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FULL PAPER Wildlife Science

Positional strategy of trunk muscles among aquatic semi-aquatic and terrestrial species in Urodela

Ayano OMURA13) Wataru ANZAI 23) Daisuke KOYABU3) and Hideki ENDO3)

1)Graduate School of Agricultural and Life Sciences The University Museum The University of Tokyo 7ndash3ndash1 Hongo Bunkyo-ku Tokyo 113ndash0033 Japan

2)Graduate School of Biological Sciences The University Museum The University of Tokyo 7ndash3ndash1 Hongo Bunkyo-ku Tokyo 113ndash0033 Japan

3)The University Museum The University of Tokyo 7ndash3ndash1 Hongo Bunkyo-ku Tokyo 113ndash0033 Japan

(Received 21 June 2014Accepted 23 March 2015Published online in J-STAGE 6 April 2015)

ABSTRACT Clarificationof thetrunkstructureinUrodelais important inunderstandingthelocomotiveevolutionofbasal tetrapodsThecomponentsofthemusculartrunkwallamongUrodelausingdifferentmodesoflocomotionwerecomparedSincethewholetrunkmaybeusedforswimmingandtheeffectoflimbsmaybesmallinthemoreaquaticspeciestheyshowedsmallerdifferencesinthetrunkmusclesamonganteriormiddleandposteriorsectionsofthetrunkBycontrastinthemoreterrestrialspeciesthedorsalandabdominalmusclesarelargerinthemiddlesectionthanthoseintheanteriorandposteriorsectionsHighcompressivestressesoccurinthesupportinglimbsandtheirinsertionatthetrunkontheventralsideandspreadfromtheforelimbsalongthebacktothesupportinghindlimbsonthedorsalsideTensilestressesoccurinthemiddleventralpartThecomponentsofthetrunkmusclesamongthethreesectionsmayreflectdifferencesinstressesoccurringinthetrunkofthemoreterrestrialspeciesThefindingsalsosuggestthatinthemiddlesectionlargerdorsalmusclesforstiffeningthebacktomaintainpostureandlargerabdominalmusclesareresponsibleforbalancingthebodyweightwhileitissupportedbythelimbsinthemoreterrestrialspeciesKEY WORDS axialmuscleCaudatalocomotionsalamanderUrodela

doi101292jvms14-0320 J Vet Med Sci 77(9) 1043ndash1048 2015

Urodeleamphibiansareoftenchosenasmodels to infertheevolutionoflocomotionofearlytetrapodsbecausetheypossessaldquogeneralizedrdquobodyformandamphibiouslifestyle[5] Urodela use their body in an undulatory manner forlocomotionbothinwaterandonground[5]Forunderstand-ingthelocomotiveevolutionofbasaltetrapodsclarificationofthetrunkstructureinurodelesisessentialThe relationship between trunk muscles and ecological

habitats was previously investigated by examining cross-sectionsofmid-trunk[18]Thoseauthorssuggestedthatter-restrialspecieshavethinnerhypaxialmuscles thanaquaticspeciesOmuraet al[1314]quantifiedtheweightratiosatthemiddleofthetrunkTheydividedthetrunkmusclesintothreecomponentsnamelydorsalmuscles lateralhypaxialmusclesandabdominalmuscleTheyshowedthatmoreter-restrial species possess larger dorsalmuscles and a largerabdominalmusclewhichisseparatedfromlateralhypaxialmuscles by fascia whereas aquatic species possess largerandthickerlateralhypaxialmusclesandasmallerabdominalmusclewhichisnotclearlyseparatedfromlateralhypaxialmusclesbyfasciaItwassuggestedthatthemoreterrestrial

speciesusetheirenlargeddorsalandabdominalmusclestostabilize the body and to sustain the animalrsquos ownweightagainstgravitywhereastheaquaticspeciesuselargerlateralhypaxialmusclesaspowerful locomotorapparatus for lat-eralbendingduringswimming[1314]These studies regarded the trunk as one unit during

