Asexual reproductionreproduction andand thethe..turbellarian turbellarian archetypearchetype

Reinhard M.M . RiegerRieger Department ofoj Zoology,Zoology, UniversityUniversity ojof NorthNorth Carolina, ChapelChapel Hill,Hill,NC NC USAUSA

Present address:address: Zoologisches Institutlnstitut der Universitdt,Universitiit, UniversitiitstrasseUniversitdtstrasse 4,4,A·6020 A-6020 lnnsbruck,Innsbruck, AustriaAustria

Keywords:Keywords : Thrbellaria,Tilrbellaria,asexual asexualreproduction, reproduction,paratomy, paratomy,Myomacrostomum, Myomacrostomum, archetypearchetype

Abstract

The turbellarianturbellarian archetype is widelywidely believedbelieved totohave have beenbeen aa hermaphroditehermaphrodite lackinglacking asexualasexual reproduction,reproduction, and suchsuch asexualasexual reproductionreproduction asas isis nownow seenseen inin thethe TurbellariaTurbellaria (as(as paratomy andand architomy)architomy) is isgenerallygenerally assumed toto havehave arisenarisen secondarilysecondarily several timestimes independently.independently. AsexualAsexual reproduction clearlyclearly prevailsprevailsamong among the most primitiveprimitive metazoans suchsuch asas thethe placozoans,placozoans, sponges,sponges, andand radiates,radiates, however,however, andand ifif thethe Platyhel-Platyhel· minthes isis indeed anan earlyearly offshootoffshoot ofof bilaterianbilaterian evolution,evolution, asas somesome havehave claimed,claimed, thenthen it itisis reasonablereasonable toto expect asexual reproduction toto bebe aa primitiveprimitive featurefeature ofof thethe TurbellariaThrbellaria.. AsexualAsexual reproductionreproduction byby paratomyparatomy or architomy isis foundfound inin all threethreemain main evolutionaryevolutionary lineslinesof ofthe theThrbellaria Tlrrbellariaand andis mostis mostcommon common amongamong primitive groupsgroups suchsuch asasthe theCatenulida Catenulidaand andMacrostomida. Macrostomida. TheThe discoverydiscovery of a new, apparentlyapparently primitiveprimitive marine genus of Macrostomida havinghaving paratomyparatomy widenswidens thethe knownknown incidenceincidence ofof asexualasexual reproductionreproduction withinwithin that order.order. TheThe presencepresence of aa musclemuscle ring aroundaround thethe gut of several distantlydistantlyrelated relatedgenera generaof ofthe theMacrostom­ Macrostom- ida and similaritiessimilarities thisthisring ringshows shows withwith septasepta in thethe divisiondivision planeplane ofof paratomizingparatomizing speciesspecies areare evidenceevidence thatthat paratomy waswas a afeaturefeature ofof thethe stemstem speciesspecies forfor thisthis orderorder - a feature onlyonly secondarily lost inin mostmost macro­macro- stomids - and suggestsuggest thatthat asexualasexual reproductionreproduction isis a primitive featurefeature forforthe thePlatyhelminthes Platyhelminthes asas aa whole.whole.

Introduction primitive outgroups forfor thethe BilateriaBilateria (the(the Placozoa, Parazoa, andand Radiata),Radiata), asas wellwell asas inin thethe unicellularunicellular Presently all reconstructionsreconstructions ofof thethe stemstem groupgroup eukaryoteseukaryotes.. IfIf thethe turbellariansturbellarians occupyoccupy aa primitiveprimitive for thethe ThrbellariaTurbellaria assumeassume it comprisedcomprised small,small, ben­ben- position inin thethe Bilateria,Bilateria, asas hashas beenbeen reiteratedreiterated re-re­ thic organismsorganisms that moved byby ciliaryciliary glidinggliding and cently by AxAx (1985),(1985), wewe may expectexpect toto findfind evidence that had aa simplesimple lifelife cycle withoutwithout larvallarval forms oror that such anan alternationalternation of asexualasexual andand sexualsexual asexual reproduction (Bresslau(Bresslau & Reisinger,Reisinger, 19281928;; reproduction hashas beenbeen secondarilysecondarily lostlost inin turbellar-turbellar­ Hyman, 19511951;; Ax,Ax, 1961;1961 ; IvanovIvanov& & Mamkaev,Mamkaev, 19731973;; iansians.. Kading,Karling, 1974).1974) . JaegerstenJaegersten (1972)(1972) hashas beenbeen the only Just suchsuch evidence can bebe foundfound inin thethe orderorder one toto argue thatthat the turbellarian stemstem groupgroup had MacrostomidaMacrostomida.. DiscoveryDiscovery ofof aa newnew genusgenus ofof Mac­Mac- a complete lifelife cycle with a planktotrophicplanktotrophic larva,larva, rostomida withwith paratomy,paratomy, anan apparentlyapparently primitiveprimitive but his proposal hashas yetyet toto bebe criticallycritically reviewedreviewed byby member ofof thethe order,order, has prompted meme toto recon-recon­ specialists of of thethe groupgroup (see BallBall && Galleni, 1984)1984).. sider thethe phylogeneticphylogenetic importanceimportance of paratomy inin Asexual reproductionreproduction isis generallygenerally believedbelieved toto have this group.group. InIn manymany respects,respects, thethe MacrostomidaMacrostomida isis arisen secondarily severalseveral timestimes withinwithin thethe Titrbel-Thrbel­ primitive within the rhabditophoranrhabditophoran lineline ofof turbel-turbeI­ laria (Ax(Ax && Schulz, 1959)1959).. larian evolution (see Ehlers, 19851985;; SmithSmith et alal.,., On thethe otherother hand, anan alternationalternation of asexualasexual andand 1985);1985); and,and, therefore,therefore, aa betterbetterunderstanding understanding ofof the sexual reproduction dominatesdominates in inlifelife cyclescycles of thethe evolutionary originsorigins of asexual reproduction inin thethe

