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Number 3241, 25 pp., 10 figures, 1 table Month 00, 1998

Homology of Intermuscular Bones inAcanthomorph Fishes

SVEN GEMBALLA1 AND RALF BRITZ2

ABSTRACT

Myosepta of selected representatives of the fol-lowing acanthomorph taxa were investigated: Po-lymixiiformes, Lampridiformes, Paracanthoptery-gii, Beryciformes, Atherinomorpha, and Perco-morpha. A new technique, microdissection of al-cohol-stored specimens and polarized-lightmicroscopy, was applied to study the three-di-mensional architecture of connective tissue fibersin epaxial parts of myosepta. Several invariablesimilarities were present in all taxa: an epineuralseries of tendons or bones and a tendinous seriesof lateral bands in the epaxial part of the myosep-tum, and an epicentral series of tendons or bonesin the horizontal septum.

Patterson and Johnson’s (1995) hypothesis thatthe single bony series of intermusculars in higheracanthomorphs is the homolog of epineurals oflower teleosts is tested. Our results contradicttheir hypothesis at essential points because wediscovered epineural tendons in the normal epax-ial position in different acanthomorphs that wereconsidered to lack these. We conclude that thefirst intermuscular bone of Polymixia is an epi-central, the single series of intermuscular bonesof Holacanthopterygii are epicentrals, and theneoneurals of some percomorphs are normal epi-neurals. Phylogenetic implications of our resultsare discussed.

INTRODUCTION

Since Owen’s initial studies (1846, 1866),three series of intermuscular bones of teleostshave been known as epineurals, epicentrals,and epipleurals. In some teleosts two addi-tional bony series are developed and are

1 Lehrstuhl Spezielle Zoologie, Universitat Tubingen, D–72076 Tubingen, Germany.2 Postdoctoral Fellow and Research Associate, Department of Herpetology and Ichthyology, American Museum of

Natural History. Present address: Lehrstuhl Spezielle Zoologie, Universitat Tubingen, D–72076 Tubingen, Germany.

known as myorhabdoi (Chapman, 1944). Re-cently, Johnson and Patterson (1993) andPatterson and Johnson (1995) studied inter-muscular bones among major teleosteangroups. One important result of their exten-

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sive investigation is that the bony intermus-culars are usually continued by fiber bundlesof connective tissue in the same series as thebones. They extended the terms epineural,epicentral, and epipleural to include these in-termuscular fiber bundles that they named in-termuscular ligaments. They concluded thatthe single bony series found in higher acan-thomorphs is not composed of epipleurals asgenerally assumed, but is the homolog of theepineural series of lower teleosts that is shift-ed ventrally into the horizontal septum.

They conceded that ‘‘recording details ofthe form and distribution of those structures[i.e., intermuscular bones and fiber bundles]in cleared and stained specimens can be ex-tremely difficult . . .’’ (1995: 1) and that ‘‘ob-servations of ligaments . . . include an un-known quantity of subjectivity, and we willbe glad to see all or any of them checked byothers’’ (1995: 4).

Doubts concerning their homology ofacanthomorph intermusculars with epineuralsarose during the doctoral study of Gemballa(1995). He developed a technique to studyindividual myosepta and reliably identify in-termuscular fiber bundles by polarized light.This new technique revealed epineural fiberbundles in taxa declared to lack them by Pat-terson and Johnson (1995). Thus, it appearedto us that their applied technique misled Pat-terson and Johnson into homologizing thesingle series of intermuscular bones in acan-thomorphs with the epineural series of lowerteleosts.

The aim of our paper is to reinvestigatemyoseptal structures of representatives ofmajor acanthomorph subgroups using Gem-balla’s technique (1995) to test Patterson andJohnson’s hypothesis. Anatomical data ob-tained by this technique form the basis forunequivocal identification of the bony seriesin acanthomorphs as the homolog either ofthe epineural or the epicentral series of lowerteleosts. This is valuable because these struc-tures are incorporated in phylogenetic hy-potheses (Johnson and Patterson, 1993) thatare weakened or strengthened depending onthe homologization (see Discussion).

A detailed anatomical description of myo-septal structures is also of interest to func-tional morphologists because these structuresare involved in transmission of muscular

forces during locomotion (Nursall, 1956,Willemse, 1972, Wainwright, 1983, Westneatet al., 1993). Collagenous fiber bundles ascomponents of myosepta were not describedin detail or well understood until recently(Gemballa, 1995). Descriptions for the di-verse group of acanthomorph fishes are stilllacking and some are presented here for thefirst time. The functional significance of theresults presented here is the topic of a forth-coming paper.

ACKNOWLEDGMENTS

We thank Karsten Hartel (MCZ, Harvard),and Norma Feinberg, Barbara Brown, Me-lanie Stiassny, and Gareth Nelson (AMNH,New York) for the loan of study material. Weexpress deepest gratitude to Dave Johnsonand Colin Patterson for providing a copy oftheir interesting and stimulating paper on te-leost intermusculars before publication. Theirenormous survey facilitated our work andwas the basis for constant checking of ourdata. We enjoyed discussion with Dave John-son during a short visit to the Division ofFishes, National Museum of Natural History,Washington. We thank Gareth Nelson andRadford Arindell for inviting RB to jointhem for fieldwork in Alabama and for theirinterest in our ‘‘intermuscular problem’’. Weare grateful to Bob Shipp and the numerousindividuals who helped in many ways duringthe 64th Annual Deep Sea Fishing Rodeo,1996, on Dauphin Island, Alabama. EduardSerrat and fishermen from Palamos (Catal-unya, Spain) made fishing boat trips possiblefor SG. We thank Wolfgang Maier, Lehrstuhlfur Spezielle Zoologie, Universitat Tubingen,for his interest in our project, and MelanieStiassny for her support. Barbara Brown,Radford Arindell, and Martina Hohloch pro-vided technical assistance. The paper profitedfrom the critical reading and commenting byPeter Bartsch, Gareth Nelson, and MarceloSanchez-Villagra and the reviews of An-thony Gill (BMNH, London) and Mark W.Westneat (FMNH, Chicago). MWW drewour attention to the terminology problemswith tendons and ligaments.

This work was supported by the SFB 230and a collection study grant from the Amer-ican Museum of Natural History to SG, a

1998 3GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

TABLE 1Investigated Specimensa

Specimen SL(mm) Family

Polymixia lowei AMNH 49674Polymixia lowei AMNH 086102

10397

POLYMIXIIDAE (c&s)POLYMIXIIDAE (c&s)

Velifer hypselopterus AMNH 49575Metavelifer multiradiatus AMNH 92080Lampris guttatus AMNH 79669Aphredoderus sayanus AMNH 55089Molva dipterygia personal coll.Gephyroberyx philippinus AMNH 49701

115;279;820

74220

85

VELIFERIDAE (c&s)VELIFERIDAE (skel.)LAMPRIDAE (skel.)APHREDODERIDAE (c&s)GADIDAE (c&s, part)TRACHICHTHYIDAE (c&s)

Hoplostethus mediterraneus personal coll.Holocentrus rufus AMNH 25919Holocentrinae indet. MCZ 85252Holocentrinae indet. MCZ 85252Holocentrinae indet. MCZ 85252Holocentrinae indet. MCZ 51911

106124

1111.513.518

TRACHICHTHYIDAE (c&s)HOLOCENTRIDAE (c&s)HOLOCENTRIDAE (c&s)HOLOCENTRIDAE (c&s)HOLOCENTRIDAE (c&s)HOLOCENTRIDAE (c&s)

Holocentrinae indet. MCZ 51911Bedotia geayi personal coll.Scomberesox saurus personal coll.Channa obscura personal coll.Channa obscura personal coll.