quantificationHowever the internal structureof the trunkmay differ according to positions because muscle usageand structure differ according to the environmentThus aquantificationofthedifferencesbetweenthecomponentsoftrunkmusclesamong the locomotor types is requiredThemethodsofcopingwithbuoyancyandgravityconsiderablydifferbetweenaquaticand terrestrialspecies [11]Thede-greeoflimbdevelopmentalsodiffersamongaquaticsemi-aquaticandterrestrialspeciesandthesefactorsmayaffectthestructureofthetrunkBecausethecomponentsoftrunkmusculature differ according to the effects of gravity andthedistancefromthe limbsobservationsofonly themid-trunk region and comparison of thewholeweight of eachtrunkmusclearenotsufficienttounderstandthefunctionalsignificanceoftrunkstructureTrunkmusclesshouldbeaf-fectedandvaryinsizedependingontheirpositiontheusemadeofthetrunkandtheexternalforcesactingagainstthetrunk[17]Furthermoretherelativelengthofthetrunkdif-fersamongurodelespeciesalthoughtheyallhaveelongatedbodies During swimming elongated tetrapods performanguilliformlocomotion[26819]Gary[7]showedthatsirenidsalamandersswimbypropagatingundulatorywavesposteriorlyalongthebodyIncontrastdifferentstressesoc-curdependingontrunkpositioninterrestrialtetrapodswith

CorrespondenCe to OmuraAGraduateSchoolofAgriculturaland Life Sciences The UniversityMuseum The University ofTokyo7ndash3ndash1HongoBunkyo-kuTokyo113ndash0033Japan e-mailayanodesuyoroshikuyahoocojpcopy2015TheJapaneseSocietyofVeterinaryScienceThisisanopen-accessarticledistributedunderthetermsoftheCreativeCommons Attribution Non-Commercial No Derivatives (by-nc-nd)Licenselthttpcreativecommonsorglicensesby-nc-nd30gt

A OMURA W ANZAI D KOYABU AND H ENDO1044

elongatedbodies[9]Monitorlizardswhichhaveelongatedbodies similar to salamanders are under the influences ofdownwardgravityandsustainbodyweightbyventralmus-cleswhentheyrestquadrupedallyontheir2pairsoflimbs[16]Herethetypesofstress(tensionorcompression)andtheamountofstressvarywithbodyposition[16]A previous study examined the fiber types of trunk

muscles in 2 terrestrial salamandersAmbystoma tigrinum and A maculatum [17] The distribution of fiber types ofperivertebral muscles along the body axis was similar inthe 2 terrestrial speciesHowever only the distribution offiber type was examined in that study and the positionaldifferenceofalltrunkmuscleswasnotincludedThustherelationshipbetweenthepatternsofvariationinmuscleposi-tion and habitatswas unclear In this studywe comparedthedifferencesofcomponentsoftrunkmusculatureamongdifferentmodesoflocomotionamongUrodelawithdifferenthabitats to examine the relationships between locomotivemodeandtrunkmorphology

MATERIALS AND METHODS

Sixspeciesofadultsalamandersrepresenting5familiesand3differenthabitats(aquaticsemi-aquaticandterrestrial)wereusedinthisstudy(Table1)Thespecimenswereindi-vidualsamplesfromthecollectionbelongingtoDrOmuraThespecimenswerefixedinastraightbodypositionin10formalinandweremaintainedin70ethanolsolutionCross-sectionsincludingonevertebrawerecutfromthe

anteriormiddleandposteriorpartsof thefixedspecimensusingascalpel(Fig1)Theanteriorsectionwascutposte-riortothefrontlimbsTheposteriorsectionwascutfromthefrontofthepelvicgirdleThemiddlesectionwascutfromapositionhalfwaybetweentheanteriorandposteriorsectionsCross-sectional images were recorded using a single-lensreflex cameraTrunkmuscleswere treated as 3 groups ofmusclesnamelythedorsalmuscleslateralhypaxialmusclesandabdominalmusclesfromthefunctionsandpositionsac-cording toNaylor [12] andOmura et al [13 14] Dorsal muscles lateral hypaxial muscles and abdominal muscleswereexaminedfromthecross-sectionalimages(Fig2) The areasofthesemusclegroupsweremeasuredonthedigitalimageof thecross-sectionusingPhotoshopCS5Thearearatioofeachmusclegrouptotheareaofalltrunkmusclesinonesectionwascalculatedtodeterminethecomponentrateofeachmusclegroup

Statistical analyses were performed to test significant dif-ferences in the muscle area ratios among Urodela of different lifestylesaquaticsemi-aquaticandterrestrialHomogeneityofvariancesandmeansbetweenspecieswereexaminedbyanalysisofvariance(ANOVA)WhensignificantdifferencesweredetectedbyANOVAthesedifferenceswereidentifiedbyusingTukeyrsquostest