Hydrobiologia 132, 3535-45-45 (1986)(1986).. © Dr W.W. JunkJunk Publishers, Dordrecht.Dordrecht. PrintedPrinted ininThe The Netherlands.Netherlands . 363 6

Macrostomida isis importantimportant forfor reconsideringreconsidering thethe of anan intestinalintestinal musclemuscle ringring inin twotwo relativelyrelatively phylogenetic significancesignificance ofof thisthis process inin otherother distantly related lineslines ofof evolutionevolution inin thethe M-acro­Macro- lines ofof thethe Platyhelminthes.Platyhelminthes . stomidastomida.. TheThe onlyonly alternativealternative isis to consider thesethese muscle ringsrings toto havehave arisenarisen convergently, an un-un­ likely eventevent inin thatthat suchsuch aa structurestructure isisnot not knownknown Paratomy andand itsits derivativesderivatives withinwithin thethe Macro­Macro- in other turbellarians.turbellarians. (The(The muscular specializationspecialization stomida in the anterior gut region ofof thethe catenulidcatenulid genusgenus MyostenostomumMyostenostomum isis clearlyclearly different, having aa In thethe Macrostomida,Macrostomida, only members ofof thethe fami-fami­ bilateral organizationorganization andand aa differentdifferent position position.).) ly MicrostomidaeMicrostomidae have beenbeen known toto reproducereproduce New informationinformation onon thethe structurestructure ofof thethe musclemuscle asexually byby paratomy.paratomy. ButBut inin certaincertainmembers members ofof ring in thethe DolichomacrostomidaeDolichomacrostomidae furtherfurther under-under­ the Dolichomacrostomidae (Myozonaria and scores the structuralstructural similarity between itit andand thethe Paramyozonaria) and thethe aberrant genus MyozonaMyozona muscle ringring inin Myozona (Fig(Fig.. 1)1).. BasementBasement mem­mem- of the Macrostomidae, whatwhat appearsappears toto bebe thethe brane ofof thethe gutgut isis eithereither partially or fully exposedexposed homoiolog rudimentrudiment ofof a amuscularmuscular divisiondivision sep-sep­ along thethe musclemuscle ringring ofof dolichomacrostomidsdolichomacrostomids tum isis manifest asasa a peculiarpeculiar sphincter in the mid­mid- (Fig(Fig.. 1), and thethe latterlatter conditioncondition (that(that ofof dle of the gutgut. . Accordingly,Accordingly, II (Rieger, 1971a) have ParamyozonariaParamyozonaria simplex,simplex, FigFig.. lb)Ib) isis essentially in-in­ proposed thatthat the common ancestorancestor ofof thesethese distinguishable fromfrom thatthat ofof thethe musclemuscle ringring inin cer-cer­ genera had paratomyparatomy similarsimilar toto thatthat ofof thethe tain species of Myozona (cf(cf.,., ee.g.,.g ., Ax,Ax, 1956,1956, and Microstomidae andand thatthat musclesmuscles ofof thethe septumseptum Marcus, 1948)1948).. were retainedretained as a sphincter on the gut because ofof Furthermore, recentrecent TEM studiesstudies havehave verifiedverified the advantage thisthis musculaturemusculature offeredoffered inin servingserving the existence ofof musclesmuscles in the division septumseptum ofof as a gizzard for breaking diatomdiatom frustulesfrustules oror inin paratomizing Microstomum linearelineare (Palmberg(Palmberg & moving foodfood inin very elongated formsforms such as somesome Reuter, 1983,1983, andand Reuter & Palmberg,Palmberg, 1983)1983).. This species ofof the genera Myozonaria andand Paramy-Paramy­ septum consistsconsists of twotwo muscular sheathssheaths betweenbetween ozonaria.ozonaria . This hypothesishypothesis explainsexplains the occurrence which areare nervenerve cellscells.. MyMy ownown reinvestigationreinvestigation ofof thethe

A

FigFig.. 11.. Intestinal muscle muscle ringringin inDolichomacrostomidae. Dolichomacrostomidae. A)A) MyozonariaMyozonaria bermudensisbermudensisRieger,Rieger, 19771977;; longitudinallongitudinal sectionsection.. Note thatthat thethe gastrodermisgastrodermis isis interrupted at the muscle ringring only onon oneone sideside.. B)B) ParamyozonariaParamyozonaria simplexsimplexRiegerRieger && Tyler,lYler, 19771977;; cross sectionsection.. Note that the gastrodermis isis completely lackinglacking atat thethe ringring.. NucleiNuclei to outside ofof circularcircular muscles areare nucleinuclei ofof musclemuscle cellscells.. I114 µm=1,4m= 1,4 mm.mm. 37

