biol 2015 – evolution and diversity
TRANSCRIPT
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BIOL2015–EvolutionandDiversityLab8:Lophotrochozoa–PartI:
Platyhelminthes,Rotifera,Bryozoa,andBrachiopoda
Introduction
Lastweekwestudiedtheporiferaandthecnideria.Spongeslacktruetissueandanysymmetricalorganizationofthebody,whilecnideriahastruetissue(withtwogermlayers)andradialsymmetry.Todaywebeginthestudyoftheanimalswithbilateralsymmetry(Bilateria).Allbilateralanimalshavethreegermlayers(endoderm,mesodermandectoderm).Thebilateriaaredividedintotwomajorgroups(seeFigure1),theprotostomesandthedeuterostomes(thesenamesrefertowherethemouthformsintheearlyembryo.SeeFigure32.7ofyour2010textbook).
Theanimalphylogenyhasbeenrevolutionizedinthelast15yearsbymolecularphylogeneticstudies,whichhavechangedmanyoftheacceptedrelationshipsthatwerebasedsolelyonmorphologicaldata.OnesuchgroupistheLophotrochozoa(Figure1),whicharealargegroupofanimalswithintheprotostomesandwillbeourfocusforthenexttwolabs.Figure2showsamoredetailedphylogenyofthemajorgroupsintheLophotrochozoa–theyarediverse!ThenameLophotrochozoacomesfromthenamesofthetwomajoranimalgroupsincluded:theLophophorataandtheTrochozoa.
Trochozoa:Manyofthemembersareworm-like,thoughnotallofthemarefamiliarorcommon.ThetwolargestgroupsoftrochozoansaretheMollusca(mollusks)andtheAnnelida(segmentedworms).
Itmightseemstrangeatfirsttogroupearthwormsandsquidstogether.Theycertainlydon'tlookmuchalike,butthatisonlytruewhenlookingattheadultform;theyshareafundamentalfeatureoftheirlife
Figure1.Asimplifiedphylogenetictreeofanimals.
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Figure2.AphylogenetictreebasedonDNAdata.Dunnetal.2008.
Trochozoa
Lophophorata
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history.Manyannelidsandmolluskssharepatternsofdevelopmentinearlyembryonicstages.Whentheirlarvaehatch,eachisamicroscopicswimmerknownasatrochophorelarva.Thelarvahastwobandsofciliaaroundthemiddlethatareusedforswimmingandforgatheringfood,andatthe"top"isaclusteroflongerflagellae.Sothelarvaeofthesegroupsarenearlyidentical,eventhoughtheymatureintoverydifferentadultforms.Untilveryrecently,theArthropoda(insects&crustaceans)wereconsideredcloserelativesoftheAnnelida,basedonthefactthatbothgroupsaresegmented,butnoarthropodhasatrochophorelarvaandnomolecularstudiessupportacloserelationshipbetweenthem.
Lophophorata:ThisgroupincludesthephylaPhoronida,Entoprocta(bothsmallgroups),Bryozoa(formerlycalledEctoprocta)andBrachiopoda,withthelattertwohavinganextensivefossilrecord.Thefeaturesharedbythisgroupisthelophophore,anunusualfeedingappendagebearinghollowtentacles.
WhiletheLophophorataareawell-recognizedgroup,phylogeneticstudiesdonotyetagreeontheidentityoftheirclosestrelatives.TheseanimalswereonceincludedinthePseudocoelomata,becausetheydonothaveadistinctinternalbodycavityliketheTrochozoa,butthisgroupingdoesnotholdtogetherinmodernstudies.
Platyhelminthes(Flatworms)
Flatwormsareanatomicallysimplecomparedtonematodesandannelids.Theylackcirculatoryandrespiratorysystems.Manymembersofthisphylumareparasitic(e.g.,flukesandtapeworms)buttherearealsoanumberofspeciesthatarefree-living,liketheplanarianswewillhaveinlabthisweek.Thesenon-parasiticPlatyhelminthesarefoundinfreshwater,marineandinsometerrestrialenvironmentsandarefuntowatch.
Flatwormfeatures:
• Threetissuelayersinembryo.Almostallanimalssharethisbasicfeature;thespongesandcnidariansareexceptions.
