Table Of Content©EntomologicaFennica.15March2007
Preimaginal feeding habits of Dictyogenus fontium
(Plecoptera, Perlodidae) in an alpine brook in NW Italy
StefanoFenoglio*,TizianoBoandGiorgioMalacarne
Fenoglio,S.,Bo,T.&Malacarne,G.2007:PreimaginalfeedinghabitsofDictyo-
genusfontium(Plecoptera,Perlodidae)inanalpinebrookinNWItaly.—Ento-
mol.Fennica18:27–31.
ThefeedinghabitsofDictyogenusfontiumnymphswereinvestigatedintheRio
Olen, an alpine brook in NW Italy. In an analysis of the gut contents of 84
nymphs,wedetectedevidenttrophicpreferences:Chironomidae(Diptera)were
themajorcomponentofthepreyingested,independentlyoftheiravailabilityin
thesubstratum.Plantdetritusandalgaewerealsoanimportantpartofthedietof
thiscarnivorousstonefly.Thefindingsarediscussedonthebasisofecological
considerations.
S.Fenoglio,T.Bo&G.Malacarne,UniversityofPiemonteOrientale,Di.S.A.V.,
Via Bellini n. 25, 15100 Alessandria, Italy; *corresponding author’s e-mail:
[email protected]
Received5December2005,accepted16March2006
1.Introduction Plecoptera nymphs are the most abundant
predators in many lotic systems. In particular,
Benthiccommunitiesinrunningwaterarecom- PlecopteraSystellognathaisthedominantpreda-
posedalmostexclusivelyofinsects:theyconsti- tor group in fishless alpine streams, with a key
tute an estimated 70–90% of the macro- ecological role as top-down control elements in
invertebratespeciesinstreamsandrivers(Min- benthic communities (Zanetell & Peckarsky
shall1969).Plecopteraisoneofthemostimpor- 1996,Wipfli&Gregovich2002).Peckarskyand
tantordersintheseecosystems,indeedoftenthe Dodson(1980)analysedthecolonisationprocess
dominantorder(Zwick2000, Elliott2004). For ofinstreamartificialsubstrataandfoundthatsub-
this reason, many studies have investigated dif- stratacontainingpredaceousstoneflieswereless
ferent aspects of the ecology and life-history of colonised, confirming that these predators can
thisgroup(TiernodeFigueroa&Fochetti2001, significantlyinfluencebenthiccommunitystruc-
Stewart&Stark2002). ture. Optimal foraging theory, as formulated by
Inrecentyears,therehasbeenincreasingin- Krebs(1978),predictsthatpredatorsoptforthe
terestinstoneflyfoodhabits.Sincethepioneer- most advantageous prey according to different
ing study of Richardson and Gaufin (1971), re- factors,suchasencounterrate,preydensity,en-
search has been conducted on food selection ergy contents, handling time and others. Fuller
(Feminella&Stewart1986),ontogeneticshiftsin andStewart(1977,1979)investigatedfoodhab-
the diet (Bo & Fenoglio 2005) and functional its of many Plecoptera species, quantifying
grouping(Short&Ward1981–feedingmecha- electivityandfoodpreferencesofstoneflylarvae
nisms, according to theRCC, River Continuum inrelationtofoodavailability:theydemonstrated
Concept,Vannoteetal.1980). theexistenceofseasonalandontogeneticshifts,
28 Fenoglioetal. (cid:127) ENTOMOL.FENNICAVol.18
dietary overlaps,daily periodicity andprey size Table1.Mainabioticparametersinthesamplingsta-
electivity. Predaceous stoneflies exhibit a great tion,RioOlenbrook(NWItaly).