Fig. 2. Microstomum hama1um.hamatum. A) langemlalTangential sectlonsection through secondaT)'secondary divIsiondivision plane (5&lion(section fromfrom E. Westblad). Note thethe circularcircular arrangement of muscle fibers in the plane of septum (V). B) Sagittal section through the primary division plane (ventral side). Note cross arrangement ofmuscle fibers in the plane of septum (V). B) Sagittal section through the primary division plane (ventral side). Note cross sectionssections of muscle fibers in divisiondivision plane (V).(V). The fibers have different staining properties fromfrom thethe body wall musculature. br=br=brain,brain, t=testis.t= testis, cm:circuJarcm=circular muscles of bodybody wall, 1m:lm=longitudinallongitudinal muscles of body wall.wall, g==g=gut.gut. 383 8 divisiondivision planeplane ofof M.M. hamatumhamatum atat thethe lightlight Phylogentic positionpositionof ofMyomacrostomum Myomacrostomum microscopicmicroscopic levellevel suggestssuggests musclemuscle fibersfibers oriented circumferentiallycircumferentiallyaround around thethe gutgut areare thethe dominatedominate The malemale genitalgenital organsorgans in thethe newnew genusgenus My­My- constituent.constituent .Palmberg Palmberg && ReuterReuter (1983)(1983) dodo notnotmen­ men- omacrostomum are similar toto those ofof certaincertain tiontion thethe orientationorientation ofof thethe musclemuscle fibersfibers in thethe dou­dou- members ofof thethe Macrostomidae (Myozona styl-styl­ ble septumseptum ofof M . lineare,lineare, but their Fig.Fig . lala suggestssuggests ifera,ifera, BradynectesBradynectes sterreri)sterren) andand toto thosethose of the thethe fibers originateoriginate inin thethe circularcircularmuscles muscles ofof the Microstomidae:Microstomidae: thethe gonadgonad isis unpairedunpaired andand isis situat-situat­ body wallwall andand continuecontinue inin anan obliqueoblique radialradial direc-direc­ ed caudally close toto thethe copulatorycopulatory organorgan and thethe tiontion toto the circular layerlayer ofof thethe gutgut musculature.musculature . stylet pointspoints rostrallyrostrally.. ThatThat thethe vesiculavesicula granulo­granulo- Clearly moremore TEMTEM datadata isis needed onon structurestructure andand rum andand seminalisseminalis areare separatedseparated suggestssuggests closercloser tiesties development of the septum inin microstomidsmicrostomids forfor to membersmembers ofof the familyfamily Macrostomidae.Macrostomidae. Howev­Howev- further comparisoncomparison withwith thethe intestinalintestinal musclemuscle ringring.. er, becausebecause the Macrostomidae isis ratherrather heter-heter­ Most importantimportant forfor thethe interpretationinterpretation ofof thethe in-in­ ogeneous asas itit nownow stands,stands, identifyingidentifying tiesties toto itit asas testinal musclemuscle ring isisthe thediscovery discovery ofof aanew new genus,genus, a whole areare notnot asas meaningfulmeaningful asas tiesties toto individualindividual Myomacrostomum, which, byby virtuevirtue ofof thethe pres-pres­ macrostomid generagenera.. ence of a caudalcaudal sensorysensory organorgan (Rieger(Rieger && Tyler,TYler, Myomacrostomum isis clearly not,not, however, aa 1974)1974) and ofof similaritiessimilarities in adhesiveadhesive organsorgans (Tyler,(TYler, member ofof thethe other other remainingremaining familyfamily inin the Mac­Mac- 1977), isis clearlyclearlyaligned alignedwith with thethe Dolichomacro­Dolichomacro- rostomida, thethe Dolichomacrostomidae,Dolichomacrostomidae, inasmuchinasmuch stomidae and thethe genus BradynectesBradynectes.. Myomacro­Myomacro- as there is nono evidence thatthat itit has secondarilysecondarily lostlost stomum (see Appendix forfor diagnosis)diagnosis) isis repre-repre­ the complex genitalgenital apparatusapparatus characteristiccharacteristic ofof allall sented by twotwo species,species, MM.. unichaetaunichaeta and MM.. known membersmembers of ofthisthis familyfamily (see(see Rieger,Rieger, 1971a,1971a, bichaeta, both ofof whichwhich reproducereproduce asexuallyasexually byby 1971b)1971b).. AtAt thethe samesame time, evidenceevidence fromfrom twotwo otherother paratomy (Fig(Fig.. 3).3) . WhileWhile thethe possibilitypossibility thesethese twotwo sets ofof characterscharacters -- namely presencepresence ofof thethe caudal species representrepresent twotwo morphs inin a acomplexcomplex lifelife cyclecycle sensory organ (Rieger(Rieger & Tyler,Tyler, 1974)1974) andand morphol­morphol- cannot bebe excluded,excluded, thethe availableavailable datadata warrantwarrant es-es­ ogy ofof adhesiveadhesive organsorgans (Tyler,(Tyler, 1977)1977) - shows My-My­ tablishment ofof twotwo separateseparate taxataxa. . TheThe two speciesspecies omacrostomum belongs to a monophyletic taxontaxon are distinguished primarily byby paired-vs-unpairedpaired-vs-unpaired that includesincludes thethe DolichomacrostomidaeDolichomacrostomidae andand thethe condition ofof thethe caudalcaudal organ,organ, a afeaturefeature thatthat isis ofof genus BradynectesBradynectes.. InIn thisthis taxon,taxon, Myomacrosto-Myomacrosto­ generic-level significancesignificancein in thethe Dolichomacro­Dolichomacro- mum andand BradynectesBradynectes appearappear asas primitiveprimitive sistersister stomidaestomidae (see(see Rieger & Tyler,Tyler, 1974)1974).. In addition groupsgroups toto the DolichomacrostomidaeDolichomacrostomidae asas II hypothe-hypothe­ the twotwo speciesspecies differdiffer inin bodybody size,size, inin lengthlength ofof sized earlierearlier (Rieger,(Rieger, 1971c)1971c).. BecauseBecause My-My­ rhammites, andand possiblypossibly inin positionposition ofof thethe femalefemale omacrostomumomacrostomum alsoalso hashas characterscharacters inin itsits male gonad (see(see AppendixAppendix forfor furtherfurther discussion)discussion).. reproductivereproductive system intermediate between thosethose ofof AsAs forfor thethe questionquestion ofof thethe originorigin ofof intestinalintestinal thethe Macrostomidae andand MicrostomidaeMicrostomidae (i(Le..e. dis-dis­ musclemuscle rings,rings, itit isis significantsignificantthat thatspecimens specimens ofof MM . tincttinct vv.. granulorum andand vv.. seminalis as inin Macro­Macro- bichaetabichaeta havehave been foundfound whichwhich havehave aa musclemuscle ringring stomidaestomidae;; closeclose spatialspatial relationship betweenbetween testistestis inin the samesame positionposition asas otherother individualsindividuals havinghaving anan andand copulatorycopulatory organorgan asas inin Microstomidae),Microstomidae), this this incipientincipient secondarysecondary divisiondivision planeplane (Fig(Fig.. 3A,3A, B)B);; newnew genusgenus appearsappears toto bebe mostmost similarsimilar toto thethe otherother specimensspecimens withwith twotwo zooidszooids eithereither havehave clearclear hypotheticalhypothetical stem species ofof thethe MacrostomidaMacrostomida asas signssigns of a secondary divisiondivision planeplane oror lacklack suchsuch aa aa wholewhole.. planeplane entirelyentirely (Fig(Fig.. 3)3).. Preliminary studiesstudies indicateindicate thethe muscle ringring hashas thethe samesame basicbasic structurestructure (spe-(spe­ cificallycifically interruptioninterruption of thethe intestinalintestinal epitheliumepithelium LossLoss ofof paratomyparatomy inin thethe Macrostomida andand otherother andand exposureexposure toto thethe gutgut lumenlumen asas ininMyozonaMyozona andand platyhelminthsplatyhelminths ParamyozonariaParamyozonaria simplexsimplex.. ThusThus thethe discoverydiscovery ofof MyomacrostomumMyomacrostomum stronglystrongly supports thethe hypothe-hypothe­ InIn lightlight ofof thethe primitiveprimitive positionposition ofof My-My­ sissis (Rieger,(Rieger, 1971a)1971a) thatthat the musclemuscle ringring inin omacrostomum,omacrostomum, itsits ability toto reproducereproduce asexuallyasexually DolichomacrostomidaeDolichomacrostomidae isis thethe remnantremnant ofof aa muscu-muscu­ maymay indicateindicate thatthat asexualasexual reproductionreproduction isis primitiveprimitive larlar division septum similarsimilar toto thatthat ofof thethe Micro-Micro­ inin the order MacrostomidaMacrostomida andand thatthat thethe absenceabsence ofof stomidaestomidae.. paratomyparatomy inin mostmost otherother membersmembers of ofthethe orderorder isis thethe