• Acoelomate:Flatwormsdon'thaveanykindofcoelomorpseudocoelom;theirbodiesarebasicallysolid.ThissimplebodystructureledbiologiststobelievethatthephylumPlatyhelminthesbranchedofffromtherestoftheanimalsbeforetheevolutionofthecoelom.However,newphylogeneticstudieshaveledsomeresearcherstoconcludethatflatwormsdescendedfromanancestorthathadacoelom,andlaterlostthecoelom.
• Gastrovascularcavity:Thedigestivetracthasonlyoneopening,andbranchesthroughoutthebody.Flatwormshaveextracellulardigestion,likemostanimals(butunlikesponges).
• Pharynx:amusculartubethroughwhichtheflatwormcansuckfoodintoitsgastrovascularcavity.Theopeningintothepharynxcouldbeconsideredthemouth,butsincethisanimalhasatwo-waygut,thatopeningalsomustfunctionastheanus.
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Trepaxonemata
Thereareapproximately4,500speciesofplanarians.Mostarefree-livingandmarine,althoughyoucanalsofindtheminfreshwaterandonland.Manyarepredators,theyfeedonprotozoans,smallinvertebratesordeadanimals.Thecanclonethemselvesbylongitudinalortransversalpartitioning;theyarealsohermaphrodites.
Dugesia(Planarians)
ObservelivingDugesiaatadissectingmicroscope.Noticetheflattenedbodiesoftheseanimals,andtheirgracefulmovements.Athickmucouscoatinganddenselyciliatedepidermisontheirventralsideallowsthemtoglidealongsurfaces.Alsonotethattheyhavea“head-end”wherephotosensitiveeyespotsorocelliarelocated(lendingtheDugesiaa"cross-eyed"appearance),whichhelpthemavoidbrightlight.
Findthefollowingslides:
• Findapreparedslidelabeled"Planariaw.m.".• Findapreparedslidelabeled"Planariacombinationw.m.".• Findapreparedslidelabeled"Planaria
c.s.".
Thisspecimenisstainedtohighlightthegastrovascularcavity,showingthesmallbranchescalleddiverticula.Notethatthereisonemainbranchofthegastrovascularcavityintheanteriorpartofthebody,buttwomainbranchesposteriortothepharynx.
Theeyespotsaresimpleanddon'tformanimage;that'swhytheyarecalledeyespotsinsteadofeyes.However,theyareslightlycup-shapedandfacetowardthesides.Withthisarrangement,theflatwormcantelllightfromdarkandmovetowardthedark.Cup-shaped
Figure3.AwholemountviewofaPlanaria.
Figure4.Cross-sectionviewsofPlanaria.
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eyesarejustonestepinthetransitionfromphotoreceptivecellstoeyeballs.
Planariacrosssections:ThismicroscopeslidehasseveraldifferentcrosssectionsofaPlanaria,showingdifferentregionsofthebody.Ontheventralsurfacesyoucanobserveciliausedforlocomotion.
Anteriorregion:Thissectionanteriorto(infrontof)thepharynx.Thebodyismoreorlesssolid;theonlyopeningsarethediverticulaofthegastrovascularcavity.
Pharynxregion:Thiscrosssectionshowsthepharynx,whichisretractedintothepharyngealcavity(theemptywhitespacesurroundingthepharynx).Thepharynxitselfisathick,musculartube;whenPlanariaeats,itevertsthepharynx,stickingitoutofthebodytosuckupbitsoffood.
Posteriorregion:Thegastrovascularcavityisdividedintotwomainbranches,withsmallerdiverticula.Theverticalpinklinesaredorsoventralmuscles.
Trematoda(Flukes)
TheflatwormsoftheclassTrematoda,alsocalledflukes,areinternalparasitesofmollusksandvertebrates.Thereareapproximately18,000to24,000species.Theyhaveoralsuckers,sometimessupplementedbyhooks,withwhichtheyattachtotheirvertebratehosts.Trematodeshaveretainedthesamebodyformanddigestivecavityastheturbellarians.However,practicallytheentireinteriorisoccupiedbythereproductivesystem;theseorganismsarecapableofproducinghugenumbersofoffspring.TrematodesoftheorderDigeneahavecomplexlifecyclesinvolvingtwoormorehosts.Thelarvalwormsoccupysmallanimals,typicallysnailsandfish,andtheadultwormsareinternalparasitesofvertebrates.Manyspecies,suchastheliverflukeClonorchissinensisandthebloodfluke(Schistosoma),causeseriousdiseasesinhumans.
