specimens : five species of adult salamanders representing five families 
 were examined in this study ( table 1table 1.specimens used in this studyspecieshabitatsvl * ( mm)amphiuma tridactylumaquatic408434471necturus maculosusaquatic175172168cynops pyrrhogastersemi - aquatic534845hynobius nigrescensterrestrial707273ambystoma tigrinumterrestrial11110386*snout - vent length . ) .
aquatic species included amphiuma tridactylum ( n=3 ) and 
 necturus maculosus ( n=3 ) , which have elongated bodies and shortened limbs 
 ( a. tridactylum more so than n. maculosus ) .
semi - aquatic 
 species were represented by cynops pyrrhogaster ( n=3 ) , which has a short 
 body and relatively robust limbs .
terrestrial species included hynobius 
 nigrescens ( n=3 ) and ambystoma tigrinum ( n=3 ) , which have short 
 trunks and robust limbs .
specimens were fixed in straight body position using 10% formalin and 
 preserved in 70% ethanol solution .
m. dorsalis trunci 
 makes up most of the epaxial muscle mass and m. interspinalis , m. 
 intertransversarius and m. subvertebralis are perivertebral 
 muscles .
m. obliquus externus , m. obliquus externus 
 superficialis , m. obliquus externus profundus , m. 
 obliquus internus and m. transversus abdominis are grouped into 
 lateral hypaxial muscles , and m. rectus lateralis is found along the trunk 
 on the lateral hypaxial muscles .
the following trunk muscles were examined in this study : m. dorsalis 
 trunci , m. interspinalis , m. 
 intertransversarius , m. subvertebralis , m. obliquus 
 externus , m. obliquus externus superficialis , m. 
 obliquus externus profundus , m. obliquus internus , m. 
 transversus abdominis , m. rectus profundus , m. rectus 
 lateralis and m. rectus abdominis . each trunk muscle on the left 
 side
was dissected using tweezers in the five species , and specimens were observed from the 
 lateral view ( fig .
1.ambystoma tigrinum ( a ) : lateral view of trunk musculature ( b ) : 
 dorsal view of trunk musculature after removal of m. dorsalis trunci . 

subsequently , cross - sections were obtained from the middle of the trunk between the 
 pectoral and pelvic girdles on the right side of the body .
images of the lateral view and 
 cross - sections were observed using a microscope with a single - lens reflex camera .
ambystoma tigrinum ( a ) : lateral view of trunk musculature ( b ) : 
 dorsal view of trunk musculature after removal of m. dorsalis trunci . 

, specimens were moistened with water to avoid drying , which may cause 
 measurement error .
each trunk muscle was weighed to an accuracy of 0.01 mg using an 
 electronic balance shimadzu auw220d ( simadzu , kyoto , japan ) .
the ratio of the weight of each 
 muscle to the weight of all trunk muscles was calculated as a percentage . in this study , to 
 calculate these ratios , the weight of all trunk muscles was treated except that of 
 m. rectus lateralis , because of its small size . for distinction among the 
 five species , values for some muscles
were grouped according to anatomical position , running 
 direction and function : m. obliquus externus superficialis and m. 
 obliquus externus profundus ; m. obliquus internus and m. 
 transversus abdominis ; m. rectus profundus and m. rectus 
 abdominis .
these tests 
 were conducted using statistical processing software ( jmp pro 9 , sas institute japan , tokyo , 
 japan ) .
homogeneity of variance and means between species were confirmed by analysis of 
 variance ( anova ) .
when significant differences were identified by anova , differences between 
 species were estimated using tukey s test .
francis , naylor 
  and maurer [ 12 , 13 ] were used as references for the 
 following muscle descriptions .
two superficial lateral hypaxial layers consist of m. obliquus 
 externus superficialis and m. obliquus externus profundus .
these 
 muscle fibers run from the craniodorsal region caudoventrally towards the pubic bone . 

m. obliquus internus and m. transversus abdominis make 
 up the inner lateral hypaxial layers .
these muscle fibers run from the cranioventral trunk 
 region to the dorsal parts of the hip girdle .
some species have m. obliquus externus instead of m. obliquus 
 externus superficialis and m. obliquus externus profundus . 