393 9 A

400 11mpm

40µm

x

FigFig.. 3.3 . MyomacrostomumMyomacrostomum novnov.. gen.gen. A)A) MM bichaeta novnov.. spec.spec. DrawnDrawn fromfrom squeezesqueeze preparationspreparations ofof livingliving specimens.specimens . B)B) MM unichaetaunichaeta novnov.. spec.spec . DrawnDrawn fromfrom squeezesqueeze preparationspreparations ofof livingliving specimens.specimens . C) MM. bichaeta, musclemuscle ringring inin livingliving specimens;specimens ; phase contrastcontrast.. D) M. unichaeta, stylet inin livingliving specimens;specimens ; phase contrastcontrast;; x-x, distancedistance forfor measuringmeasuring styletstylet lengthlength..

result ofof secondarysecondary lossloss. . The systematicsystematic distribu-distribu­ al reproductionreproduction is primitiveprimitivewithin withinthe theMacrostomi­ Macrostomi- tiontion of anan intestinalintestinal muscle ringring suggestssuggests thatthat lossloss da,da, itit isis likelylikelythat thatthe thephylum phylum PlatyhelminthesPlatyhelminthes asas of paratomy hashas occurredoccurred at atleastleast twicetwice inin thethe or-or­ a whole isis derived fromfrom anan ancestorancestor havinghaving asexualasexual der,der, onceonce ininthe theDolichomacrostomidae Dolichomacrostomidae andand onceonce inin reproduction,reproduction, perhapsperhaps a aprimitiveprimitive fissionfission oror per-per­ thethe evolutionary lineline leading toto MyozonaMyozona.. IfIf asexu- haps eveneven paratomyparatomy (see(see AxAx & &Schulz,Schulz, 1959,1959, forfor re-re- 40 view ofof the terminology ofof asexualasexual reproductionreproduction inin rantedranted.. OnOn thethe contrary,contrary, itit isis likelylikely thatthat suchsuch Turbellaria)Turbellaria).. EvidenceEvidence forfor this alsoalsocomes comes from thethe reproductive powerspowers werewere presentpresent inin thethe stemstem spe-spe­ factfact thatthat asexualasexual reproductionreproduction isis knownknown inin thethe otherother cies ofof thethe phylum.phylum . ConsideringConsidering thethe dominancedominance ofof two majormajor evolutionaryevolutionary lineslines ofof thethe TurbellariaTurbellaria (see(see an alternationalternation of asexual and sexualsexual reproductionreproduction Smith etet alal.,., 1985, forfor definitiondefinition ofof thesethese lines)lines).. In inin life cyclescyclesof of thethe moremore primitiveprimitive phylaphyla Porifera,Porifera, the Catenulida, asexualasexual reproductionreproduction isis very com-com­ Cnidaria, andand Placozoa,Placozoa, discoverydiscovery ofof a abilaterianbilaterian monmon.. SuppressionSuppression ofof asexualasexual reproductionreproduction intointo thethe that reproduces asexuallyasexually andand thatthat isis asas primitiveprimitive aa juvenile stagestage isisknown known inin the genus RhynchoscolexRhynchoscolex member ofof thethe TurbellariaTurbellaria asas MyomacrostomumMyomacrostomum (Reisinger, 1924)1924);; asexualasexual reproductionreproduction isis notnot ostensibly is has far-reachingfar-reaching consequences forfor known inin thethe marinemarine familyfamily Retronectidae,Retronectidae, butbut thethe questions ofof thethe originorigin ofof thethe BilateriaBilateria as aa whole.whole . reproductive biologybiology asas aa wholewhole ofof thisthis familyfamily isis enigmatic (see(see DoeDoe && Rieger,Rieger, 1977)1977). . InIn thethe Nemertodermatida-Acoela lineline ofof evolution,evolution, asexu-asexu­ Appendix al reproductionreproduction is knownknown onlyonly inin thethe derivedderived orderorder Acoela, butbut inin this groupgroup such reproduction (in(in thethe MyomacrostomumMyomacrostomum unichaetaunichaeta novnov. gengen.. nov.nov. spec.spec. form ofof paratomy) occursoccurs inin a genus (Paratomella) which appearsappears toto bebe mostmost similarsimilar toto thethe stemstem spe-spe­ MaterialMaterial'• 3 male-maturemale-mature specimens and 1 imma­imma- cies onon thethe basisbasis ofof characterscharacters ofof thethe digestivedigestive ture specimen, studiedstudied alivealive;; 1 male-male- andand female-female­ parenchyma (Smith(Smith & &Tyler,lYler, 1985)1985).. mature specimen,specimen, seriallyserially sectioned,sectioned, holotypeholotype Among closercloser relativesrelatives of the Macrostomida, AMNH # #12331233;; 1 male-mature specimen,specimen, seriallyserially specifically inin free-livingfree-livinggroups groups ofof thethe Polycladida­Polycladida- sectionedsectioned;; paratypeparatypeAMNH AMNH ## 12341234.. Neoophoran line,line, asexualasexual reproduction is knownknown LocalityLocality:: sandbank, 1 milemile fromfrom mouthmouth of NewNew only inin the Tricladida, andand herehere thethe likelihoodlikelihood ofof River InletInlet atat New RiverRiver estuaryestuary NN.C.,.C ., USA, secondary originorigin hashas beenbeen interpretedinterpreted asas highhigh (see(see southern shore; finefinesand, sand,0- 0-55 cmcm sediment depth Ax && Schulz,Schulz, 1959)1959).. It is worth notingnoting here,here, at LTL,LTL, summersummer 19721972.. however, thatthat the frequency ofof occurrenceoccurrence ofof fissionfission EtymologyEtymology:: 'Myomacrostomum' - habitus likelike is highesthighest inin thethe Dugesiidae, nextnext highesthighest inin thethe Macrostomum and intestinalintestinal muscle ringring asas inin My-My­ Planariidae andand absentabsent inin the DendrocoelididaeDendrocoelididae ozona and MyozonariaMyozonaria;; 'unichaeta'`unichaeta' -- unpairedunpaired (see Calow,Calow, 1985).1985). FurtherFurther study isis needed, there-there­ tuft ofof caudalcaudal sensorysensory organ.organ . fore, ofof suchsuch structuresstructures asas thethe 'muscle`muscle septum'septum' ofof Description:Description: Male-mature specimens about certain marinemarine proseriatesproseriates (see Karling, 1966;1966 ; ownown 1 mmmm long,long, withwith twotwo zooidszooids (see(see FigFig.. 3b);3b) ; posteriorposterior unpublished observations onon MonocelididaeMonocelididae from zooid with male reproductivereproductive organorgan atat UU 9595 andand in-in­ Bermuda) asas thethe plesiomorphplesiomorph sistersister groupgroup toto thethe cipient secondsecond pharynx (Fig(Fig.3b).. 3b) . TWoTwo specimensspecimens Tricladida.Tricladida . with a secondary divisiondivision planeplane inin processprocess ofof for-for­ Asexual reproduction is,is, of ofcourse, course,very verycommon common mationmation;; aa thirdthird specimenspecimen with secondarysecondary divisiondivision amongamong membersmembers of ofthethe parasiticparasitic classesclasses of thethe plane presentpresent only inin anterior zooid.zooid . MaleMale copula-copula­ Platyhelminthes (Hyman,(Hyman, 1951)1951).. ShouldShould furtherfurther tory organ withwith simplesimple stylet,stylet, 2525 /lmµm longlong (Fig(Fig.. 3d,3d, evidenceevidence support thethe hypothesishypothesis thatthat asexualasexual 6b).6b). TestisTestis smallsmall andand spherical,spherical, closeclose toto thethe leftleft sideside reproductionreproduction isis aa primitiveprimitive featurefeature ofof free-livingfree-living of the styletstylet.. AnAn oocyte occupying aboutabout oneone thirdthird acoelomatesacoelomates (see(see also,also, inin thisthis respect,respect, Berg, 1985,1985, of the body diameterdiameter presentpresent inin bothboth sectionedsectioned for Nemertea),Nemertea), thenthen thisthis groupgroup couldcould bebe seenseen asas es-es­ specimens immediatelyimmediately inin frontfront ofof the male organsorgans.. pecially adaptedadapted for the evolution of parasitismparasitism.. In oneone sectionedsectioned specimen, aa secondsecond malemale reproductive system was foundfound atat thethe caudal endend ofof the anterior zooid.zooid . CiliaryCiliary tufttuft ofof caudalcaudal sensory ConclusionConclusion organorgan 250-250- 350350 /lmµm longlong.. InIn oneone specimen,specimen, aa ciliaryciliary tuft waswas clearlyclearly seen also in the anterioranterior zooid WhileWhile it isis commonlycommonly assumedassumed thatthat asexualasexual (Fig.(Fig . 3b).3b) . OneOne immature specimenspecimen found,found, aboutabout reproductionreproduction arosearose asas a secondary specializationspecialization in 600600 /lIDµm inin length,length, withwith developingdeveloping primary divisiondivision thethe phylum Platyhelminthes,Platyhel,minthes, discoverydiscovery of My­My- planeplane and aa muscle ringring inin each ofof the two develop-develop­ omacrostomumomacrostomum showsshows thisthis assumptionassumption isis notnot war-war- inging zooids;zooids ; caudalcaudal sensorysensory organ clearlyclearly unpaired 4141 A