2698

240121126

HOLOCENTRIDAE (c&s)BEDOTIIDAE (c&s)SCOMBERESOCIDAE (c&s)CHANNIDAE (c&s)CHANNIDAE (c&s)

Anthias pleurotaenia AMNH 38119Serranus hepatus personal coll.Lates calcarifer AMNH 37836Centropomus robalito AMNH 32925Morone americana AMNH 57453

100728099

112

SERRRANIDAE (c&s)SERRANIDAE (c&s)CENTROPOMIDAEb (c&s)CENTROPOMIDAE (c&s)MORONIDAE (c&s)

Percopsis omiscomaycus AMNH 27265Oligoplites saurus AMNH 45080Parupeneus barberinoides AMNH 43106Kuhlia rupestris AMNH 17954Arnoglossus laterna personal coll.

86132

619653

PERCOPSIDAE (c&s)CARANGIDAE (c&s)MULLIDAE (c&s)KUHLIIDAE (c&s)BOTHIDAE (c&s)

ac&s 5 cleared and double stained, skel 5 skeletonized specimens, SL 5 standard length.b Mooi and Gill (1995) gave full familial status to the centropomid Latinae.

dissertational grant to RB from the Landes-graduiertenforderung Baden-Wurttembergand subsequently a postdoctoral fellowshipin the Department of Herpetology and Ich-thyology, American Museum of Natural His-tory, New York.

MATERIAL AND METHODS

Most specimens (see table 1) were clearedand double-stained according to the proce-dure of Dingerkus and Uhler (1977). A fewdata were gathered from large, skeletonizedspecimens. Length is given as standardlength in millimeters.

Glycerin-stored cleared and stained speci-mens were transferred to 100% or 70% etha-nol because connective tissue elements are

much easier to identify in ethanol. A retrans-fer into glycerin is possible whenever nec-essary. After a close examination, individualmyosepta were removed with fine iris springscissors by cutting close to their insertion onthe axial skeleton. Removed myosepta weremounted on slides, and studied under astereomicroscope in transmitted light and un-der a light microscope using transmitted lightor difference-interference contrast. Storageof individual myosepta was in 100% ethanol.

Polarized light microscopy has an advan-tage in visualizing fiber pathways within amyoseptum, because of double-refraction ofcollagen fibers. Collagen fibers are observedas bright strands against a black background.We photographed myosepta with a polarized

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light microscope (ZEISS Axioplan with cam-era MC-100). Several photographs weretaken to cover the epaxial part of a myosep-tum. Prints were later mounted on blackcardboard to show the complete epaxialmyoseptum.

Polarized light has some disadvantages.Thick intermuscular bones are not translu-cent, appear black, and thus are difficult torecognize. Thin intermuscular bones may behard to distinguish from tendons in the pho-tographs. However, ossifications were easy toidentify by their red Alizarin stain during dis-section. There are problems when collagenfibers at angles of about 45 or 1358 cross,because of double refraction of collagen. Oneof the two directions then appears almostblack and may seem to be lost in these cross-ing areas. Closer differential interferencecontrast (DIC) examination revealed this tobe an artifact and fiber directions were al-ways continuous. Nevertheless, it was some-times difficult to find an orientation that re-vealed all fiber directions to the same extent.

Individual myosepta of the followingfreshly caught species supplement the list intable 1: Caranx hippos (Carangidae) andSphyraena barracuda (Sphyraenidae). Myo-septa were removed from adult specimens ofthe two species during the 64th Annual Al-abama Deep Sea Fishing Rodeo on DauphinIsland, July 19–21, 1996. First, the muscletissue was scooped out with a spoon. Thenindividual myosepta were cut close to the ax-ial skeleton and preserved in 4% formalin.Remnants of muscle tissue were digested ina trypsin solution and the myosepta rinsed inwater and photographed with incident lightin front of a blackboard.

TERMINOLOGY

Distinct bundles of connective tissue fibersoccur in epaxial and hypaxial parts of myo-septa and in the horizontal septum. Pattersonand Johnson (1995) applied different termsto these fiber bundles naming them epineural,epipleural and epicentral ligaments and pos-terior oblique tendons (POTs). Westneat (per-sonal commun.) pointed out to us the factthat the proper anatomical term for these fi-ber bundles is tendon and not ligament. Ten-dons connect muscle to bone, whereas liga-

ments connect bone to bone. Therefore weuse the term ‘‘intermuscular tendons’’ in-stead of Patterson and Johnson’s ‘‘intermus-cular ligaments.’’ Accordingly, individual fi-ber bundles are named epineural tendons, ep-icentral tendons (5 anterior oblique ten-dons), epipleural tendons, and posterioroblique tendons.

We maintain that fiber bundles fit the def-inition of tendon, not ligament, e.g., epineu-ral tendons connect epaxial musculature tothe axial skeleton. Intermuscular tendonstransfer muscular forces from myomeres tothe axial skeleton and thus act as tendons.When parts of the tendons ossify, the result-ing intermuscular bones may sometimes beconnected to the axial skeleton via a shortbundle of fibers. Patterson and Johnson(1995) stressed that in such case the exactterm for this proximal part is ligament be-cause then it connects bone to bone, i.e., in-termuscular bone to axial skeleton. Ontoge-netically, however, it is the proximal portionof a tendon and we see no necessity to applya different term to the same structure at alater point in ontogeny.

NUMBERING SEGMENTALLY ARRANGED

STRUCTURES

The area of attachment of one myoseptumencompasses several vertebrae and thus num-bering a myoseptum is problematic. For con-venience, we number a given myoseptum ac-cording to the most anterior vertebra attachedto the anterior cone of the myoseptum. In allinvestigated specimens this is the same ver-tebra attached to the epineural bone or epi-neural tendon.

ABBREVIATIONS

BL Baudelot’s ligamentDAC dorsal anterior coneDPC dorsal posterior coneecb epicentral boneect epicentral tendonenb epineural boneent epineural tendonEFP epaxial flanking partESP epaxial sloping partHFP hypaxial flanking partHSP hypaxial sloping partlb lateral bandSH horizontal septum

1998 5GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

VAC ventral anterior coneVPC ventral posterior cone

RESULTSTHREE-DIMENSIONAL SHAPE OF MYOSEPTUM

Patterson and Johnson (1995) pointed outthat intermuscular bones ossifying in themyoseptum are associated with strands ofstrong fibers of connective tissue, their ‘‘in-termuscular ligaments.’’ These fiber bundlesof connective tissue that we call intermus-cular tendons (see above), are only parts ofthe complete myoseptum.

Our description of a typical myoseptumfrom the midbody region is based on findingsof Gemballa (1995) and on additional datafrom our study. Gemballa investigated rep-resentatives of all major actinopterygian sub-groups and found several invariable similar-ities in the basic architecture of the myosep-tum, i.e., the three dimensional shape and ar-rangement of connective tissue fibers. Wehave confirmed his results. Terminology forthe different parts of the myoseptum is partlyadopted and modified from Alexander(1969), Willemse (1972) and Westneat et al.(1993).

