofactive feeding. carboxymethyl cellulose (CM-cellulose). Laminarinase ac-
When baited traps were used, the bait was positioned such tivity was measured using laminarin as the substrate.
that crabs could not ingest itand thereby bias the analysisof We defined one enzyme unit (U) as the amount of en-
38 D. JOHNSTON AND J. FREEMAN
zyme that catalyzed the release of 1 /u,mol of product per (species) centroids were plotted using the unstandardized
minute, which we calculated using the appropriate molar canonical discriminant functions evaluated at group means,
extinction coefficient (e) or a standard curve. Specific ac- and each circle indicates the 95% confidence elipses. Su-
tivity was defined as enzyme activity (U) per milligram of perimposed on this plot is the association between the new
digestive gland protein (U mg protein '). Protein concen- axes and the enzymes that were measured. This is displayed
tration was determined using the method of Bradford as a vectordiagram in which the direction and length ofthe
(1977). using bovine serum albumin as the standard. En- vector is a measure of the association between the enzyme
zyme assays were performed at 30 C and the absorbances and the axes. Those groups in which ellipsoids are not
read using a TECAN Spectro Rainbow Thermo microplate overlapping signify differences between species. The cor-
reader (trypsin. a-amylase. a-glucosidase, /3-glucosidase, relation between the position ofeach species relative to the
chitinase) or a UNICAM 8625 UV/visible spectrophotom- vector diagram determines the enzyme or enzymes respon-
eter (total protease, cellulase. laminarinase). Data points are sible for its separation.
the mean ofduplicate assays accounting for the appropriate
blanks, and each assay reports the mean standard errorof Results
five replicate crabs for each species, with the exception of
N. tubercitlosiis, for which 10 individuals were used. Stomach contents
Stomachs of all crabs had a large proportion of uniden-
Statistical anulvsis
tifiable material that was either semi-digested or detritivo-
Based on the activities foreach enzyme within a species, rous in nature. Stomachs of Nectocarcinus integrifons and
differences between species were analyzed using a multi- Petrolisthes elongatus contained no animal material. N.
variate analysis ofvariance (MANOVA). Unlike univariate integrifons had large quantities of vascular plant material
analyses, this analysis allows for the simultaneous compar- removed from either living or recently detached plants,
ison of species means for each enzyme while maintaining whereas the stomachs of P. elongatim consisted largely of
=
the chosen magnitude oftype 1 error (P 0.05) as well as brown and green phytoplankton and larger algal pieces
considering the con-elation between enzymes within a spe- (Table 1 ). Stomachs ofCarcinux nwenasandNectocarcinus
cies. Following MANOVA. significant differences were tiiberciilosus had both plant and animal material (Table 1).
explored using a canonical discriminant analysis (CDA). Stomachs completely full of gastropod shells and bivalves
Each species was plotted in the reduced multivariate space, were common in both crab species. Plant material was less
in whichthe new axes (CDA 1. CDA 2, and CDA 3) explain common and consisted ofsmall pieces ofvascular material.
a proportion of the total variability in the data. Group Stomachs of Plagusiu chabrus had very little identifiable
Table 1
Gut content itcin\ that were identified in lite stomach ofindividual crah.\ with n stomach fullness greaterthan 3 (>50%full), andthe corresponding
dietfrom the literature
Stomach content items
Species Animal Plant Literature diet Reference Classification
Nectocarcinus integrifons
DIGESTIVE ENZYMES IN SIX CRABS 39
plant matter and contained fragments of animal material,
(A)
possibly small encrusting species of bryozoans and hy-
droids, as well as exoskeletons of small crustaceans and
some fish parts (Table 1).
Digestive enzyme activity 08
Proteases. The highest protease activity was displayed by
Leptograpsus variegatus (1.19 0.02 units mg~') and P. .O)
chabrus (0.99 0.05 units mg~'). Lowest activity was E 04
measured in N. integrifons (0.34 0.05 units mg~'), N.
tuberciilosus (0.39 0.02 units mg~'), and C. maenas
(0.46 0.02 units mg~'). with less than halfthe activity of > oo
L. variegatus and P. chabrus (Fig. 1A). The highest trypsin <J (B)
activity was exhibited by P. chabrus (0.73 0.12 units
mg~') and L. variegatus (0.46 0.03 units mg~') and the o
CD
lowest activity by N. integrifons (0.14 0.03 units mg~') 0.
(Fig. IB).