observations of muscle morphology : cross - sectional areas and lateral views 
 of the trunk muscles for the five species of salamanders are shown in figs . 2 and 3fig .
2.cross-sections through the midtrunk of ( a ) : amphiuma tridactylum , 
 ( b ) : necturus maculosus , ( c ) : cynops pyrrhogaster , 
 ( d ) : hynobius nigrescens , ( e ) : ambystoma tigrinum . 
 scale bar=5 mm.fig .
3.lateral view of ( a ) : amphiuma tridactylum , ( b ) : necturus 
 maculosus , ( c ) : cynops pyrrhogaster , ( d ) : hynobius 
 nigrescens , ( e ) : ambystoma tigrinum .
the number of muscle layers and specialization of each muscle differed 
 by species . in a. tridactylum , an aquatic species , four layers ( m. 
 obliquus externus superficialis , m. obliquus externus profundus , 
 m. obliquus internus and m. transversus abdominis ) 
 composed the lateral hypaxial musculature .
m. rectus abdominis in 
 a. tridactylum could not be distinguished from m. obliquus 
 externus profundus and m. obliquus internus .
the lateral 
 hypaxial muscles of a. tridactylum were relatively thick and covered the 
 m. dorsalis trunci . in n. maculosus , an aquatic species , 
 three layers ( m. obliquus externus , m. obliquus internus 
 and m. transversus abdominis ) made up the lateral hypaxial muscles , which 
 were relatively thick .
the m. rectus abdominis of n. 
 maculosus was not separated from m. obliquus externus and 
 m. obliquus internus .
the lateral hypaxial muscles of c. 
 pyrrhogaster , a semi - aquatic species , consisted of four layers : m. 
 obliquus externus superficialis , m. obliquus externus profundus , 
 m. obliquus internus and m. transversus abdominis .
m. rectus abdominis was easily separated from m. obliquus 
 externus profundus and m. obliquus internus .
nigrescens , a terrestrial species , two layers were evident in the lateral 
 hypaxial muscles : m. obliquus externus and m. transversus 
 abdominis .
h. nigrescens possessed m. rectus profundus and an 
 isolated m. rectus abdominis . in a. tigrinum , a 
 terrestrial species , four layers ( m. obliquus externus superficialis , 
 m. obliquus externus profundus , m. obliquus internus and 
 m. transversus abdominis ) made up the lateral hypaxial muscles , which 
 were relatively thin . m. rectus abdominis of a. tigrinum 
 separated from other muscular structures .
cross - sections through the midtrunk of ( a ) : amphiuma tridactylum , 
 ( b ) : necturus maculosus , ( c ) : cynops pyrrhogaster , 
 ( d ) : hynobius nigrescens , ( e ) : ambystoma tigrinum . 

. lateral view of ( a ) : amphiuma tridactylum , ( b ) : necturus 
 maculosus , ( c ) : cynops pyrrhogaster , ( d ) : hynobius 
 nigrescens , ( e ) : ambystoma tigrinum . scale bar=5 mm .
muscle weight : weight ratios of trunk muscles to overall muscle weight are 
 represented in table 2table 2.muscle weight ratios ( % ) measured at midtrunk ( mean  sem)speciesm .
rectus abdominis , m. rectus 
 profundus 
 + m. rectus abdominisamphiuma tridactylum33.3  0.83.2  0.42.1  0.39.5  1.323.2  1.225.0  1.93.7  0.4necturus maculosus34.0  1.83.0  0.52.0  0.310.4  0.620.6  0.825.8  2.94.0
1.814.5  1.99.4  0.4hynobius nigrescens46.4  2.35.0  0.33.4  0.514.4  0.510.4  2.67.3  1.213.1  1.5ambystoma tigrinum45.7 
3.86.8  1.23.6  0.216.9  2.16.0  0.47.7  2.612.3  0.2different superscript letters indicate significant differences ( anova and tukey s 
 test , p<0.05 ) . and
4.muscle weight ratios of each trunk muscle in a. tridactylum , 
 n. maculosus , c. pyrrhogaster , h. 
 nigrescens and a. tigrinum . different superscript letters 
 indicate significant differences ( anova and tukey s test , p<0.05 ) . 