RESULTS

The differences of trunk muscles among the locomo-tormodesare shown inTable2andFig2Withinaquatic

species thedifferencesof trunkmusclesweresmaller thanthosewithin terrestrial species InS intermedia the dorsal musclesaccountedforapproximately53ofthetotalmus-culature on all sections S intermediahadalmost43lateralhypaxialmusclesandapproximately3abdominalmuscleon all sections In A tridactylumnosignificantdifferenceswere observed among the three sections in dorsalmusclesandlateralhypaxialmusclesInadditionalthoughsignificantdifferences between the anterior andmiddle sections wereobservedintheabdominalmuscletherangeinarearatiosoftheabdominalmuscleinallsectionswasnarrow(32ndash39)In C pyrrhogastertherewasnosignificantdifferenceamongthe 3 sections of the dorsal muscles The lateral hypaxialmusclesweresignificantlylargerinthemiddlesectionthanintheanteriorandposteriorsectionsandtheabdominalmusclewas smaller in the anterior sectionof the trunk than in themiddleandposteriorsectionsInC ensicauda lateral hypaxi-almuscleswerelargerintheanteriorandposteriorsectionsofthetrunk(formedover37)thaninthemiddlesectionInH nigrescens the abdominalmusclewas significantly smallerintheanteriorandposteriorsectionsofthetrunkthaninthemiddlesectionThedorsalmuscleswerelargerinthemiddlesection(over67)thanintheanteriorandposteriorsectionsInadditionsignificantlylargerlateralhypaxialmuscleswerepresentintheanteriorandposteriorsections(over30)thanin the middle section Finally the abdominal muscle wasfoundtobelargerinthemiddlesection(over15)thanintheanterior and posterior sections in H nigrescens A tigrinum hadsignificantlylargerdorsalmusclesinthemiddlesection(over37)thanintheanteriorandposteriorsectionsLateralhypaxialmuscleswerelargerintheanteriorandposteriorsec-tionsthaninthemiddlesectioninA tigrinumTheabdominalmusclewaslarger(over12)inthemiddlesectionthanintheanteriorandposteriorsectionsofthetrunk

Table1 Specimensusedinthisstudy

Species Habitat SVL(mm)Siren intermedia Aquatic 261

230242

Amphiuma tridactylum Aquatic 408434471

Cynops pyrrhogaster Semi-aquatic 534845

Cynops ensicauda Semi-aquatic 525154

Hynobius nigrescens Terrestrial 707273

Ambystoma tigrinum Terrestrial 11110386

Snout-ventlength

TRUNK MUSCLES IN URODELA 1045

DISCUSSION

InwaterUrodelahavetocopewiththedensityandvis-cosityofwaterWhentheyswimtrunkmusclesproduceatravelingwaveoflateralflexions[6]Fromanelectromyo-graphical studyweknowthatldquoactivation travels froman-teriortoposteriorpartoftrunk[6]rdquoIncontrastongroundUrodelahavetokeeptheirposturebysustainingtheirownweightbecauseoftheeffectofgravityFurthermorewhentheywalkongroundgroundreactionforcesariseasacon-sequence of gravity and locomotion in the limbs [4]Theanimalrsquosbodyhastoresistgroundreactionforces[4]There-fore tensile aswell as compressive stresses derived from

bendingofthetrunkarelargerinterrestrialthaninaquaticforms[16]Inthisstudythedifferencesinthecomponentsoftrunk

musclesamongthe3sectionsofthetrunkweresmallerinthemoreaquatic species than those in themore terrestrialspecies In themore terrestrial species themiddle sectionpossessed larger dorsal and abdominal muscles than theanterior and posterior sectionsAquaticspecieshaveelongatedbodiesandless-developed

limbs for anguilliform swimming that means by flexingtheirbodysteminamedio-lateralplaneTailpossessingam-phibiansswimusingaposteriortravelingwavealongtheirbodies [5 6] This suggests that aquatic species may use

Fig1 A Lateral view of skinned Ambystoma tigrinum B Cross-sectional view of anterior part of trunk CCross-sectionalviewofmiddlepartoftrunkCCross-sectionalviewofposteriorpartoftrunkScalebar=5mm