The myoseptum is attached to the axialskeleton medially. Its area of attachment ex-tends over several vertebrae, commonlythree, sometimes up to five. The lateral areaof attachment is the integument. When su-perimposed in lateral view (fig. 1A), both ar-eas of attachment have the shape of ( ).The..

myoseptum extends between the two areas ofattachment. It is separated by the horizontalseptum into epaxial and hypaxial parts. Bothepaxial and hypaxial parts of the anteriormyoseptum point into dorsal and ventral an-terior cones (DAC and VAC) close to thehorizontal septum (SH). The posterior tips ofthe ( ) point into dorsal and ventral posterior..

cones (DPC and VPC). The relative size ofanterior and posterior cones varies along thebody and among taxa. Between anterior andposterior cones are epaxial or hypaxial slop-ing parts (ESP or HSP). From the posteriorcones the myoseptum flexes anteriorly in theepaxial and hypaxial regions and forms epax-ial and hypaxial flanking parts (EFP or HFP).

CONNECTIVE TISSUE FIBERS

IN EPAXIAL MYOSEPTUM

When the epaxial part of the myoseptumbetween the horizontal septum and the dorsalposterior cone is removed and spread out,usually two fiber bundles are recognized.They are illustrated schematically in figure1B and are shown for several species in sub-sequent figures. One bundle of fibers origi-nates on the neural arch of the vertebra, fre-quently as a distinct tendon, and extends pos-terolaterally toward the integument, where itfans out distally. This bundle is the epineuralligament of Patterson and Johnson (1995)and is here referred to as epineural tendon(see paragraph on terminology above). Thesecond bundle originates at the dorsal ante-rior cone and also runs posterolaterally butthen curves dorsally and fans out into thedorsal posterior cone. We call this structurethe lateral band because it never shows thecharacteristic convergence of fibers that ap-pear as a distinct tendon. It is more or lessprominent and resembles an aponeurosis insome species. Such a structure was not re-ported by Patterson and Johnson (1995) forany teleost that they studied.

Ossifications can be found in both of thetwo fiber bundles or only in one of them.Ossification in both fiber bundles results in acharacteristic Y-shaped bone with proximalforking. The anteromedial branch of the boneis embedded in the epineural tendon. Thisbone is attached either directly or via theproximal part of the epineural tendon to therespective vertebra. The anterolateral branchof the bone is embedded in the anterior partof the lateral band and ends free in the dorsalanterior cone. Ossification in only one fiberbundle causes the regular slightly curvedsplint of bone. Both types of ossifications arereferred to as epineural bones by Pattersonand Johnson (1995).

CONNECTIVE TISSUE FIBER BUNDLES

IN HORIZONTAL SEPTUM

The horizontal septum separates hypaxialand epaxial parts of the myosepta and myo-meres. Two distinct fiber bundles with dif-ferent orientations may be developed in thehorizontal septum. They were called anterioroblique tendons (AOTs) and posterior

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Fig. 1. A, Schematic representation of typical myoseptum from midbody in lateral view, anterior tothe left. Position of cones in epaxial part (DAC, DPC) and hypaxial part (VAC, VPC) and epaxial andhypaxial flanking parts (EFP, HFP) are shown. Areas of attachment are -shaped. Dashed line represents..

medial area of attachment at axial skeleton; solid line lateral, area of attachment at integument. Areabetween anterior and posterior cone is called epaxial sloping part (ESP) and hypaxial sloping part (HSP),respectively (after Gemballa, 1995). B, Schematic representation of ESP of midbody myoseptum of leftside, anterodorsal view. Arrangement of epineural tendon (ent) and lateral band (lb) is shown. Arrowpoints to position of epicentral bone or tendon that runs in horizontal septum. Its proximal part coveredby DAC.

1998 7GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

oblique tendons (POTs) (Kafuku, 1950, ascited in Patterson and Johnson, 1995). TheAOTs were called epicentral ligaments byPatterson and Johnson (1995), here referredto as epicentral tendons (see above). The os-sifications within these tendons are the epi-central bones. Epicentral bones or tendonsare oriented posterolaterally. They arecrossed by the POTs that are oriented antero-laterally.

CONNECTIVE TISSUE FIBERS

IN HYPAXIAL MYOSEPTUM

The hypaxial part of the myoseptum is ba-sically a mirror image of the epaxial part.This is not the case in the more anterior myo-septa, where the abdominal cavity occupiesmore space and restricts the hypaxial mus-culature to narrow strips lateral to the ribs.Exactly as in the epaxial part, the hypaxialpart of the myoseptum shows two prominentfiber bundles. One bundle originates on therib or the dorsal part of the hemal arch andruns posterolaterally to the integument. Itwas called epipleural ligament by Pattersonand Johnson (1995) but, for reasons given, istermed epipleural tendon. The other bundleoriginates more dorsally and extends like-wise posterolaterally and ventrally into theventral posterior cone. There it flexes into thehypaxial flanking part. We call this the lateralband of the hypaxial myoseptum. Ossifica-tions can be developed in both bundles re-sulting in a proximally forked bone, or onlyin one bundle resulting in a slightly curvedunforked bone. Both types are referred to asepipleural bones by Patterson and Johnson(1995).

FIBER BUNDLES IN FLANKING PARTS

(EFP, HFP)

There are no distinct tendons in either theepaxial or the hypaxial flanking parts of themyoseptum. The fibers commonly run lon-gitudinally forming broad bands. Thesebands may partly ossify in some teleosts andthe resulting bones are called myorhabdoi af-ter Chapman (1944).

INTERMUSCULAR BONES AND TENDONS IN

ACANTHOMORPHS

In the following paragraphs we present adetailed description of the arrangement of

connective tissue fibers in the epaxial part ofmyosepta of selected taxa. We describe a typ-ical myoseptum from the midbody region ofa given taxon and then record changes thatoccur in anterior myosepta.

POLYMIXIIFORMES: Polymixia lowei. Patter-son and Johnson (1995) described the inter-musculars of Polymixia in detail. There aretwo series, partly bony and partly tendinous(epineurals and epipleurals), and one tendi-nous series of epicentrals. The first intermus-cular bone of Polymixia is stronger than theothers, lies in the horizontal septum, and ar-ticulates in a groove on the neural arch of thefirst vertebra.

Myoseptum 13 (fig. 2A) bears a short butwell-developed epicentral tendon in the hor-izontal septum. The epineural tendon is stoutand its posterior part approaches the lateralband. Individual fibers diverge along itslength from the epineural tendon and crossthe lateral band. The lateral band is broadwithout any condensations of fibers to dis-tinct bundles.

Rostrally, epineural tendon 13 is well inline with the epineural tendons or bones ofthe more anterior myosepta. Both bones andtendons insert on the neural arch of the re-spective vertebrae. Epicentral tendon 13 islikewise in line with epicentrals of anteriorsegments up to 2. In the first myoseptum wefind a different situation (fig. 2B, C). Whenobserved in alcohol under the stereomicro-scope, or with polarized light under the mi-croscope, a distinct tendinous structure ispresent in the epineural position of myosep-tum 1. It is in line with epineurals of all sub-sequent vertebrae (fig. 2C). We thereforeconsider this tendon to be the epineural ofmyoseptum 1. It inserts dorsal to the inter-muscular bone of the first vertebra. The in-termuscular bone is in the epicentral positionin the horizontal septum and is in line withepicentral tendons of posterior segments. Weconsider this to be the bony epicentral ofmyoseptum 1 (see below).