black bar : aquatic species , gray bar : semi - aquatic species , white bar : terrestrial 
 species .. considerable variations were observed between these salamanders .
significant 
 differences were shown among species in m. dorsalis trunci weight ratios [ f 
 ( 4 , 10)=22.79 , p<0.01 ] (  f  explains f - value of anova , and numbers in 
 bracket mean degrees of freedom ) . for m. dorsalis trunci , weight ratios 
 were smaller in aquatic species ( a. tridactylum of 33.3% and n. 
 maculosus of 34.0% ) and significantly smaller than those of terrestrial species 
 ( p<0.05 ) . for the terrestrial species , h. nigrescens 
 and a. tigrinum , m. dorsalis trunci accounted for more 
 than 46% of muscle weight . among the more aquatic species , m. interspinalis 
 occupied a smaller percentage of total muscle weight than that of more terrestrial species , 
 and significant interspecific differences
were demonstrated [ f ( 4 , 10)=17.05 , 
 p<0.01 ] . in a. tigrinum , which is a terrestrial 
 species , m. interspinalis made up less than 7% of total muscle weight , but 
 the value for this muscle was significantly larger than that of aquatic species .
significant 
 differences were exhibited among species [ f ( 4 , 10)=15.82 , p<0.01 ] in 
 m. intertransversarius weight ratios . in more aquatic species ,
the weight ratio of m. 
 intertransversarius in h. nigrescens and a. 
 tigrinum was significantly larger than in aquatic species 
 ( p<0.05 ) .
for m. subvertebralis , significant 
 differences in weight ratios among species were shown [ f ( 4 , 10)=19.87 , 
 p<0.01 ] .
in aquatic species , the weight ratio for m. 
 subvertebralis was significantly smaller than that of terrestrial species 
 ( p<0.05 ) .
different superscript letters indicate significant differences ( anova and tukey s 
 test , p<0.05 ) .
muscle weight ratios of each trunk muscle in a. tridactylum , 
 n. maculosus , c. pyrrhogaster , h. 
 nigrescens and a. tigrinum .
different superscript letters 
 indicate significant differences ( anova and tukey s test , p<0.05 ) . 

black bar : aquatic species , gray bar : semi - aquatic species , white bar : terrestrial 
 species .
significant interspecific differences in weight ratio of m. obliquus 
 externus were seen among species [ f ( 4 , 10)=61.20 , p<0.01 ] . 

weight ratios for m. obliquus externus were significantly larger in more 
 aquatic species ( p<0.05 ) .
m. obliquus externus weight 
 ratios were significantly larger than 20% in aquatic species ( a. 
 tridactylum and n. maculosus ) .
significant differences in weight 
 ratios of m. transversus abdominis were observed among species [ f ( 4 , 
 10)=51.68 , p<0.01 ] .
similarly , in more aquatic species , weight ratios 
 for m. transversus abdominis were larger .
in a. 
 tridactylum and n. maculosus , weight ratios for m. 
 transversus abdominis were significantly larger than in other species 
 ( p<0.05 ) .
in terrestrial species ( h. nigrescens and 
 a. tigrinum ) , m. transversus abdominis occupied less 
 than 8% of total muscle weight , a significantly smaller percentage than in other species . 