A OMURA W ANZAI D KOYABU AND H ENDO1046

theirwholetrunkforswimmingAlthoughaquaticorsemi-aquatic speciesalsoutilize their limbs foraquaticwalkingonthesubstrata[13]groundreactionforcesduringaquaticwalkingareveryprobablysmallerthanthoseduringterres-trialwalkingAmphiuma and Sirenpossessgreatlyreducedlimbs[10]andtheirlimbsaresuggestedtobefunctionallyineffectual[15]Inaccordancewiththiswehavefoundthatamongthe3sectionsofthetrunkminordifferencesoccurinthemoreaquaticspeciescomparedtothoseinthemoreterrestrial speciesBy contrast on the ground themore terrestrial species

needtoresistdownwardgravityStressesdifferaccordingtothesectionsofthetrunkinterrestrialspecies[16](Fig3) Inthedorsalmusclesmaintainingposturerequirestensionabove the shoulder and pelvic region The length of seg-ments in which the tensile forces are required can differaccording to locomotor habitats limb lengths or distancebetweenshoulderandpelvicjointThemiddleofthetrunk

tendstobebentdorsallyconcavebygravityandthesectioncanbeshorterorlongerInthemiddlesectiontensilestressoccursontheventral

side[16](Fig3B)andtheabdominalmusclepreventsdor-sally concave against gravityDorsalmuscles stabilize thebodyinadorso-ventralplane[15]ThedorsalmusclesmustbeactivewhenthetailisliftedfromthegroundThismaycausedorsalandabdominalmusclesinthemiddlesectiontobelargerthantheanteriorandposteriorsections

In the anterior and posterior sections above the shoulder andpelvicregionsaresubjecttotensilestress(Fig3Aand3C)Another studyhas shown that compressive stress oc-curs between the supporting forelimbs on the ventral side[9] Since stronger pectoral and pelvic girdles are subjecttohigherstress theresponsibilityofresistingtheeffectofgravityandstressmaybesmallerthanthatinthemiddlesec-tionThiswouldexplainwhyourfindingsshowedthatthedorsalandabdominalmusclesintheanteriorandposterior

Fig2-1Themajorcomponentsofthetrunkmusculatureamong3lifestylesaquaticsemi-aquaticandterrestrial(ANOVAandTurkeyrsquostestPgt005)

TRUNK MUSCLES IN URODELA 1047

sectionsweresmallerthanthoseinthemiddlesectionofthemoreterrestrialspeciesInconclusionthedifferencesbetweenthemajorcompo-

nentsofthetrunkmusculatureobviouslydifferaccordingtohabitatsandlocomotioninurodelesOurfindingsclarifytherelationshipsbetween locomotormode and trunk structurein basal tetrapods

ACKNOWLEDGMENTSWethankDrShin-ichiroKawa-daofNationalMuseumofScienceandNatureTokyo forgivingusforvaluablespecimensWealsothankDrYukioAizawaoftheNipponDentalUniversityDrKenjiroKine-buchiofNiigataSeiryoUniversityDrNatsuhikoYoshikawaofKyotoUniversityandDrYasuchikaMisawaforprovidingspecimensWethankDrTakenoriSasakiofTheUniversityMuseumTheUniversityofTokyoforlendingsomeequip-ment for functionalmorphologicalanalysisWealso thankMrTakashiYoshimineofMicroscopeNetworkforproviding

Fig2-2Themajorcomponentsofthetrunkmusculatureamong3lifestylesaquaticsemi-aquaticandterrestrial(ANOVAandTurkeyrsquostestPgt005)

Fig3 Restingquadrupedon2pairsoflimbswithdistributionofstresses derived from bodyweightWhite arrows show tensionBlackdistributionsrepresentcompressionModifiedfromPreus-choftet al [16]

A OMURA W ANZAI D KOYABU AND H ENDO1048

adapterofmicroscopeWethankAsazooDrYukiTaguchiMsAyakoNodaandMrJunHataseforencouragingusandprovidingsomespecimens

REFERENCES

1 AziziEandHortpnJM2004Patternofaxialandappendicu-larmovementsduringaquaticwalkinginthesalamanderSiren lacertina Zoology 107 111ndash120 [Medline] [CrossRef]

2 AndrewRB1977Themuscularcontrolofvertebrateswim-mingmovementsBiol Rev Camb Philos Soc 52 181ndash218 [CrossRef]

3 BrandLR1996VariationsinsalamandertrackwaysresultingfromsubstratedifferencesJ Paleontol 70 1004ndash1010

4 CarrierDR1993Actionofthehypaxialmusclesduringwalk-ingandswimming in thesalamanderDicamptodon ensatus J Exp Biol 180 75ndash83

5 DebanSMandSchillingN2009ActivityoftrunkmusclesduringaquaticandterrestriallocomotioninAmbystoma macula-tum J Exp Biol 212 2949ndash2959 [Medline] [CrossRef]

6 Gary BG 1996Undulatory locomotion in elongate aquaticvertebratesanguilliformswimmingAm Zool 36 656ndash665

7 Gary B G 1997 Anguilliform locomotion in an elongatesalamnder (Siren intermedia)effectof speedonaxialundula-torymovementsJ Exp Biol 200 767ndash784 [Medline]