LAMPRIDIFORMES: Velifer hypselopterus.Velifer is a deep-bodied fish and, as is fre-quently the case in myosepta of such fishes,anterior cones are not developed. We showmyoseptum 11 (fig. 3A) of the midbody re-gion. There is an epineural tendon with astout proximal end that inserts on the neural

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Fig. 2. Polymixia lowei. A, B. Micrographs of epaxial myosepta. Here and in all subsequent micro-graphs myosepta spread out: medial area of attachment to the left. A, Myoseptum 13. B, Myoseptum1. C, Spatial relationship of epaxial myoseptum 1 and axial skeleton, lateral view, parts behind myo-septum illustrated in gray, modified after Patterson and Johnson (1995).

1998 9GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

spine and with a distal end that fans out. Thebroad lateral band coming from the DPCcrosses the epineural tendon and insertsalong the height of the centrum. It appearsto be of equal thickness. In the horizontalseptum there is an epicentral tendon that iscrossed by three POTs.

In anterior myosepta some gradualchanges can be noticed. They are shown byphotographs of myosepta 9, 4, and 1 (fig.3B–D). Epineural tendon 11 is aligned withthe series of epineural bones. The site of at-tachment shifts from the neural spine in 9 tothe neural arch in 1. Epineural bones 10–7insert with a short proximal tendinous sheathand those of 6 to 1 articulate directly withvertebrae. We identified epicentral tendonsfor myosepta 11–2 (see fig. 3B, C for myo-septa 9 and 4) but failed to do so for myo-septum 1. There it seems to be representedby a diffuse bundle of fibers at the level ofBaudelot’s ligament (fig. 3D).

Lampris guttatus. We were not able tostudy myosepta of this lampridiform butfound differences from Patterson and John-son’s (1995) account of its intermusculars.The two anterior intermuscular bones of ourlarge specimen of L. guttatus (AMNH79669) are attached to centra and not to neu-ral arches or spines. As already noted by Pat-terson and Johnson, all following intermus-cular bones are attached to proximal portionof ribs.

BERYCIFORMES: Hoplostethus mediterra-neus. Examination of myosepta 9–1 revealsthe basic arrangement of the lateral band,epineural, and epicentral tendons. Myosep-tum 8 of Hoplostethus (fig. 4A) is thus sim-ilar to myosepta of the midbody region ofVelifer (9, 11, fig. 3A, B) or Polymixia (13,fig. 2A). It has a broad epicentral tendon inthe horizontal septum. In this area of thetrunk, vertebrae have long hemal spines. Thehorizontal septum and epicentral tendons aredisplaced ventrally and attach at their distalends. Epineural tendon 8 inserts on the cen-trum dorsal to the epicentral. Its fibers di-verge in its middle part and eventually fanout at its distal end. The remaining part ofthe myoseptum consists of very fine, thin fi-bers that can be recognized only under po-larized light. Consequently, they form a veryfaint lateral band when compared to the for-

mation in Velifer or Polymixia. Epineuraltendon of myoseptum 6 (fig. 4B) is not asdistinct in its proximal part as that of 8. Inaddition, distance between the insertions ofepicentral and epineural has doubled com-pared to 8. This is caused by a dorsal shiftof the insertion of the epineural from the cen-trum in 8 to the neural arch in 6. In myosep-tum 3 (fig. 4C) the epicentral tendon attachesto the ventral part of the centrum and thedistance between the insertions of epineuraland epicentral tendons has increased further.Epicentral tendons 8–3 are in line with epi-central bones in 2 and 1 (fig. 4D). Also epi-neural tendons 1–8 are aligned and thus twocomplete series are present.

We observed a ventral shift of the epi-neural and epicentral series toward the mid-body region in our specimen of Hoplostethusmediterraneus. The shift starts approximatelyat vertebra 6, which bears the first hemalspine. As the hemal spine lengthens in sub-sequent segments, the epicentral series shiftsfurther ventrally to the level of the distal endof the hemal spines. Insertion of epineuralelements shifts ventrally from the neural archto the centrum. Further caudally we recordeda dorsal shift of the two series. The epineuraltendon is attached from vertebra 14 to theneural arch. The insertion of the epicentralgradually shifts up to the centrum from ver-tebra 16.

Gephyroberyx philippinus (not figured).The pattern of tendons and bones in Gephy-roberyx is similar to that in Hoplostethus.There is ventral shift of the middle part ofthe whole series. The bony epicentrals of thefirst two myosepta attach to the neural arch,followed by a series of epicentral tendons.We failed to remove myoseptum 1 for a clos-er examination but we were able to detectepineural tendons dorsal to epicentrals in allremaining myosepta. There is also a lateralband in the normal position in each myosep-tum.

Holocentrus rufus. Epineural 10 (fig. 5A)is completely tendinous as in the posteriorsegments. It inserts on the neural arch of ver-tebra 10. There is an epicentral tendon in thehorizontal septum. We found the same ar-rangement in myosepta 9, 8, and 7. In 8 (fig.5B) the epineural tendon bears a small ossi-fication in its distal part, whereas the proxi-

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Fig. 3. Velifer hypselopterus. Micrographs of epaxial myosepta A, Myoseptum 11. B, Myoseptum 9.C, Myoseptum 4. D, Myoseptum 1.

1998 11GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

Fig. 4. Hoplostethus mediterraneus. Micrographs of epaxial myosepta. A, Myoseptum 8. B, Myo-septum 6. C, Myoseptum 3. D, Myoseptum 1, epicentral bone broken twice.

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mal part remains tendinous. In 7 (fig. 5C) theepineural bone is attached directly to the ax-ial skeleton. The site of insertion is shiftedventrally from the neural arch in 10, and thecentrum in 9, to the proximal part of the ven-tral rib in 8 and 7. The basic arrangement offibers in these myosepta of the midbody re-gion is almost identical to that described forPolymixia (13, fig. 2A), Velifer (9, 11; fig.3A, B), and Hoplostethus (8, fig. 4A).

Anteriorly, each myoseptum from 6 to 3bears an epicentral tendon (see fig. 5D–F). Inmyosepta 2 and 1, epicentrals are ossified.We found some peculiar changes of the in-termuscular bones in myosepta 6 (fig. 5D) to1, which we did not observe in other teleosts.Starting with 6, a ventral shift of the epi-neural bone can be noticed that involves theproximal part of the bone. This intermuscularbone 6 is located in the lateral band but stillattached to the rib as in 7. In myosepta 5, 4,and 3 (marked by *, see fig. 5E, F) the boneshave lost contact with the axial skeleton andare shifted further ventrally. They are in aposition neither epineural nor epicentral butsomewhat intermediate. There is still an ep-icentral tendon beneath these bones. Thestrong epineural tendon of the midbody myo-septa is not present here. Inserting at the cen-trum, there is a set of fibers that resemblesthe epineural fiber direction but the fibers donot extend to the integument. Bones of my-osepta 2 and 1 (fig. 5G, H) are strongly de-veloped in sharp contrast to those of 5, 4,and 3. They attach to centrum 2 or to neuralarch 1. They are in the horizontal septum andtherefore we consider them epicentrals.