Carbohydrases. Carbohydrase activity was present in all 0.4-
crabs, with hydrolysis of both a- and ^-linked substrates
recorded in all species. a-Amylase activity was about three
times higher in Petrolisthes elongatus (0.29 0.04 units
mg~') than in N. tuberculosus (0.09 0.02 units mg~')
(Fig. 2A). a-Glucosidase specific activity was highest in L.
variegatus (0.0022 0.0002 units mg~') and was about In! For Lep Velv Eur Plag
twice the level recorded for all other species. Petrolisthes Species
elongatus had negligible a-glucosidase activity (Fig. 2B).
j3-Glucosidase activity was highest in P. elongatus Figure 1. Specific activity of(A) total protease and (B) trypsin mea-
(0.025 0.005 units mg~'). about three times greater than fsuereeddinfgrhoambictrs.udVeadliugeesstairvee gmleaannde(uxntirtasctmogf~'si)x craSb.Es.pecwiheesrewitunhitdsifafreerenitn
in L. variegatus (0.007 0.0008 units mg~') (Fig. 3A). jumol p-nitroanalide min~' for trypsin and /xg tyrosine min~' for total
Although variable, laminarinase activity was highest in P. protease. Key to crab species: Int = Nectocarcinus integrifons (n = 5).
elongatus (0.35 0.08 units mg~'), and L. variegatus also For= Petrolistheselongatus(n = 5),Lep =Leptograpsusvariegatus(n =
Thahdesoutbhsetrantfioaulracstpievciiteys(0h.a1d8 com0p.a0r2atuinivteslymgl~o'w)er(Faicgt.iv3iBt)y., 5()n,=Vel5)v,=PlaNgec=tocPalracgiunsuisatcuhbaebrrcuuslo(snus=(n5)=. 10),Eur = Carcinusmaenas
ranging between 0.058 0.01 (N. tuberculosus) and
0wa.na0ds1l6hoiwgehse0ts.ti0n0fo3Cr.(NmC..aeimnnataeesgnra(is0f).on0su0n1(i4t0s.0m10g9.~0'0.006C.e0ul0nli4utlsuanmsiegts~a'cm)tgi~vai'nt)dy 6fi)r.stThaexisg,reaCtDesAt di1ff(e.vreanxcies)a,mwohnigchtheexsppleaciinesedwa5s1a%loonfg tthhee
PChliatgiunsaisaechacatbirvuitsy(0w.a0s01s9imil0ar.0f0o1r4 munoistts msgpe'ci1e)s,(Firga.n4gAi)n.g vraatriioantibonet(wFeige.n5t)h.eTshpiescideisffoenretnhceebwaassisloafrgtehley hdiugehtaoctsievpiat-y
between 0.023 0.003 (P. chabrus) and 0.041 0.005 (L. of laminarinase and /3-glucosidase in P. elongatus, and the
evlaornigeagtautsu,s)whuincithshamdg~a's.ubsTthaentieaxlcleypltoiwoenrwchaistinPaesteroalcitsitvhietsy Tbhiehgtehwesaeecnctiovtnihdteyasxopifesc.ai-eCgsDlauAncods2iadc(avcsoaeuxnidtsi)esdpallfasoyored2s1hb.oy2w%eLd.ofvdaivrfaifreeigraaettniucosen.s,
(0.006 0.001 units mg~') (Fig. 4B). with Plagusia chabrus being separated from other species
Relationship between enzyme complement and crab by its high activity of trypsin and total protease, while N.
integrifons and N. tuberculosus were separated by their
species
activity of cellulase and chitinase (Fig. 5). The third axis
Significant differences were found between crab species (CDA 3)explained 18.4% ofthe variation and, whenplotted
when the specific activities of all enzymes were compared with CDA 2, shows a separation between N. integrifons and
uPsi<ng0M.0A01N).OTVhAe C(PDilAlaie'xspTlraaicneed=723..324%7;ofF(t4h0e,3v0a)ri=ati6o.n58o1n, Nr.atetdubaelrocnugloCsDusA(F2igb.y6t)h.eiTrhceeslelultawsoe aspnedcicehsitairneasestialcltisveiptay-,
the firstand secondaxes (CDA 1 andCDA 2) and 39.6% on but are now separated from each other along CDA 3 by the
the second and third axes (CDA 2 and CDA 3) (Figs. 5 and higher activity ofcellulase in N. integrifons and the higher
40 D. JOHNSTON AND J. FREEMAN
35-
030-
DIGESTIVE ENZYMES IN SIX CRABS 41
U.UOJ
42 D. JOHNSTON AND J. FREEMAN
1=P.chabrus(n=5| Carcinus maenas (green cruh) and Nectocarcinus
32=-LCvmaaneengaatsu(sn(=n5=)5) tuberculosus (velvet cmh)
4-P elongalus(n=5)
65==NN-itnulbeegrncluolnossu(sn=(5n)=10) Cureinns maenas and N. tuberculosus had both plant and
animal (gastropod shells and bivalves) material within their
stomachs (Table 1 ), suggesting they are omnivores. Carei-
niis maenas is well studied and is described as a voracious
predator feeding primarily on bivalve molluscs, poly-
chaetes. and small crustaceans (Elner, 1981: MacKinnon,
4 CDA3-184% 1997) (Table 1 ). However, algae have also been frequently
observed in its stomach but generally do not contribute
more than about 10% to the total volume of stomach con-
tents (Elner, 1981 ). The diet ofA7, tuberculosus has not been
studied, but the morphology ofits mouthparts, in particular
the specialized mandibles, appears suited to grinding hard
animals (such as molluscs) and vascular plant material
-8 -L CHIT (Salindeho and Johnston, 2003).