200 J.Jrn c

200 fJrn

Pig.Fig. 4. Photomicrographs of squeezed animals. A) M. unichaeto,unichaetu, mature specimen; note primary division planeplane andand the secondarysecondary divi·divi- sion plane inin each zooid. B) M. bichtleta;bichuetu; note primary division plane andand two muscle rings (V).(V). C)C) M.M unichtlda,unichaeta, immature specimen; enlargement of caudalcaudal zooid showingshowing muscle ring (V). 4242

Fig. 5. A) M. bichuetu.bichaela. Brain and rhammiterbammile strands. B) B. unichuetu.rmichaela. RbammitcRhammite glands enlarged. in this immature specimen. female-maturefemale-mature (see(see Fig. 3a), studied alive:alive; 1 imma­imma- Colorless, with slightly yellowish brown intes­intes- ture speciment serially sectioned (holotype AMNH tine, and spotted appearance due to rhabdite bun­bun- /I# 1235); 1 immature specimen serially sectioned dles. Rhabdite glands scattered throughout the (para(paratypetype AMNH # 1236). body wall and sunken below the circular muscle Locality:LocaliJy: same as for M. unichaeta. layer; rhabdites appear in bundles. RhammireRhammite Etymology: 'bichaeta'‘bichaeta’ - paired ciliary tufts on glands well developed, extending laterally as far the caudal sensory organ. posterior as the secondary division plane or muscle Description: The largest specimen found was ring and apparently penetrating the brain. Rham­Rham- 600 !tmpm in length [paired nature of the caudal sen­sen- mites 20- 30 pm!tm long. Gland ring of pharynx with sory organ in this specimen determined only on the two distinctly different gland types.types. Rhabdite basis of the asymmetrical position and length of its glands clearly sunken below the circular muscle lay-lay~ one ciliary tuft]; it consisted of two zooids, each er. Position of longitudinal nerve cords could not with a muscle ring (Fig. 3a). Of four specimens ap-ap­ proximately 450 pm in length, one contained struc- be determined. proximately 450 ILm in length, one contained struc­ turestures that could possibly be maturing oocytes, one Myomacrostomum bichaeta nov. spec. with two zooids had no signs of secondary division, one had secondary division planes developing inin MateriakMaterial: 7 immatureimmature specimens, 1J possibly both of twotwo zooids, and the thirdthird had a muscle ring 43

Fig. 6. A) M. unichaetq.unichaetu. Primary division plane, immature specimen. B) Stylet. developing inin a position equivalent to that of the ring with two types of glands. With paired caudal secondary division plane in other specimens. Two1\vo sensory organ, sensory tufts 50-50-100 100 #J-ffipm long. other specimens (one studied alive, one studied in Muscle ring clearly interrupting the gastrodermis serial sections) approximately 350 JLffipm inin length as viewed in sections, a feature not obvious in one consisted also of two zooids and had a secondary of the live specimens (see Fig. 3c).3~). No signs of male division plane in the posterior zooid and a muscle organs in specimens available. ring in the anterior zooid (Fig. 3a). TwolWo specimens studied alive consisted only of a single zooid and Diagnosis of new genus Myomacrostomum were between 250 and 300 pm in length; one of were between 250 and 300 JLm in length; one of these had a typical muscle ring; the other did not Macrostomida with rhammites penetrating the (Fig. 3a). brain. With asexual reproduction by paratomy, cau-cau~ da1 sensory organ, and dolichomacrostomid type With rhabdite bundles scattered throughout the dal sensory organ, and dolichomacrostomid type body wall, as in h4.M. unichaeta. Rhammites of adhesive organ (with flower-like folding of 4- 11 pm long, less than half the size of those in A4. anchor-cell collar). Male copulatory organ with 4-II #-LID long, less than half the size ofthose in M. anchor-eell collar). Male copulatory organ with vesicula seminalis, vesicula granulorum, and unichae/a.unichaeta. They penetrate the brain, but are less vesicula seminalis, vesicula granulorum. and numerous than in M. unichaeta. Pharyngeal gland anteriad-pointing stylet. Unpaired testis justjust in 44