8 GrayJ1933Themovementoffishwithspecialreferencetotheeel J Exp Biol 10 88ndash104

9 Hohn-SchulteBPreuschoftHWitzelUandDistler-Hoff-mannC2013Biomechanicsandfunctionalpreconditionsforterrestriallifestyleinbasaltetrapodswithspecialconsiderationof Tiktaalik roseae Historical Biology Int J Paleobiol 25 1ndash15

10 KentwoodDW2007TheEcologyandBehaviorofAmphib-iansUnivChicagoPressChicago

11 LiemKFWalkerWFBemisWEandGrandeL2001FunctionalAnatomy of the Vertebrates an Evolutionary Per-spectiveHarcourtCollegePressPhiladelphia

12 NaylorBG1978TheSystematicsofFossilandRecentSala-manders (Amphibia Caudata) -with Special Reference to theVertebralColumn andTrunkMusculatureUnpublishedPhDThesisUniversityofAlberta

13 OmuraAAnzaiWandEndoH2014aFunctionalandmor-phologicalvarietyintrunkmusclesofUrodelaJ Vet Med Sci 76 159ndash167 [Medline] [CrossRef]

14 Omura A Ejima K Honda K Anzai W Taguchi YKoyabu D and Endo H 2014bLocomotionpatternandtrunkmuisculoskeletal architecture among Urodela Acta Zool (in press)

15 OrsquoReillyJCSummersAPandRitterDA2000Theevo-lutionofthefunctionalroleoftrunkmusclesduringlocomotioninadultamphibiansAm Zool 40 123ndash135 [CrossRef]

16 PreuschoftHSchulteDDistlerCWitzelUandHohnB2007BobyshapeandlocomotioninmonitorlizardsMertensi-ella 16 59ndash78

17 SchillingNandDabanSM2010Fiber-typedistributionoftheperivertebralmusculatureinAmbystomaJ Morphol 271 200ndash214 [Medline]

18 SimonsRS andBrainerdEL1999Morphologicalvaria-tion of hypaxial musculature in salamanders (LissamphibiaCaudata) J Morphol 241 153ndash164 [Medline] [CrossRef]

19 WardleCSVidelerJ JandAltringhamJD1995Turn-ingintofishswimmingwavesbodyformswimmingmodeandmusclefunctionJ Exp Biol 198 1629ndash1636 [Medline]

Table2 Muscle area ratios () measured at three parts in trunk(meanplusmnSEM)

Species Positionoftrunk Muscle area ratioSiren intermedia dorsalmuscles anterior 534plusmn07

middle 532plusmn04posterior 516plusmn09

lateral hypaxial muscles

anterior 437plusmn07middle 423plusmn14posterior 442plusmn14

abdominalmuscle

anterior 27plusmn01middle 33plusmn02posterior 28plusmn02

Amphiuma tridactylum

dorsalmuscles anterior 526plusmn05middle 532plusmn04posterior 516plusmn09

lateral hypaxial muscles

anterior 441plusmn07middle 428plusmn05posterior 447plusmn09

abdominalmuscle

anterior 32plusmn02middle 39plusmn01posterior 36plusmn02

Cynops pyrrhogaster

dorsalmuscles anterior 578plusmn19middle 625plusmn21posterior 576plusmn33

lateral hypaxial muscles

anterior 359plusmn17middle 269plusmn15posterior 334plusmn29

abdominalmuscle

anterior 62plusmn01middle 104plusmn05posterior 89plusmn11

Cynops ensicauda dorsalmuscles anterior 550plusmn03middle 592plusmn14posterior 536plusmn03

lateral hypaxial muscles

anterior 373plusmn09middle 308plusmn11posterior 386plusmn04

abdominalmuscle

anterior 76plusmn07middle 99plusmn02posterior 77plusmn06

Hynobius nigrescens

dorsalmuscles anterior 575plusmn09middle 677plusmn12posterior 596plusmn05

lateral hypaxial muscles

anterior 332plusmn17middle 169plusmn19posterior 306plusmn12

abdominalmuscle

anterior 92plusmn09middle 153plusmn31posterior 98plusmn08

Ambystoma tigrinum

dorsalmuscles anterior 627plusmn03middle 731plusmn10posterior 629plusmn09

lateral hypaxial muscles

anterior 293plusmn05middle 145plusmn12posterior 288plusmn06

abdominalmuscle

anterior 79plusmn05middle 123plusmn06posterior 82plusmn06