Figure6. Resultsofthecanonical discriminant analysis are displayed Although our stomach content analysis suggests both
lor the second (CDA 2) and third (CDA 3) canonical discriminant func- species to be omnivorous, their digestive enzyme comple-
tions evaluated at group means. Group means are central to the 95% ment differs from that of the omnivorous L. variegatus.
confidence ellipses. In the bottom right corner ofeach graph is a vector Protease activity for both N. tuberculosus and C. maenas
diagram for the enzymes measured. The direction and length for each was less than halfthat ofL. variegatus. This may be due to
en/ymeisanindicationoftheassociationbetweentheen/ymeandtheaxes
and can be used to interpret differences among the species. Key to vector a reliance on plant material with lower protein content than
diagram: a-GLU = a-glucosidase,AMYL = a-amylase,|3-GLU = /3-glu- the sea lettuce consumed by L. variegatus. The protease
cosidase. CELL = cellulase, CHIT = chitinase. LAM = laniinarmase. levels ofN. tuberculosus and C. maenas are similar to that
PROT = total protease. TRYP = trypsin. of the scavenging omnivorous redclaw crayfish Cherax
quadricarinatus (0.236 units mg ') (Figueiredo et al..
2001) (Table 2).
ately consistent with an omnivorous feeding strategy. Al- Despite the plant material in the stomach ofthese species,
though it is generally accepted that high protease activity laminarinase activity was low, suggesting that brown algae
reflects a carnivorous diet, other studies have also found are not an important dietary component ofeither species. In
high proteolytic activity in omnivores (Jonas el al., 1983; the MANOVA. N. tuberculosus is separated from the other
Hidalgo et ul., 1999). The sea lettuce (Ulvn Icictncu). which crab species by its cellulase and chitinase activity, which
is ingested in large quantities by L. variegatus (Lobhan and reflects its ability to break down and digest plant cellulose
Harrison. 1997). is high in protein (15% of the organic as well as the chitinous shells of molluscs and other inver-
matter) and may contribute to the high protease activity in tebrates. Interestingly, C. maenas lies fairly centrally on the
this crab. axis of both CDA plots (Figs. 5, 6), which suggests they
More in line with an omnivorous diet, the a-glucosidase have intermediate levels ofall digestive enzymes compared
activity was about twice as high in L. variegatus as in any to the other crab species. C. maenas may be more of a
other species we studied. This high a-glucosidase activity generalist feeder, utilizing a broader spectrum of dietary
was responsible for separating L. variegatus from all other items. Such a strategy would help to explain its incredible
crab species in the MANOVA and, coupled with substantial success in a wide range of habitats (Cohen et al., 1995).
a-amylase activity, suggests that L. variegatus is well
equipped to utilize the carbohydrates within its diet. The Nectocarcinus integrifons (red rock crab)
strong activity ofa-enzymes indicates that a-linked storage Herbivory is common in crabs the consumption ofvas-
carbohydrates are important in its diet. Such storage prod- cularplantshas been observed in several species (Giddins et
ucts are present in both green algae (i.e., starch, which is a al.. 1986; Kyomo. 1992; Woods and Schiel. 1997). The
mixture of amylose and amylopectin) and red algae (i.e., stomachs ofN. integrifons contained no animal material but
floridean starch, a branched glucan similar to amylopectin) didcontain large quantities ofvascularplant material (Table
(Lobban and Harrison, 1997). L. variegatus also had sub- 1). Klumpp and Nichols (1983) found the seagrass Posi-
stantial laminarinase activity, which suggests that this crab ilonin anstralis to occur in the stomachs of 93% of the N.
may also ingest brown algae, a component not described in integrifons individuals sampled and tooccupy 85% ofstom-
previous dietary studies (Griffin. 1971; Skilleterand Ander- ach volume. Consistent with a diet low in protein (seagrass
son. 1986). is only 7% protein), protease and trypsin activities were
DIGESTIVE ENZYMES IN SIX CRABS 43
Table 2
Comparison ofspecificactivities ofcrustaceanproteases amicarbohydrasesfrom crude digestiveglamle.\tract\
Enzyme
44 D. JOHNSTON AND J. FREEMAN
the chelipeds to chop pieces of algae for ingestion or the appear suitable as indicators of the dietary preference for
feeding on detritus, may account for the occurrence ot brown algae (Figueiredo et al., 2001: Wigglesworth and
multieellular algae and detritus in the stomachs of P. elon- Griffith. 1994).
gatus (Kropp. 1981 ).