This positional difference of intermuscularbones 1 and 2 (epicentrals) from those of 3,4, and 5 and from posterior ones (epineurals6 on) is reflected also by an ontogenetic dif-ference. In a series of five specimens of un-determined Holocentrinae ranging from 11 to26 mm SL, we found that intermuscularbones 1 and 2 are well ossified in all speci-mens and attach to the neural arch. However,bones posterior to 2 are not developed untilthe 26 mm stage. At this length we found sixadditional ossifications (3–8), none of themreaching the vertebral column.

PARACANTHOPTERYGII: Aphredoderus say-anus. We found some differences from Pat-terson and Johnson’s (1995) report of the in-

termusculars of this species. Our specimenhad nine bony intermusculars in the horizon-tal septum that represent the epicentral series.Bones 1 and 2 insert on the neural arch andposterior bones on the rib. This bony seriesis continued caudally by well-developed epi-central tendons. Myoseptum 15 (fig. 6A) isan example from the midbody region. Thereis a strong epicentral tendon with few POTsattached to it, a lateral band of evenly dis-tributed fibers, and an epineural tendon withconverging fibers. The myoseptum is thin,and the epineural tendon is visible only undera microscope.

Molva dipterygia. As in Aphredoderus,there is a bony epicentral series in the hori-zontal septum. We show a myoseptum fromthe midbody region (fig. 6B). It bears a smallepicentral ossification in the horizontal sep-tum that is embedded in a tendinous sheathand not attached to the axial skeleton. Wellabove the horizontal septum there is a broadepineural tendon with the fibers convergingslightly. They cross fibers of an aponeurosis-like, elongated lateral band.

ATHERINOMORPHA: Bedotia geayi. As re-corded by Patterson and Johnson (1995),there is a long series of bones in the hori-zontal septum of Bedotia. We show myosep-tum 15 (fig. 6C). There is a well-developedbony epicentral in the horizontal septum withPOTs crossing it. The lateral band is broad.The epineural is present but inconspicuous.

Scomberesox saurus. All myosepta of thiselongate species exhibit long DACs andDPCs which is reflected by the presence ofa very long lateral band. As in Bedotia thereis a long series of epicentrals in the horizon-tal septum. We show myoseptum 11 (fig. 6D)of this species. A bony epicentral is presentand a conspicuous lateral band. The epineur-al is a faint structure consisting only of fewstrands of fibers.

PERCOMORPHA: Lates calcarifer. As de-scribed by Patterson and Johnson (1995), La-tes has a short series of four intermuscularbones in the horizontal septum of the firstfour vertebra which we homologize with epi-centrals (see below). Bones 1 and 2 attach toneural arches and 3 and 4 to ribs. Thesebones are continued by epicentral tendonsthat are hard to recognize from 4 through 9.In figure 6E the weak tendon of 8 is shown.

1998 13GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

Fig 5. Holocentrus rufus. Micrographs of epaxial myosepta. A, Myoseptum 10. B, Myoseptum 8.C, Myoseptum 7. D, Myoseptum 6. E, Myoseptum 5. F, Myoseptum 4. G, Myoseptum 2. H, Myoseptum1. Note bony element (distal end marked by asterisk) shifting ventrally from margin of lateral band inD to area between epicentral tendon and lateral band in F (see discussion). Proximal ends of epineuraltendons in D–H marked by arrowhead.

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1998 15GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

←

Fig. 6. Micrographs of epaxial myosepta. A, Aphredoderus sayanus, myoseptum 15. B, Molva di-pterygia, myoseptum of midbody region. C, Bedotia geayi, myoseptum 15. D, Scomberesox saurus,myoseptum of midbody region. E, F. Lates calcarifer. E, Myoseptum 8. F, Myoseptum 2.

More posteriorly, these tendons becomemore pronounced and were recorded by Pat-terson and Johnson (1995) for vertebra 12and posterior segments. In addition to thisepicentral series there is an epineural seriesdorsal to the former. In myoseptum 8, an ex-ample for the midbody region, the epineuraltendon consists of numerous converging fi-bers. The lateral band is developed as thenormal broad band of evenly distributed fi-bers. This series of epineural tendons is con-tinued anteriorly as shown by myoseptum 2(fig. 6F). Here the epineural tendon consistsof fewer fibers than the epineural tendon onvertebra 8. The epicentral bone is located be-low the epineural and crossed by few POTs.

Centropomus robalito. Centropomus has apattern of intermuscular bones and tendonslike that of Lates. Epicentral bones are re-stricted to the anterior eight vertebrae with 1and 2 attached to neural arches and the sub-sequent ones to ribs. Bony epicentrals arecontinued by tendons that are weakly devel-oped. We show myoseptum 9 (fig. 7A) as anexample of this region. In contrast, epineuraltendons are conspicuous and consist of nu-merous converging fibers. They are accom-panied by a broad lateral band. Anteriorly,epineural tendons gradually change from dis-tinct tendons to broad bands with fibers con-verging only slightly. This can be noted inmyoseptum 3 in figure 7B. Below the epi-neural tendon there is an epicentral bone.

Morone americana. Intermusculars of M.americana strongly resemble those of Latesand Centropomus. The seven anterior myo-septa have epicentral bones in the horizontalseptum that are continued posteriorly by ten-dons, as shown by myoseptum 11 (fig. 7C).Epicentral and epineural tendons can be eas-ily recognized. The lateral band is present. Inmyoseptum 6 (fig. 7D), the epicentral is bonyand the epineural tendon consists of fewerfibers than that of 11. In myoseptum 2 (fig.7E), the epicentral bone is longer than in 6and attached to the neural arch. The epi-neural tendon is well-developed and inserts

above the epicentral. Myoseptum 1 (fig. 7F)has the same arrangement as 2, but the epi-neural tendon has a more diffuse appearance.

Serranus hepatus and Pseudanthias pleu-rotaenia. We found 11 epicentral bones inthe horizontal septum in Serranus and 12 inPseudanthias. Figure 8A shows myoseptum15 of Serranus. There is a distinct epineuraltendon with strongly converging fibers. Theepicentral tendon is weakly developed. Bothepicentral and epineural series are continuedanteriorly. In myosepta 8 (fig. 8B) and 2 (fig.8C), the epicentral is ossified but there is noremarkable change in the epineural tendon.The situation is very similar in Pseudanthias.In both myosepta 8 (fig. 8D) and 3 (fig. 8E)the epineural tendon is well above the epi-central bone. The lateral band is only weaklydeveloped.

Parupeneus barberinoides. Our specimenof Parupeneus bears a series of 14 epicentralbones in the horizontal septum. Their site ofattachment on the axial skeleton is the sameas reported for intermuscular bones (theretermed epineurals) of Upeneus by Pattersonand Johnson (1995). In both genera this se-ries is continued by tendons. There is a sec-ond series of intermusculars above the hori-zontal septum, the epineural tendons,whichcan be traced rostrally to the second segment.