Clonorchissinensis(ChineseLiverFluke)
Findapreparedslidelabeled"Clonorchissinensisw.m.".
Figure5.Cross-sectionoftheposteriorregionofPlanaria.
Figure6.AwholemountviewoftheChineseliverfluke.
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Someoftheseslidescontainlargeranimals,somesmallerones–makesuretolookatboth. Findtheoralsucker;theacetabulumorventralsucker;andthepairedandelongateintestines(labeled“gut”inFigure6).Notethatithasreduceddigestivesystemandalso,howmuchofthespacewithintheadultflukeisdevotedtoreproduction.
NoticealsothatTrematodeshavebothmaleandfemalereproductiveorgans(theyarehermaphrodites).Thelargeuterus(nothomologouswithouruterus)isfilledwitheggsandislocatedanteriortothetestes,whichproducesperm.Whydoyouthinkthattheorganisminvestssomuchenergyonreproduction?
Fasciolahepatica(CommonLiverFluke)
Thecommonliverflukeorsheepliverflukeisgloballydistributed,andinfectstheliversofvariousmammals,includinghumans.Fasciolahasacomplexlifecyclewithmultipleintermediatestagesandhosts(Figure7).Thediseasecausedbytheflukeiscalledfascioliasisandcausesgreateconomiclossesinsheepandcattle.ObservethepreservedspecimensofFasciolahepaticaondisplayandbeabletoidentifyit.
Findapreparedslidelabeled"Fasciolahepaticamiracidiaw.m.".
Fasciolatakeondifferentformsdependingonthestageoftheirlifecycle.Miracidiaarethefree-livingmotileform,coveredwithcilia,andsettleintoamolluskhost.Theyaresmallandvariableinform,soobservemorethanone.
Findapreparedslidelabeled"Fasciolahepaticarediaew.m."and"Fasciolahepaticametacercariaw.m."toseewhatformthisparasitetakesinitsintermediatehosts(Figure7).
Figure7.Fasciolalifecycle.
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Schistosomamansoni-BloodFluke
Thisflukecausesthediseaseschistosomiasis,whichaffectsmorethan20millionpeopleayear.Schistosomesareatypicaltrematodesinthattheadultstageshavetwosexes(dioecious)andarelocatedinbloodvesselsofthedefinitivehost(mostothertrematodesarehermaphroditicandarefoundintheintestinaltractorinorgans,suchastheliver).Schistosomeshavetwohosts:adefinitivehost(i.e.human)wheretheparasiteundergoessexualreproduction,andasingleintermediatesnailhostwherethereareanumberofasexualreproductivestages.
Findapreparedslide(demonstration)labeled"Schistosomamansonimaleandfemalew.m.".Youwillobserveapairofmatingflukes.Duringcopulation,thefemaleisheldinaspecialchannel(thegynecophoralgroove).
Cestoda(Tapeworms)
Cestodaisthenamegiventoaclassofparasiticflatworms,commonlycalledtapeworms.Asadultstheyliveinthedigestivetractofvertebrates,andofteninthebodiesofvariousanimalsasjuveniles.Overathousandspecieshavebeendescribed,andallvertebratespeciescanbeparasitizedbyatleastonespeciesoftapeworm!Infectioninhumansoccursbecauseofconsumptionofimproperlycookedorprocessedmeat(pork,beef,orfish,forexample).Taeniasaginata,thebeeftapeworm,cangrowupto20minthegutofitshost,butthewhaletapeworm,Polygonoporusgiganticus,cangrowtoover30m!Cestodeeggshaveevenbeendiscoveredinfossilsharkfeces(coprolites)datingtothemid-tolatePermian,some270mya.
Figure8.LifecycleofSchistosomamansoni.
Figure9.ThisisanadultTaeniasaginatatapeworm.Whichsideisthehead?
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Tapewormslackadigestivetrack.Allnutrientsareabsorbeddirectlythroughthebodysurface.Thehost’sdigestiveenzymesdoallthework.Theheadregion,orscolex,ismodifiedforattachmenttotheintestinalwall.Thescolexhassuckers,andoftenhasaringofhooksaswell.