variety of food habits: some species seemto be
Temperature(°C) 4.8
extremely selective, feeding mainly on certain
pH 7.7
prey(Fenoglio&Bo2004),whileotherspecies
Conductivity(µS/cm) 78.0
aremoreopportunistic,eatingthemostabundant Alkalinity(mgHCO–/l) 26.0
organisms in the benthic community (Allan & Cl–(mg/l) 3 0.5
Flecker1988,Dudgeon2000).Moreover,studies SO––(mg/l) 4.2
PO4–––(mg/l) <0.1
haveshownthatseveralSystellognathahavevar- NO4–(mg/l) 0.6
iedfoodhabits,feedingnotonlyonlivinganimal NO3–(mg/l) <0.01
prey but also on plant detritus (Lillehammer NH2+(mg/l) <0.4
4
1988).Forthisreason,StewartandStark(2002)
stated that some stonefly species would not fit
intoanyofthestereotypedcategoriesdefinedby size of 500 µm) early in the morning, because
dominant food or feeding mechanisms, e.g. the Systellognatha are considered mainly nocturnal
fivefunctionalfeedinggroupssensuRCC(scrap- feeders (Vaught & Stewart 1974). We also col-
ers, shredders, predators, filterers and collector- lectedSurbersamplesinthesameriffletoassess
gatherers). Hence, the food habits of unstudied thepresenceandabundanceoftaxainthenatural
species cannot beinferred fromtheir phylogen- benthic invertebrate population. Samples were
eticposition. preserved in 90% ethanol. In the laboratory, all
Dictyogenusfontium(Ris,1896)isatypical organismswerecountedandidentifiedtothege-
orophilous species, endemic to the Alps (Con- nuslevel,exceptforLumbriculidaeandearlyin-
siglio1980,Ravizza&Ravizza-Dematteis1991, stars of some Diptera, which were identified to
Fochetti 1995, Brittain & Saltveit 1996). As its thefamilylevel.
specific name suggests, this mountain stonefly The total length of D. fontium nymphs was
livesinsmallspringsandbrooksataltitudesrang- measured (0.1 mm accuracy) and the nymphs
ingfrom1,000to2,600ma.s.l.,whereitisoften were processed to assess food consumption by
theonlyPerlodidae(Ravizza&Ravizza-Demat- means of gut contents analysis. Guts were re-
teis1990). moved and the contents of the alimentary canal
Theaimofthisstudywastoanalysethedietof were analysed by the transparency method for
D. fontium nymphs in a typical alpine environ- slides.Fragmentsofanimalpreywereidentified
ment. tothelowesttaxonomiclevelpossible.Identifica-
tionofpreywasbasedonsclerotizedbodyparts,
particularly head capsules, mouthparts and leg
2.Materialandmethods fragments. Stewart and Stark (2002) stated that
thecountofsclerotizedfragments(i.e.headcap-
On 29–30 September 2005, D. fontium nymphs sules) can give a reasonably accurate count of
werecollectedina100muniformriffleoftheRio preyconsumed,andweightsorvolumescouldbe
Olen brook (UTM 414652–5079947; 2,100 m extrapolated, if necessary, from morphometric
a.s.l.). The sampling station is a typical alpine weight-volumerelationshipsofprey.Toinvesti-
lotic environment, characterized by high slope, gatepossiblefeedingpreferences,wecompared
coarseriverbedandfastflowingwater.Riparian thegutcontentswiththenaturalcompositionand
vegetation is almost absent, except for some abundanceofmacroinvertebratecommunitiesin
Sorbusaucuparia.Thisloticsystemshowsgood theriverbedusingthetrophicelectivityindexof
environmentalquality,correspondingtoanenvi- Ivlev(1961):
ronmentwithouttracesofhuman-inducedalter-
ation(firstclassintheItalianExtendedBioticIn- E=(ri–pi)/(ri+pi)
dex, Ghetti 1997). The main abiotic parameters
arereportedinTable1.Samplingswerecarried whereri=theproportionofingestedspeciesand
out with a kick-net sampler (20 x 20 cm; mesh pi=therelativeabundanceinthebenthiccommu-
ENTOMOL.FENNICAVol.18 (cid:127) PreimaginalfeedingofDictyogenusfontium 29
nity.Feedingpreferenceswerealsoquantifiedus- Table2.Relativeabundance(%valueinthecommu-
inganotherelectivityindex(McCormick1991): nity)ofmacroinvertebratescollectedinthenatural
riverbed.