Kuhlia rupestris. Kuhlia has a series ofseven epicentral bones in the horizontal sep-tum. As evidenced by myoseptum 5 (fig.9A), there is an epineural tendon above theepicentral bone. It consists of several strongconverging fibers. Myoseptum 8 (fig. 9B) isalmost identical to 5 but the epicentral is ten-dinous here.

Oligoplites saurus. The epicentral series ofOligoplites consists of nine bony elementscontinued caudally by tendons. We show my-osepta 10 from the midbody and 2 from theanterior region. Myoseptum 2 (fig. 9C) has abony epicentral. The lateral band is broadand consists of thick fibers. The epineuraltendon is conspicuous and inserts on the neu-ral arch. Myoseptum 10 (fig. 9D) is different

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Fig 7. Micrographs of epaxial myosepta. A, B. Centropomus robalito. A, Myoseptum 9. B, Myo-septum 3. C–F. Morone americana. C, Myoseptum 11. D, Myoseptum 6. E, Myoseptum 2. F, Myo-septum 1.

1998 17GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

Fig 8. Micrographs of epaxial myosepta. A–C. Serranus hepatus. A, Myoseptum 15. B, Myoseptum8. C, Myoseptum 2. D, E. Pseudanthias pleurotaenia. D, Myoseptum 8. E, Myoseptum 3.

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Fig 9. Micrographs of epaxial myosepta. A, B. Kuhlia rupestris. A, Myoseptum 5. B, Myoseptum8. C, D. Oligoplites saurus. C, Myoseptum 2. D, Myoseptum 10.

from 2 in that it is more elongate and has adistinct DAC. The epicentral is tendinousand has numerous POTs crossing it. The lat-eral band is a long and strong fiber bundle.The epineural tendon consists of severalthick converging fibers that attach to the neu-ral spine.

Caranx hippos. We show myoseptum 2and one from the anterior abdominal region(precise segment unknown; fig. 10A, B) re-moved from an adult specimen. The arrange-

ment of bones and tendons in the two myo-septa is very similar. There is a strong epi-central bone in both. The epineural tendon isdistinct at its proximal end and fans out to-ward its distal part. The lateral band is dis-tinct and has the appearance of an aponeu-rosis.

Sphyraena barracuda. We show a myo-septum of the anterior body region of a largeadult (fig. 10C). There is an epicentral bonewith one POT attached to it. Similarly as in

1998 19GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

Fig. 10. A–C. Photographs of epaxial myosepta. A, B. Caranx hippos. A, Myoseptum 2. B, Myo-septum from anterior body. C, Sphyraena barracuda, myoseptum from midbody. D–E. Channa obscura.D, Micrograph of horizontal septum from midbody, dorsal view, right side, anterior to the left. E,Micrograph of epaxial myoseptum 19.

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Caranx, the epineural tendon is distinct at itsproximal end and fans out toward its distalpart. The lateral band is a well-defined broadband of dense fibers.

Channa obscura. This species bears a se-ries of about 19 epicentral bones that is con-tinued posteriorly by a series of epicentraltendons. As shown by the micrograph of thehorizontal septum (fig. 10D), the series iscrossed by POTs. Above the series in theepaxial part of each myoseptum we again candemonstrate epineural tendon and lateralband (fig. 10E). Even if less pronounced,they are present in all body segments.

Arnoglossus laterna. This is the only per-comorph we investigated bearing epineuralossifications. There are Y-shaped epineuralossifications in the anterior 12 segments. Insubsequent myosepta only the lateral band(but not the anteromedial branch) is ossified.Thus the ossifications are C-shaped. Epicen-trals of the first 12 segments are ossified.

DISCUSSION

Johnson and Patterson (1993) and Patter-son and Johnson (1995) concluded that thesingle bony series of intermusculars of mostacanthomorphs is not the homolog of the epi-pleural series of non-acanthomorphs as wasgenerally thought. We agree with this con-clusion. Primitively, in acanthomorphs thereare three separate series of intermusculars,which in their unmodified state are easy todistinguish by their position: epineurals arein the epaxial myoseptum, epicentrals in thehorizontal septum, and epipleurals in thehypaxial myoseptum. However, that criteriondoes not work in most acanthomorph fishesbecause there is only a single series of bones.Even assuming that bony series may shiftduring phylogeny, as already noted by Owen(1846, 1866), one problem of homology re-mains: Is the single bony series in most acan-thomorphs the homolog of the epicentral orof the epineural series of lower teleosts?

In each series, a bone or a tendon corre-sponds to a bone or a tendon in about thesame position in the adjacent myosepta.(Only Holocentrus seems to be an exception,see discussion below.) We present evidencethat there are two series, either tendinous orbony, in all acanthomorph species investi-

gated. One series is situated in the horizontalseptum, and the other dorsal to that in theepaxial part of the myosepta. Thus we canuse the position as a simple criterion and ho-mologize the series in the horizontal septumof acanthomorphs (anterior elements usuallybony) with the epicentral series of lower tel-eosts. The series (usually tendinous) in theepaxial myosepta dorsal to the epicentral se-ries is homologized with the epineural seriesof lower teleosts.

These homologies stand in contrast to thefindings of Patterson and Johnson (1995)who hypothesized that the single bony seriesin the horizontal septum is the epineural se-ries that is shifted ventrally. In the subse-quent discussion we explain our line of evi-dence and discuss their hypothesis in light ofour findings.

PATTERSON AND JOHNSON’S (1995)HYPOTHESIS:

VENTRAL SHIFT OF EPINEURALS

INTO HORIZONTAL SEPTUM

Patterson and Johnson (1995) comparedthe situation of intermusculars in Polymixia(considered the primitive euacanthomorphpattern), Holocentrus (primitive acantho-pterygian pattern), Lates (primitive perco-morph pattern), and Pseudanthias (derivedpercomorph pattern). Polymixia was hypoth-esized to show the most primitive stateamong euacanthomorphs, because there arethree series of intermusculars: an epineuralseries, predominantly bony, an epicentral se-ries, completely tendinous, and an epipleuralseries, partly bony and partly tendinous.However, compared to what they take to beprimitive for acanthomorphs, Polymixiashows one derived feature: The first inter-muscular bone, which they homologizedwith the epineural of the first vertebra, nolonger lies in the epaxial myoseptum but isshifted ventrally into the horizontal septum.In Holocentrus, considered by them to ex-emplify the primitive state for acanthopter-ygians, not only one but the first five to sevenintermuscular bones, which they considerepineurals, are shifted ventrally into the hor-izontal septum.

Eventually, in percomorphs all intermus-cular bones, which they claim to be epineu-

1998 21GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

rals, lie in the horizontal septum. Therefore,they hypothesized that all epineurals areshifted ventrally into the horizontal septum.Accordingly, this ventral displacement of theepineurals into the horizontal septum hastaken place in three steps, which Johnson andPatterson also used to define monophyleticgroups (see below).

The first step, ventral displacement of thefirst epineural into horizontal septum (John-son and Patterson, 1993: 601, character 8), isconsidered a synapomorphy for their Eu-acanthomorpha (Acanthomorpha minusLampridiformes). The second step, ventralshift of anterior epineurals into horizontalseptum (ibid., p. 603, character 14), is con-sidered a synapomorphy for their Holacan-thopterygii (Acanthomorpha minus Lampri-diformes and Polymixiiformes). The finalstep, ventral displacement of all epineuralsinto horizontal septum (ibid.,p. 615, charac-ter 32), is considered a synapomorphy oftheir Percomorpha (which includes Atheri-nomorpha).