Surprisingly, the herbivorous P. elongutus had high total Conclusions digestive enzyme complement as indicator
protease activity (Table 2). There are two explanations for ofdiet type
this. Firstly, high protease activity may be a physiological Each species of crab studied had a complex suite of
adaptation to maximize digestion of small amounts of pro- digestive enzymes, the relative activities of which reflected
tein from large volumes of ingested plankton. Micropha- species-specific dietary niches. As opportunistic feeders.
gous feeding in adult porcelain crabs is similar to plank- crabs have a wide range ofdigestive enzymes. However, it
tivorous (phytoplankton) feeding by larval crustaceans. is clear from this study that the specific enzymes dominant
Comparative studies on the digestiveenzymes ofcrustacean within each crab species are consistent with their particular
larvae indicate that protease activities may be higher in diets. The porcelain crab P. elongatus has high activities of
animals that consume phytoplankton than in carnivorous laminarinase and /3-glucosidase for digesting dietary brown
rlaarpviadel.y Hexitgrhactprtohteearseelaatcitvievliytysmmalalyperontaebilnectohmespeonsepnectiefsrotmo adilggeaset(tlhaemivnaasrciunl)a.rHsieagghracsesll(uclealsleuloascet)ividtiyetiosfntehceesrseadrryoctko
large volumes of food, so there is a net energy gain despite crab N. integrifons. Significant trypsin and total protease
a relatively low overall assimilation efficiency (Kumlu and activities break down the high-protein diet of the speedy
Jones. 1997; Le Vay er <//.. 2001). Secondly, it is possible crab P. chabrus. For the swift-footed shore crab L. rarie-
ttehvoaentr,cPo.zueollodopnlhgaaanvtkeutsobniesewancatsiunaglnelosytteoidmdnedniutvriofirineogdusfiinlatnetrdhefteheasdttioznmgo.aocphHlsaonwko--f ggair-ietauemsny.ladalisggeaelsatcidtioienvtitoifiesst.haecThhsieteavrvecedhlvievnitaictsrhaipbgrheNd.ao-mngitlnbueacrncotisliiyldoasriseteds aahnnadds
animals sampled in this study. Furthermore. P. elongatus high cellulase activity to digest the cellulose ofits plant diet
had substantially lower chitinase activity than the other and high chitinase activity to digest the chitinous shells of
species studied here, suggesting a poor capacity to break the molluscs and other invertebrates that it also consumes.
dzooowpnlacnhkittionn, aansdtroutchteurralincvoermtpeobnraetnets.ofOmtnhieveoxroosukselestpoencieost eTnhceousnpteecirfeidchnearteuraeppoefartshetoefnazvoyrmeasspiencifmiocstfeecdrianbgsbpeehcaive-s
that do ingest shelled invertebrates, such as L. variegatus ior and dietary preference, and demonstrates different strat-
and N. tuberculosits, possess considerable chitinase perfor- egies of resource use. In contrast to the other species, the
mance. green crab C. maenas did not appear to have a dominant
The activities of the carbohydrases (indicative of plant enzyme, which suggests that it is a generalist feeder that
digestion) were mixed, giving us an insight into the specific- utilizes a broad range of dietary items, which may help to
carbohydrates assimilated by P. elongatus. a-Amylase ac- explain its incredible success in a range ofdiverse habitats.
tivity was very high, about three times higher than in N.
tiiherciilosus. High a-amylase activity reflects the high pro- Acknowledgments
portionofstarch inplants ingestedby P. elongatus (Sabapa-
thy and Teo. 1993). Interestingly, a-glucosidase activity We thank Sam Ibbott and Craig Mackinnon (Marine
was negligible, which suggests that although P. elongatus is Research Laboratories. Tasmanian Aquaculture and Fisher-
highly efficient at digesting large structural polysaccharides ies Institute), Rob Gurney (CSIRO), Dean Blunt and Barry
such as starch using a-amylase. it is less effective at digest- Stewart (Tasmanian Clean Water Oysters) for their assis-
ing smaller oligosaccharides. which are broken down using tance in the collection of crab species from sites around
a-glucosidase. Tasmania. We also thank Martin Lourey for critical review
/3-Glucosidase activity was highest in the porcelain of final versions of this manuscript.
crab about three times greater than in the next highest
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