Mostofthetapeworm’sbodyconsistsofrepeatedsegmentscalledproglottids(althoughit’snotanexampleoftruesegmentation).Eachproglottidisaself-containedpacketcontainingmostlyreproductiveorgans.Newproglottidsareproducednearthescolex.Themostmatureproglottidsareattheoppositeendoftheanimal.
Findthepreparedslideslabeled"Taeniapisiformisscolexw.m."and"Taeniapisiformismatureandgravidw.m.".Ontheformerobservethesuckersandhooks,onthelatterobservehowmuchofthebodyisdevotedtothereproductivestructures.Alsonotehowthe“segments”calledproglottidschangeshapeoverthelengthoftheanimal.
Rotifera
BdelloideaRotiferameans“wheelbearer”.Theyaregenerallymicroscopic(barelyseenwiththenakedeye),withtwomaindistinguishingfeatures.Thefirstisananteriorregioncalledthecoronathatisusedinfeedingandinlocomotion.Thesecondisamuscularpharynx(calledamastax)thatpossessescomplexjawsknownastrophi.RotifersarepseudocelomatesExaminethelivespecimensandthepreparedslidesofarotifertofamiliarizeyourselfwiththeirfunctionalmorphology.Notethe3generalregions:head,bodyandfoot(andtoes).Observealivespecimenatincreasinglyhighermagnification,ultimatelyaimingtoobservethepatternofciliationonthecorona.Asinmanyfreshwaterorganisms,manyrotiferspossess
Figure7:Rotifer
Figure10.StainedadultTaeniapisiformis.
Figure11.Aschematicdrawingofarotifer.
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remarkableadaptationstowithstandperiodsofdroughtand/orcoldtemperatures.Themostnotableofthesearetheirabilitytoundergoparthenogenesis(growthanddevelopmentofeggswithoutfertilization)andproducerestingeggs.Manyspecieshavetheabilitytoundergoanhydrobioisis,arestingstagethatoccursduringdryconditionsinwhichtheanimalcansurvivecompletelydesiccatedforlongperiodsoftime.Findapreparedslidelabeled"Rotifersw.m.".Findthesamestructuresasabove,butalsotrytofindthestomachandpseudocoel.
Bryozoa
PhylactolaemataBryozoa(formerlycalledectoprocta),means"mossanimals”).Theyareageologicallyimportantgroupofsmallanimals.ThevastmajorityofBryozoansarecolonialandmostaremarine.Superficiallytheycanresemblecorals.Bryozoansaremostabundantintemperate-tropicalwatersthatarenottooturbid.Theyrequireafirmsubstrateontowhichtheyattachorencrust,andclearagitatedwaterfromwhichtheyobtaintheirsuspendedfood.Enclosedwithinaskeletonofcalcite,Bryozoanshaveasac-likecoelomatebodywithawell-definedmouth,anus,andotherspecializedorgans.Onesuchorganisthelophophore(aciliatedstructureusedinfoodgathering)thatisattachedtotentaclesthatsurroundthemouth.Findapreparedslidelabeled"BryozoaPectinatellaw.m.".Beabletoidentifytheorganismandthelophophores.
Figure12.Aclose-upofaBryozoancolony.Thehorseshoe-shapedlophophoresareclearlyvisible.
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Brachiopoda
LingulataWhilerecentbrachiopodsarearatherrareandinsignificantgroup,theirlongfossilhistoryshowsthattheywereattimesthemostprominentanimalsintheseas.Consequently,brachiopodsreceiveonlypassinginterestfromzoologists,butagreatdealofattentionfrompaleontologists.Thephylumisquiteimportantforbiostratigraphy,paleoecology,andevolutionarystudiesbecauseitshowsagreatvarietyofchangesinformandfunctionthroughtime.Brachiopodsresembleclams,buttheirshellsaredorsal-ventral(thatis,there’satopshellandabottomshell),whereasclamshavelateralshells(there’saleftandarightshell).BrachiopodsfeedusingalophophorelikeEcoproctans,whichcreatesawatercurrentthatenablesthemtofilterfoodparticlesoutofthewater.FindpreservedspecimensofLingulaondisplay.ThegenusLingulafirstevolvedover400millionyearago,makingittheoldestlivinganimalgenusthatstillcontainsextantspecies!IdentifytheinternalstructureslabeledinFigure13onthepreservedspecimensinthelab.
Figure13.Adissectedbrachiopodshowingtheinternalstructure.