They discovered, unexpectedly, a series oftendinous structures above the epineural se-ries in a number of percomorph taxa. Ac-cording to their hypothesis, this dorsal seriesis not an epineural series because that seriesis located in the horizontal septum. Conse-quently, they concluded that the tendinousseries above the epineurals is a neoforma-tion—the so-called ‘‘neoneural ligaments.’’These ‘‘neoneural ligaments’’ may even os-sify in some bothid and samarid species andthen represent ‘‘neoneural bones.’’

Patterson and Johnson had doubts con-cerning their method of recognizing tendonsin glycerin-stored cleared-and-stained speci-mens (1995: 4) : ‘‘. . . we cannot pretend thatobserving and recording intermuscular liga-ments is an entirely objective procedure. . .’’.On the other hand they also found that‘‘Transferring cleared-and-stained specimensto alcohol may render the ligaments moreopaque, and so more visible’’ (1995: 4).

We used alcohol along with microdissec-tion, light- and DIC-microscopy, and polar-ized light to demonstrate the presence of var-ious tendinous structures apparently over-looked by them. We consider our methodmore objective, which leads to reproducibleresults.

HOMOLOGY OF INTERMUSCULAR BONE ON

FIRST VERTEBRA IN POLYMIXIA

Patterson and Johnson (1995) identifiedthe first intermuscular bone of Polymixia asan epineural that is displaced ventrally fromits position in the epaxial part of the myo-septum into the horizontal septum. Theybased their homology on two lines of evi-dence (1995: 33) : (1) the first intermuscular‘‘develops in series with the epineurals andis fully formed at 12 mm SL, before the epi-centrals are recognizable’’ and (2) ‘‘there isno lower teleost in which the epicentral se-ries contains just one bone on V1 followedby a series of ligaments’’.

We have demonstrated above that a dis-tinct tendinous structure originates dorsal tothe origin of the first intermuscular and isaligned with the subsequent epineural bones(fig. 2). We therefore interpret this tendon asthe epineural of the first vertebra. It was notreported and apparently overlooked by them.This may be caused by the different tech-nique that they applied, i.e., studying speci-mens in glycerin.

We think that an epineural tendon on thefirst vertebra in Polymixia is sufficient evi-dence that the bone ventral to this tendon isnot a ventrally shifted epineural bone, andwe want to address their argument briefly.The intermuscular bone on the first vertebra‘‘develops in series with the epineurals andis fully formed at 12 mm SL, before the epi-centrals are recognizable’’ (Patterson andJohnson, 1995: 33). We consider this argu-ment to be invalid because the identificationof epicentral tendons in 12 mm long cleared-and-stained specimens would be impossiblewith their technique. The fact that ‘‘there isno lower teleost in which the epicentral se-ries contains just one bone on V1 followedby a series of ligaments’’ (1995: 33) is notevidence that this is impossible, but ratherthat Polymixia is unique in this respect.

In summary there is strong and unambig-uous evidence that the first vertebra of Po-lymixia has an epineural tendon. Therefore,the first intermuscular bone of Polymixia isnot an epineural, as claimed by Patterson andJohnson (1995), but an epicentral.

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INTERMUSCULAR BONES IN LAMPRIDIFORMES

There is only a slight difference betweenour and Patterson and Johnson’s (1995)specimen of Velifer. We could not identifythe discrete epicentral tendon of the first ver-tebra as described by them. Both investiga-tions reveal a tendinous epicentral series inthe horizontal septum. The series dorsal tothat is tendinous in its posterior part andbony in its anterior part. It is in an epineuralposition. We thus consider it an epineural se-ries.

Differences between our and their speci-men of Lampris guttatus in the attachment ofthe first two intermuscular bones are inter-esting. We found both anterior bones at-tached to the centrum. The first stout boneeven articulates in a well-developed, deepgroove of the centrum. All subsequent inter-muscular bones attach to the proximal partsof the ribs. This arrangement of intermus-cular bones is characteristic for the epicentralbones of higher acanthomorphs (see discus-sion below). In fact, they (Johnson and Pat-terson, 1993: 558) have already pointed outthis resemblance but concluded that thisplacement is developed independently inLampris and higher acanthomorphs. It wouldbe worth investigating myosepta of this spe-cies to check the precise placement of inter-muscular bones in the myoseptum and to re-consider their possible homology with epi-centrals.

INTERMUSCULAR BONES IN

PARACANTHOPTERYGII

There are no difficulties in homologizingintermusculars in Molva and Aphredoderus,because in both genera there is a bony/ten-dinous epicentral series in the horizontal sep-tum and a tendinous epineural series in theepaxial part of the myoseptum. Patterson andJohnson (1995) identified a single bony se-ries in the horizontal septum of the paracan-thopterygians, Percopsis guttatus, Aphredo-derus sayanus, Raniceps raninus, and Gai-dropsaurus mediterraneus, and homologizedthem with epineural bones shifted ventrallyinto the horizontal septum. We did not rein-vestigate all species but our different findingsin Aphredoderus sayanus led to the conclu-

sion that Patterson and Johnson must haveoverlooked the epineural tendons.

INTERMUSCULAR BONES IN BERYCIFORMES

We described a continuous tendinous se-ries in the epineural as well as in the epicen-tral position (bony on V1 and V2) for Ho-plostethus and Gephyroberyx. These resultsallow unequivocal homology. In Pattersonand Johnson’s (1995) Hoplostethus speci-men, they recorded only a series of epicentraltendons from vertebra 9 backward and twobones on V1 and 2 in the epicentral position.They interpreted the two bones as epineuralbones shifted ventrally into the horizontalseptum. In light of our results, we think thatthey overlooked the real epineural tendons inthe anterior 8 myosepta because they wereunrecognizable in glycerin-stored cleared-and-stained specimens.

Homologization of intermuscular bones inour Holocentrus specimen is unambiguousfor myosepta from 7 backward. We found acontinuous series of tendons in the horizontalseptum preceded by bones on V1 and V2,and we regard these as homologs of epicen-tral tendons or bones. The series dorsal to theepicentrals is the epineural series. In myo-septa 6 to 1, these bony elements undergo aremarkable ventral shift. In myosepta 4 and3, they are situated even ventral to the lateralband. It is not clear whether an epineural ten-don is present as well. We are not sure howto interpret this pattern of bones lying be-tween the epineural and epicentral position.Because we have not seen a similar arrange-ment in any other species, we are inclined tointerpret these as autapomorphic for Holo-centridae.

For the two bony elements in the horizon-tal septum, the question arises whether theyare true epicentrals, as claimed above, or epi-neurals that have shifted further ventrally asclaimed by Patterson and Johnson (1995),who investigated two species, Holocentrusspinifer and Holocentrus vexillaris. Theyfailed to recognize tendinous epicentrals 3 to8 but recorded them from 9 backward. Theyconsidered the two bones on V1 and V2 asepineurals because the bones are in line withother epineural elements. We have demon-strated that these bones on V1 and V2 are in

1998 23GEMBALLA AND BRITZ: INTERMUSCULAR BONES IN ACANTHOMORPHS

line with the epicentral series. Patterson andJohnson may have overlooked epicentral ten-dons 3 to 8 because the tendons are difficultto detect by examination of glycerin speci-mens but, according to our findings, moreeasily detected by dissection and microscopy.