front ofof thethe copulatorycopulatory organ,organ, slightlyslightly displaced toto References the leftleft.. OvaryOvary medial, immediately inin frontfront ofof thethe Ax, P., Studien tiber psammobionte Thrbellaria Macrostomida testistestis.. FemaleFemale gonoducts possiblypossibly lacking.lacking. InIn ma­ma- Ax, P., Studien fiber psammobionte Tbrbellaria Macrostomida IVIV.. MyozonaMyozona styliferastylifera novnov.. spec.spec . Zoo\.Zool . Anz.Anz. 157157:: 251-260.251-260 . rine sands.sands . TYpeType of genus:genus : MM.. unichaeta.unichaeta . Ax, PP.,., 1961.1961 . VerwandtschaftsbeziehungenVerwandtschaftsbeziehungen andund PhylogeniePhylogenie derder TurbellarienTurbellarien.. Ergebn.Ergebn . BioI.Biol. 24:24 : 1-1-6868.. Diagnoses ofof thethe new speciesspecies Ax, PP.,., 1963.1963 . RelationshipsRelationships and phylogenyphylogeny ofof thethe TurbellariaTurbellaria.. In EE.. C.C . Dougherty (ed(ed.),.), The lowerlower metazoametazoa.. University ofof California Press, Berkeley, California:' 191-224. MM.. unichaeta - mature specimens 11 mmmm inin California Press, Berkeley, California : 191-224. Ax, PP.,., 1985.1985 . TheThe position ofof thethe GnathostomulidaGnathostomulida andand Platy-Platy­ length, withwith oneone unpaired caudal sensory organ helminthes in the phylogenetic system of the Bilateria. In S. bearing a ciliary tuft 250-300 ",m long. Rham­ helminthes in the phylogenetic system of the Bilateria . In S. bearing a ciliary tuft 250-300 µm long. Rham- Conway Morris,Morris, JJ.. D.D. George, R.R . Gibson,Gibson, && H.H . M.M . PlattPlatt mites 2020-30- 30 ",mµm longlong.. With characteristicallycharacteristically (eds),(eds), TheThe originsorigins andand relationshipsrelationships ofof lowerlower invertebratesinvertebrates.. Oxford University Press, Oxford. shaped copulatorycopulatory styletstylet (see(see FigFig.. 3d,3d, 6b)6b) 2525 ",mµm inin Oxford University Press, Oxford . Ax, P. & E. Schulz, 1959. Ungeschlechtliche Fortpflanzung length. Ax, P. & E . Schulz, 1959 . Ungeschlechtliche Fortpflanzung length . durch Parat6mie bie acoelen Thrbellarien. Biologischen Zen­ M. bichaeta immature specimens durch Paratomie bie acoelen Tbrbellarien. Biologischen Zen- M. bichaeta - immature specimens tralblatttralblatt 78:78 : 613613-621- 621.. 250-600250-600 µm",m in length,length, withwith pairedpaired caudalcaudal sensory Ballarin,Ballarin, LL.. & LL.. Galleni,Galleni, 19841984.. LarvalLarval developmentdevelopment inin Echinoplana celerrima. nans. am. microsc. Soc. 103: 31-37. organ bearingbearing twotwo ciliaryciliary tufts 50-10050-100 µm",m longlong.. Echinoplana celerrima. Trans. am . microsc. Soc . 103 : 31-37 . Berg, B. 1985. Annulonemertes gen. nov., a new segmented Rhammites 4-114-11 gm",m longlong.. NoNo malemale organsorgans ob-ob­ Berg, B . 1985 . Annulonemertes gen . nov ., a new segmented hoplonemerteanhoplonemertean.. In SS.. ConwayConway Morris,Morris, J1.. D.D. George, RR.. Gib­Gib- served.served . MuscleMuscle ringring onon gutgut inin sectionssections interruptinginterrupting son, & H. M. Platt (eds), The origins and relationships of the gastrodermis. son, & H . M . Platt (eds), The origins and relationships of the gastrodermis . lowerlower invertebratesinvertebrates.. Oxford UniversityUniversity Press,Press, Oxford.Oxford . Bresslau,Bresslau, E.E . && E.E. Reisinger, 19281928.. Plathelminthes.Plathelminthes . InIn W.W. Ktiken­Kfiken- thai & T. Krumbach (oos), Handbuch der Zoologie, Vo\. 2, Pt Discussion ofof speciesspecies differencesdifferences thal & T. Krumbach (eds), Handbuch der Zoologie, Vol. 2, Pt Presently available data indicates clearly that the 1,I, pp.pp. 3434-51- 51.. Presently available data indicates clearly that the Calow, P. & D. A. Read, 1985. Ontogenetic patterns and two species described here are indeed distinct spe­ Calow, P. & D. A . Read, 1985 . Ontogenetic patterns and two species described here are indeed distinct spe- phylogeneticphylogenetic trendstrends inin freshwaterfreshwater flatwormsflatworms (Tricladida)(Tricladida);; con­con- cies, eveneven withwith thethe lacklack ofof information onon genitalgenital straintstraint oror selection? ThisThis volume.volume . organs of MM. bichaetabichaeta.. DifferencesDifferences inin otherother featuresfeatures Doe,Doe, DD.. A.A. && R. M.M. Rieger, 19771977.. A newnew speciesspecies ofof thethe genusgenus Retronectes (Thrbellaria, Catenulida) from the coast of