finally , significant differences among species were exhibited in weight ratio for m. 
 rectus abdominis [ f ( 4 , 10)=90.82 , p<0.01 ] ; it was smaller in 
 more aquatic species .
in aquatic species ( a. tridactylum and n. 
 maculosus ) , m. rectus abdominis accounted for less than 5% of 
 total muscle weight , which was significantly smaller than that in other species 
 ( p<0.05 ) .
terrestrial species ( h. nigrescens and 
 a. tigrinum ) were equipped with significantly larger m. rectus 
 abdominis .
morphological trends : a comparison of trunk muscles on cross sections 
 through the midtrunk region showed interspecific variations in relative muscle thickness 
 ( fig .
thicker lateral hypaxial muscles were 
 found in more aquatic species , whereas thinner lateral hypaxial muscles were found in more 
 terrestrial species .
this finding supports the results of a previous study by simons and 
 brainerd   comparing s. 
 lacertina , a. tridactylum , cryptobranchus 
 alleganiensis and a. tigrinum . in the more developed limbs of 
 the terrestrial species , the locomotive function of the lateral hypaxial muscles shifts , and 
 their thickness decreases .
m. obliquus 
 internus , one of the lateral hypaxial muscles , is used during both swimming and 
 walking . in swimming
, m. obliquus 
 internus mainly produces lateral bending and produces or resists long - axis 
 torsion and produces sagittal flexion . in 
 terrestrial locomotion ,
m. obliquus internus plays a role in counteracting 
 long - axis torsion of the trunk .
while lateral 
 hypaxial muscles are used on the ground , more aquatic species may need larger lateral 
 hypaxial muscles for lateral bending , because of water viscosity and the resulting drag . 
 especially in a. tridactylum ,
although both a. tridactylum and n. maculosus 
 are fully aquatic species , a. tridactylum possesses smaller limbs than 
 n. maculosus .
therefore , a. tridactylum may depend more 
 than n. maculosus on dorsally enlarged lateral hypaxial muscles for 
 locomotion
. m. rectus lateralis was recognized in a. tigrinum , which 
 is a terrestrial species , and c. pyrrhogaster , which is a semi - aquatic 
 species , and m. rectus profundus was observed in h. 
 nigrescens , which is a terrestrial species .
although differences were observed in 
 the number of layers of lateral hypaxial muscles among species , no significant trend was 
 found related to habitat or predominant mode of locomotion .
these results are consistent 
 with those of simons and brainerd   for other 
 taxa .
the same authors did not find a close association between ecology and predominant mode 
 of locomotion of salamanders with the number of hypaxial muscle layers in their analysis of 
 ten families of urodela .
m. rectus abdominis was independent of lateral hypaxial muscles in 
 semi - aquatic species ( c. pyrrhogaster ) and terrestrial species ( h. 
 nigrescens and a. tigrinum ) . however , m. rectus 
 abdominis in aquatic species ( a. tridactylum and n. 
 maculosus ) was continuous with the lateral hypaxial musculature .
therefore , it is reasonable to assume a more specific function of m. rectus 
 abdominis in more terrestrial species .
m. rectus abdominis plays 
 a role in counteracting sagittal extension of the trunk produced by the action of the 
 epaxial muscles [ 5 , 20 ] and by the gravity with the elongate trunk in salamanders .
therefore , we suggest that the terrestrial species 
 examined in this study use their relatively greater and clearly isolated m. rectus 
 abdominis in resisting sagittal extension of the trunk and that this adaptation 
 is related to the amount of time they spend on land .
muscle weights : we found m. dorsalis trunci was larger in 
 more terrestrial species ( fig .
4 ) . during both 
 swimming and walking , m. dorsalis trunci produces lateral bending [ 5 , 6 , 9 ] .
in addition , most lateral strands of the autochthonous 
 m. dorsalis trunci may be in a position to exert ventral flexing of the 
 trunk ( fig .
therefore , in order to 
 counteract the effect of gravitational forces , m. dorsalis trunci was 
 greater in more terrestrial species .
m. 
 interspinalis may also contribute to flexing the trunk ventrally to counteract 
 the influence of the gravity by its position ( fig . 
 2 ) .
thus , m. interspinalis was larger in more terrestrial species , 
 which employ m. interspinalis on land
although the function of m. 
 intertransversarius has not been clarified , it seems to play a role in ensuring 