Our interpretation that the first two inter-muscular bones are epicentrals and thus dif-fer from the posterior bones is further sup-ported by ontogenetic differences that wefound between the two bone groups. The firsttwo intermuscular bones are already presentin the smallest larva available (11 mm),whereas 3 to 6 ossify in line with true epi-neural bones of 7 backward, but not beforethe 26 mm stage.

INTERMUSCULAR BONES IN ATHERINOMORPHA

In none of the atherinomorphs (Bedotiasp., Menidia peninsulae, and Exocoetus vol-itans) they investigated did Patterson andJohnson (1995) record two intermuscular el-ements within a single myoseptum. With theexception of some posterior segments in Me-nidia, they reported only a bony series in thehorizontal septum, which was interpreted asepineural shifted ventrally. Again, with ourtechnique, we detected two intermuscular el-ements in a single myoseptum for Bedotiageayi and Scomberesox saurus. We considerthe bony elements in the horizontal septumto be epicentrals, and the tendinous elementssituated above the horizontal septum to beepineurals.

INTERMUSCULAR BONES IN PERCOMORPHA

In all percomorphs there is only one seriesof intermuscular bones, which has generallybeen referred to as epipleural. Patterson andJohnson (1995) pointed out that this series isnot the homolog of the epipleural series oflower teleosts, and we agree with this state-ment.

We have demonstrated above that there aretendons in the position of epineurals dorsalto the intermuscular bones in all perco-morphs that we investigated. The tendons in-sert on the neural arches of the respectivevertebra and we homologize them with epi-neural tendons. Therefore, the intermuscularbone situated ventral to the tendons is not anepineural but an epicentral. This contradicts

the homology of Patterson and Johnson(1995). They claimed that the single series ofintermuscular bones in percomorphs is theepineural series that has shifted ventrally.This homology is based on a comparison ofintermusculars of Polymixia, Holocentrus,and Centropomus. They considered the po-sition of the intermusculars in these threetaxa to be a morphocline reflecting ventraldisplacement of epineurals into the horizon-tal septum during evolution.

However, we have seen that (1) the inter-muscular bone on V1 of Polymixia is not anepineural, (2) other beryciforms like Hoplo-stethus have epineural tendons dorsal to theirseries of intermuscular bones, and (3) centro-pomids also possess a series of epineural ten-dons dorsal to their series of intermuscularbones. For these reasons we conclude thatPatterson and Johnson (1995) were misled inhomologizing the single series of intermus-cular bones with epineurals of lower teleosts.Our results present unambiguous evidencethat there are epineural tendons in Perco-morpha and that their single bony series isactually the homolog of the epicentral seriesof lower teleosts.

Patterson and Johnson (1995) reportedtendons in the position of epineurals of lowerteleosts also in a number of percomorph rep-resentatives (Ammodytidae, Pomacentridae,Polynemidae, Mullidae) that even ossify inthe pleuronectiforms Samaris and Bothus(and Arnoglossus according to our observa-tions). Because they claimed that epineuralsof percomorphs are ventrally displaced andlocated in the horizontal septum, Pattersonand Johnson concluded that the series of ten-dons in the position of epineurals of lowerteleosts is something different. They there-fore called those tendons ‘‘neoneurals’’ andreported them in additional percomorph taxa:Teraponidae, Kuhliidae, Carangidae, Eche-neidae, Lutjanidae, Caesionidae, Gerreidae,Sparidae, Lethrinidae, Sciaenidae, Haemuli-dae, Labridae, and Scaridae.

In percomorphs, presence of tendinousstructures that occur in the position of epi-neurals of lower teleosts is additional evi-dence for our hypothesis that intermuscularbones in percomorphs are epicentrals. Wehave checked myosepta of some of these taxathat are reported to possess ‘‘neoneurals’’

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(see above Kuhlia, Parupeneus, Oligoplites,Caranx) and found the same basic pattern ofepineurals and epicentral bones in the myo-septum that we reported for other perco-morphs (see above). We therefore have nodoubt that percomorph ‘‘neoneurals’’ arenormal epineurals and that percomorph ‘‘epi-neurals’’ are epicentrals.

PHYLOGENETIC IMPLICATIONS AND

CONCLUDING REMARKS

We have presented convincing evidencethat (1) the first intermuscular bone of Po-lymixia is an epicentral and (2) the single se-ries of intermuscular bones in higher acan-thomorphs is the homolog of the epicentralseries of non acanthomorphs. If this interpre-tation is accepted then the synapomorphy‘‘First epineural displaced ventrally into thehorizontal septum’’ used by Johnson and Pat-terson (1993: 601, character 8) to supporttheir Euacanthomorpha (Acanthomorpha mi-nus Lampridiformes) is invalid. We thinkthat the phylogenetic position of Lampridi-formes basal to Polymixiiformes needs crit-ical reexamination especially in the light ofthe sister-group relationship of Polymixia andall remaining acanthomorphs proposed byother authors (Rosen, 1985; Stiassny, 1986;Patterson and Rosen, 1989; Stiassny andMoore, 1992).

Accepting our hypothesis also invalidatesthe following characters as synapomorphies:‘‘Epicentral ligaments absent anteriorly’’(Johnson and Patterson, 1993: 603, character13) and ‘‘Distal parts of anterior epineuralsdisplaced ventrally into horizontal septum’’(1993: 603, character 14) used by Johnsonand Patterson to support their Holacanthop-terygii. ‘‘Anterior epineurals displaced ven-trally on to the ribs’’ (1993: 606, character

20) used to support a group Zeiformes 1 Eu-acanthopterygii and ‘‘Point of origin of allbut the first two epineurals displaced ven-trally, and the distal parts of all epineuralsdisplaced ventrally into the horizontal sep-tum’’ (1993: 615, character 32) used to sup-port their Percomorpha (including Atherino-morpha).

A series of epicentral bones may turn outto be a synapomorphy for their Holacanthop-terygii. However, further studies are neededto support such a hypothesis.

Our hypothesis explains several of theirconflicting data.

1. The ventral position of the intermus-cular bones of Paracanthopterygii, some Ste-phanoberyciformes, and Zeiformes need notbe explained as a downward shift of epi-neurals acquired independently from that inPercomorpha (which include Atherinomor-pha). Rather it is the plesiomorphic retentionof an epicentral bony series.

2. In zeiforms and percomorphs the ante-rior POTs are thought (Patterson and John-son, 1995: ii) to ‘‘acquire a new association,attaching to epineural bones secondarily po-sitioned in the horizontal septum.’’ Our hy-pothesis does not need such an assumptionbecause the bones in the horizontal septumare epicentrals and thus the situation of POTsin zeiforms and percomorphs is a retentionof the plesiomorphic association between thetwo systems.

3. The concept of ‘‘neoneurals’’ in somepercomorphs is superfluous because the‘‘neoneurals’’ are true epineurals.

As Patterson and Johnson (1995) havestressed, intermusculars of teleosts holdmuch potential for resolution of phylogenet-ically problematic groups, and we hope thatour improvement of their method will helpin accomplishing this goal.

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