North are clearly ofof thethe sortsort knownknown toto bebe diagnosticdiagnostic forfor Retronectes (Tbrbellaria, Catenulida) from the coast of North Carolina,Carolina, USA.USA . MikrofaunaMikrofauna Meeresbodens 6666:: 549-556.549-556 . other species ofofMacrostomida Macrostomida [e[e.g.,.g ., paired and un-un­ Ehlers,Ehlers, u.,U., 1985.1985 . Phylogenetic relationshipsrelationships withinwithin thethe Platyhel-Platyhel­ paired nature of thethe caudalcaudal sensorysensory organorgan appears minthesminthes.. In SS.. Conway Morris,Morris, J1.. D.D. George, RR.. Gibson, & toto bebe aa genericgeneric characteristiccharacteristic ofof Dolichomacro­Dolichomacro- HH.. M.M . Platt (eds),(eds), TheThe origins and relationshipsrelationships ofof lowerlower in-in­ vertebrates. Oxford University Press, Oxford. stomidaestomidae (see(see RiegerRieger && Tyler,Tyler, 1974)]1974)].. RhammiteRhammite vertebrates. Oxford University Press, Oxford . length is significantly different in the two (by Hyman,Hyman, LL.. H.,H ., 19511951.. The InvertebratesInvertebrates.. IIII.. PlatyhelminthesPlatyhelminthes andand length is significantly different in the two (by Rhynchocoda. McGraw-Hill, New York: 550 pp. 3-fold), moreso than is expected of variation with­ Rhynchocoela. McGraw-Hill, New York : 550 pp. 3-fold), moreso than is expected of variation with- Ivanov,Ivanov, A.A. V.V. && YuYu.. VV.. Mamkaev,Mamkaev, 19731973.. The turbellariansturbellarians (Tur-(Thr­ in a singlesingle speciesspecies.. However,However, becausebecause wewe areare deal-deal­ bellaria)bellaria) -- TheirTheir originorigin andand evolutionevolution.. [In Russian]Russian] Akad.Akad . inging here with aa unique and newnew genusgenus havinghaving NaukaNauka USSR,USSR, Leningrad,Leningrad, 221221 pppp.. Karling T. G., 1966. On the defecation apparatus in the genus asexual reproduction, wewe cannotcannot entirelyentirely excludeexclude Karling T. G ., 1966 . On the defecation apparatus in the genus ArchimonocelisArchimonocelis (Tbrbellaria,(Thrbellaria, Monocelididae)Monocelididae).. SarsiaSarsia 2424:: thethe possibilitypossibility that these two speciesspecies actuallyactually be-be­ 37-4437-44.. long toto aa singlesingle speciesspecies with aa complexcomplex lifelife cyclecycle Karling,Karling, TT.. G.,G., 1974.1974 . On thethe anatomyanatomy andand affinitiesaffinities ofof thethe tur-tur­ alternating asexualasexual and sexualsexual phasesphases.. bellarianbellarian ordersorders.. In NN.. W.W. Riser && M.M . PP.. Morse (eds),(eds), Biology ofof thethe TbrbellariaThrbellaria.. McGraw-Hill, New YorkYork:: 1-161-16.. Marcus,Marcus, EE.,., 1948.1948 . ThrbellariaTbrbellaria do BrasilBrasil.. Zoologia UnivUniv.. SaoSiloPau-Pau­ lo Brasil 13: 111-244. Acknowledgments lo Brasil 13 : 111-244 . Acknowledgments Palmberg,Palmberg, II.. & MM.. Reuter,Reuter, 19831983.. AsexualAsexual reproductionreproduction inin MicrostomumMicrostomum linearelineare (Tbrbellaria)(Thrbellaria).. II.. An autoradiographicautoradiographic TheThe authorauthor wouldwould likelike to acknowledgeacknowledge thethe loanloan ofof andand ultrastructuralultrastructural studystudy.. IntInt.. JJ.. InvertInvert.. Reprod.Reprod. 66:: 197197-206- 206.. Reisinger, E., 1924. Der Gattung Rhynchoscolex. Z. Morpho!. EE.. Westblad'sWestblad's sectionedsectioned Macrostomid-materialMacrostornid-material Reisinger, E ., 1924 . Der Gattung Rhynchoscolex . Z . Morphol. Oekol.. TiereTiere 1 I:: 1-371-37.. fromfrom thethe NaturhistoriskaNaturhistoriska Riksmuseet,Riksmuseet, Stockholm.Stockholm Oekol . Reuter,Reuter, MM.. && II.. Palmberg,Palmberg, 19831983.. AsexualAsexual reproductionreproduction inin ThisThis workwork waswas supportedsupported byby NSF-grantsNSF-grants DEBDEB MicrostomumMicrostomum linearelineare (Tbrbellaria)(Thrbellaria).. IIII.. The nervousnervous systemsystem inin 81196528119652 (R(R.. M.M . Rieger PP.I.).I .) andand BSRBSR 81168948116894 (S(S.. thethe divisiondivision zonezone.. IntInt.. JJ.. InvertInvert.. ReprodReprod.. 66:: 207-217207-217.. TylerTYler PP.I.)..I .) . Rieger,Rieger, RR.. M.,M ., 1971a1971a.. Die Turbellarienfamilie Dolichomacro­Dolichomacro- stomidaestomidae novnov. . famfam.. (Macrostomida)(Macrostomida).. II. . TeilTei!.. Vorbemer-Vorbemer­ kungenkungen andund KarlingiinaeKarlingiinaennov.ov . subfam.subfam . II.. ZoolZoo!.. JahrbJahrb.. AbtAbt.. SystSyst.. OekolOeko!.. GeogrGengr.. TierTieree 9898:: 236-314.236-314 . 45