 spinal integrity and ventral flexing similar to m. interspinalis , because 
 of its location between vertebrae .
4 ) . functional 
 subunits that can stabilize and mobilize the trunk and adjust body stiffness compose 
 m. subvertebralis . in 
 addition
because of its role in trunk stabilization , m. subvertebralis is 
 more often utilized in terrestrial environments . thus , heavier m. 
 subvertebralis is characteristic of more terrestrial species .
lateral hypaxial muscles ( m. obliquus externus superficialis , m. 
 obliquus externus profundus , m. obliquus internus and m. 
 transversus abdominis ) were larger in more aquatic species ( fig .
lateral hypaxial muscles function in torsion control and 
 maintaining stability [ 3 , 5 ] .
as mentioned earlier , lateral hypaxial muscles are used in both 
 swimming and walking .
m. obliquus internus 
 has functions of mainly producing lateral bending and producing or resisting long - axis 
 torsion of the trunk as well as some sagittal flexion in swimming . in terrestrial locomotion ,
m. obliquus internus plays 
 a role in counteracting long - axis torsion of the trunk .
trunk muscles in species with diminutive limbs produce lateral bending actively 
 during aquatic locomotion , whereas lateral bending is produced passively by the muscle 
 action of extrinsic limbs .
aquatic species rely more on lateral bending of 
 the trunk to move forward ; therefore , weights of lateral hypaxial muscles are relatively 
 greater in more aquatic species . though all lateral hypaxial muscles play a role in 
 stabilization and torsion control on ground , muscle ratios of lateral hypaxial muscles may 
 be affected by differences in lateral bending in different environments .
since muscular forces along the ventral contour line are needed in animals with lengthy 
 trunks that are supported by limbs , the 
 m. rectus abdominis , which is the most ventral muscle , must be heavier in 
 more terrestrial species [ 5 , 17 ] .
it is suggested that the function of m. rectus 
 abdominis is to prevent sagittal trunk extension caused by the action of the 
 epaxial muscles   and by the gravity with the 
 elongate trunk in salamanders . since larger 
 weight ratios for m. dorsalis trunci
4 ) and influence of gravity occurs 
 , dorsally concave curvature of the trunk by 
 dorsal muscles may also be larger than in more aquatic species . to resist greater dorsally 
 concave curvature of the trunk , larger
the aquatic species were larger than the 
 terrestrial species , and lateral hypaxial muscles were thicker and heavier in the aquatic 
 forms .
it is considerable that the stronger development of the lateral hypaxial musculature 
 in the aquatic species may be enforced by the factor by size . in this study ,
interspecific variation in trunk musculature among urodela reflects their 
 main mode of locomotion in different ecological habitats .
 abstracttrunk musculature in urodela species varies by habitat . in this study , 
 trunk musculature 
 was examined in five species of adult salamanders representing three different habitats : 
 aquatic species , amphiuma tridactylum and necturus 
 maculosus ; semi - aquatic species , cynops pyrrhogaster ; 
 terrestrial species , hynobius nigrescens and ambystoma 
 tigrinum . 
 more terrestrial species have heavier dorsal and ventral trunk 
 muscles than more aquatic forms . by contrast , the lateral hypaxial musculature was 
 stronger in more aquatic species 
 . the number of layers of lateral hypaxial musculature 
 varied among urodela species and did not clearly correlate with their habitats . 
 the 
 m. rectus abdominis was separated from the lateral hypaxial musculature 
 in both terrestrial and semi - aquatic species . in aquatic species , 
 m. rectus 
 abdominis was not separated from lateral hypaxial musculature . 
 lateral hypaxial 
 musculature differed in thickness among species and was relatively thinner in terrestrial 
 species . in more terrestrial species , 
 dorsal muscles may be used for stabilization and 
 ventral flexing against gravity . 
 ventral muscle may be used in preventing dorsally concave 
 curvature of the trunk by dorsal muscles and by weight . the lengthy trunk supported by 
 limbs 
 needs muscular forces along the ventral contour line in more terrestrial species . 
 and , the locomotion on well - developed limbs seems to lead to a decrease of the lateral 
 hypaxial musculature . 
MATERIALS AND METHODS
RESULTS
DISCUSSION