E*=(W–1/N)/W+1/N
i i Taxa Relative FFG
abundance(%) (*)
where
Plecoptera
W =(r /p)/Sr /p Leuctrasp. 3.68 Sh
i i i i i
Nemourasp. 4.61 Sh
Protonemurasp. 2.63 Sh
In this formula, r = the proportion of ingested
i Chloroperlasusemicheli 3.16 P
species,p =therelativeabundanceinthebenthic
i Siphonoperlamontana 1.58 P
community, and N = the number of food items. Isoperlasp. 1.05 P
Bothindexesrangefrom–1.0to1.0.Avalueof– Dictyogenusfontium 5.05 P
1.0 means total avoidance, 1.0 indicates prefer- Ephemeroptera
Ecdyonurussp. 8.95 Sc
enceand0indicatesindifference.Thepresenceof
Rhithrogenasp. 1.58 Sc
algae and detritus (e.g. fragments of terrestrial
Epeorusalpicola 2.11 Sc
vegetation) was recorded and quantified on a Baetismelanonyx 5.79 Cg
scaleof0–3(0=nopresence;3=highestabun- Baetissp. 2.63 Cg
danceclass). Trichoptera
Rhyacophilasp. 11.32 P
Hyporhyacophilasp. 2.11 P
Drusussp. 5.26 Sh
3.Results
Monocentralepidoptera 2.63 Sh
Cryptothrixnebulicola 2.63 Sh
Weexaminedthegutcontentsof84Dictyogenus Diptera
Chironomidae 11.58 Cg
fontium nymphs and we collected 760 macro-
Limoniidae 2.11 P
invertebratesbelongingto26taxa.Thetaxalist
Empididae 1.05 P
andrelativemacroinvertebrateabundanceinthe Simuliidae 8.11 F
riverbedarereportedinTable2.Themeanlength Lispesp. 1.45 P
ofD.fontiumimmaturestageswas12.9mm,with Prionocerasp. 0.53 Sh
Oligochaeta
a minimumof 8.0 mmand a maximumof 17.5
Lumbriculidae 3.68 Cg
mm.
Eiseniellatetraedra 0.53 Cg
Duringthelaboratoryanalysis,wefoundfour Crustacea
Niphargussp. 0.13 Cg
Platyhelminthes
Crenobiaalpina 2.50 P
Arachnida
Hydracarina 1.58 P
(*)FFG:f unctionalfeedinggroups(Cg=collector-gatherers;F=
filterers;P=predators;Sc=scrapers;Sh=shredders.seeMerrittand
Cummins1996).
completelyemptyguts.Fineplantfragmentsand
algaewerepresentin82%oftheexaminedguts.
Furthermore,plantdetrituswasvirtuallytheonly
element found in five nymphs; interestingly,
these five nymphs were of very different size,
from10.4to17.5mm(totallength).
Resultsofthefeedingpreferenceanalysisare
Fig.1.Electivityindexesformacroinvertebratetaxain
thedietofDictyogenusfontiumnymphsinRioOlen reportedinFig.1.Consideringallspecimenscol-
brook. lected,themostimportantpreygroupwasChiro-
30 Fenoglioetal. (cid:127) ENTOMOL.FENNICAVol.18
nomidae(Diptera): it constituted 68.8% oftotal theRioOlenbrook,thetwoindexesrevealedthat
ingesteditemsandwaspresentin65.5%ofguts. theirpresenceinthedietwasrelatedtoactivese-
Ephemeroptera,mainlyBaetidae,wasthesecond lection and preference (Ivlev index = 0.79,
mostfrequentgroup,occurringin9.5%ofguts. McCormick index = 0.76). This consistent pat-
Intwospecimens,wefoundheadsandotherbody ternofelectionofChironomidaeandavoidance
partsofLimnephilidae(Trichoptera)andinone ofotherabundantorganismscouldberelatedto
gut there was an entire specimen of Hyporhya- ecologicalandbehaviouralfactors;forexample,
cophila sp. (Trichoptera). Plecoptera legs and Simuliidae could be difficult to catch because
fragmentswerepresentinonlyonegut.Interest- they inhabit microhabitats difficult to explore
ingly,someorganismsthatwerequiteabundant (uppersurfaceofstonesinfastflowingenviron-
inthenaturalenvironment,suchasRhyacophila ments). On the other hand, the relatively abun-
sp.,Hyporhyacophilasp.(Trichoptera)andPro- dant caddisflies of the family Rhyacophilidae
simuliumsp.(Diptera),werealmostabsentinthe maybeadifficultpreybecauseoftheirlargesize
dietofD.fontium. andpredaceoushabits.
In conclusion, this study provides evidence
thatthedietofD.fontiuminalpineenvironments
4.Discussion isnotstrictlycarnivorousandischaracterisedby
trophicselectionforafewselectedprey.
Several studies have examined food habits in
stoneflies (Fuller & Stewart 1979, Peckarsky Acknowledgements.WethankPaoloAgostaandMassimo
1980,Allan&Flecker1988).Theproblemofan Pessino for help during field sampling. We are also in-
debtedtoPeterZwickandananonymousrefereefortheir
accurate assessment of feeding habits and food
constructivecomments.
electivity has been pointed out in many papers
and summarized in the work of Peckarsky
(1984): the exclusion of particular prey items
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