Rieger,Rieger, R.R . M.,M ., 1971b.1971b . DieDie TurbellarienfamilieTurbellarienfamilieDolichomacro­ Dolichomacro- Smith, J.J. P.P. S.S . && S.S . lYler,Tyler, 1985.1985 . TheThe acoel turbellarians:turbellarians : kingpinskingpins stomidaestomidae RiegerRieger.. II.II . Teil.Teil . DolichomacrostominaeDolichomacrostominae II.. Zoo!.Zool . of metazoan evolutionevolution oror aa specializedspecialized offshoot? InIn SS. . Con­Con- Jahrb.Jahrb . AbtAbt Syst.Syst . Oekol.Oekol . Geogr.Geogr . TiereTiere 98:98 : 569569-703-103.. way Morris,Morris, JJ.. D.D . George,George, R.R . Gibson,Gibson, && H.H . M.M . Platt (eds),(eds), Rieger, R.R. M.,M ., 1971c.1971c . BradynectesBradynectes sterreristerreri gen.gen . nov.,nov., spec.spec. nov.,nov., The originsorigins andand relationshipsrelationships ofof lowerlower invertebratesinvertebrates.. OxfordOxford eine neueneue psammobiontepsammobionte MacrostomideMacrostomide (Thrbellaria)(lbrbellaria).. Zool.Z ool . University Press, Oxford.Oxford . Jahrb.Jahrb . Abt.Abt . Syst.Syst . Oekol.Oekol . Geogr.Geogr. TiereTiere 98:98 : 205-235.205-235 . Smith, JJ.. P.P. S.,S ., S.S. lYler,Tyler,& & R.R . M.M. Rieger,Rieger, 1985.1985 . IsIs thethe ThrbellariaTurbellaria Rieger, R.R . M.,M ., 1981.1981 . MorphologyMorphology ofof thethe TurbellariaTurbellaria atat thethe ultra-ultra­ polyphyletic? ThisThis volume.volume . structuralstructural levellevel.. HydrobiologiaHydrobiologia 84:84 : 213-229.213-229 . Tyler, S.,S., 1977.1977. UltrastructureUltrastructure andand systematics:systematics: anan exampleexample from Rieger, R.R. M.M . && S.S. lYler,Tyler, 1974.1974. AA newnew glandular sensory organorgan turbellarian adhesiveadhesive organs.organs . MikrofaunaMikrofauna Meeresboden 6161:: inin interstitialinterstitialMacrostomida. Macrostomida. I.I . Ultrastructure.Ultrastructure. MikrofaunaMikrofauna 271-286.271-286 . MeeresbodenMeeresboden 42:42 : 1-1-4141..