CYPRINIFORMES

Datovo, Aléssio & Vari, Richard P., 2014, The adductor mandibulae muscle complex in lower teleostean fishes (Osteichthyes: Actinopterygii): comparative anatomy, synonymy, and phylogenetic implications, Zoological Journal of the Linnean Society 171 (3), pp. 554-622 : 580-584

publication ID	https://doi.org/ 10.1111/zoj.12142
persistent identifier	https://treatment.plazi.org/id/03DB6116-5334-A215-FEE8-D28727B27B03
treatment provided by	Marcus
scientific name	CYPRINIFORMES
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CYPRINIFORMES View in CoL View at ENA

Description

Rasbora cephalotaenia ( Fig. 15 View Figure 15 )

The rictalis is fully subdivided into the ectorictalis and endorictalis. The ectorictalis originates from the quadrate and preopercle and inserts tendinously on the lateral face of the maxilla. Although the ventral portion of the endorictalis is partially continuous with the malaris at their origins, these two sections are completely separate from each other along most of their expanse. The endorictalis originates from the quadrate and preopercle and inserts primarily musculously on the posterior portion of the coronoid process of the lower jaw. An inconspicuous transverse raphe is present approximately at the midlength of the endorictalis.

The malaris originates from the hyomandibula, symplectic, and preopercle. Its fibres converge onto the dorsoposterior portion of the elongate intersegmental aponeurosis (= subocular tendon). Anteriorly this aponeurosis subtly divides into a thin mandibular tendon and a robust meckelian tendon that anchors to the coronomeckelian.

Albeit being largely continuous with the malaris, the stegalis is easily discernible from a medial view by its anteriorly displaced origin on the metapterygoid. Fibres of the stegalis attach along the ventral margin of the strap-like intersegmental aponeurosis.

The ramus mandibularis trigeminus nerve traverses the middle of the endorictalis in the course of its passage through the segmentum facialis.

The thin, elongate mandibular tendon serves as the axial tendon of origin for the bipinnate segmentum mandibularis. As a result of this bipinnate configuration, the coronalis and mentalis can be recognized along the posterior portion of the segmentum mandibularis. No distinction between the coronalis and mentalis is apparent anteriorly and the whole segmentum mandibularis inserts on the dentary and Meckel’s cartilage.

Danio rerio (not illustrated)

The adductor mandibulae morphology in Danio rerio is virtually identical to that of Rasbora cephalotaenia , including the presence of a transverse raphe on the endorictalis and a distinguishable stegalis. Neither feature was noted in previous descriptions of the adductor mandibulae of this taxon (cf. Hernandez et al., 2005; Hernandez, Bird & Staab, 2007; Diogo & Doadrio, 2008; Diogo et al., 2008a, b; Staab & Hernandez, 2010).

Raiamas senegalensis ( Fig. 16A View Figure 16 )

The ectorictalis originates from the quadrate and preopercle. Shortly anterior to the area of origin, the muscle fibres that arise from the region of the articular condyle of the quadrate differentiate from the remaining sections, thereby yielding a clear subdivision into an ectorictalis superioris and an ectorictalis inferioris. Most of the fibres of the ectorictalis inferioris arise from a ventral tendon attached to the quadrate and spread out anterodorsally to insert on the lateral surface of the maxilla. Towards its insertion, the endorictalis superioris passes medial to the endorictalis inferioris and inserts onto the tendon running along the ventromedial region of the rictalis.

The endorictalis, malaris, and stegalis are largely continuous with each other at their origins, which involve the quadrate, metapterygoid, hyomandibula, preopercle, and pterotic. Proximate to its insertion, the endorictalis separates from the adjacent malaris and attaches primarily musculously on the coronoid process of the dentary. The endorictalis and malaris are further separated from each other by the ramus mandibularis trigeminus nerve that passes between them. Towards its insertion, the fibres of the malaris converge onto the lateral face of the intersegmental aponeurosis.

At its origin, the stegalis is separated into an epistegalis that arises medial to the levator arcus palatini and a substegalis with an origin that can barely be differentiated from that of the malaris. Anteriorly the epistegalis and substegalis conjoin and insert on the intersegmental aponeurosis together with the malaris. Although not readily distinguished from the malaris, the presence of the substegalis section in Raiamas can be inferred based on the origin of a set of shorter fibres from the metapterygoid, a typical site of origin for the substegalis in both the Cypriniformes and most other teleosts.

The configuration of the segmentum mandibularis and the insertion of the intersegmental aponeurosis in Raiamas are basically similar to that of Rasbora .

Carassius auratus ( Fig. 16B View Figure 16 )

The ectorictalis is completely separated from the underlying facial sections and arises from the preopercle, quadrate, and angulo-articular. Towards its insertion, the ectorictalis progressively separates into two sections – the ectorictalis inferioris and ectorictalis superioris. Each subsection converges onto separate insertional tendons that cross anteriorly and attach to the lateral surface of the maxilla.

At their origins, the endorictalis and malaris cannot be differentiated from each other and thus form an endoricto-malaris. This common origin for these two sections involves the preopercle and hyomandibula. As they proceed toward their insertions, these sections separate. The endorictalis grades into an aponeurosis that inserts along the posterodorsal rim of the anguloarticular and the coronoid process of the dentary. The malaris, in turn, converges to the intersegmental aponeurosis.

The ramus mandibularis trigeminus nerve courses through the segmentum facialis between the malaris and endorictalis. The nerve then continues through the middle of the endorictalis to soon thereafter emerge ventrally between that section laterally and the intersegmental aponeurosis medially prior to its final passage to the lower jaw.

At its origin, the stegalis is readily distinguishable from the endoricto-malaris by its shorter fibres that arise from the metapterygoid. The posterodorsal portion of the stegalis is separated from the adjacent malaris but as these sections proceed anteriorly towards their insertion on the intersegmental aponeurosis, they become fully continuous and indistinguishable from each other. The major part of the intersegmental aponeurosis converges onto the coronomeckelian, thus forming a meckelian tendon. The mandibular tendon is represented solely by a delicate, flat connective tissue band that serves as the site of origin for the tiny segmentum mandibularis.

The segmentum mandibularis consists of an extremely small and delicate bundle of parallel fibres. It originates from the mandibular tendon and inserts on Meckel’s cartilage.

Remarks

The segmentum facialis exhibits an outstanding degree of morphological diversification across the Cypriniformes , with the scale of the modifications especially pronounced in the Cobitoidea (sensu Conway, 2011). Elucidation of muscle division homologies is challenging when confronted with morphological diversity at this scale. Notwithstanding these complications, tentative hypotheses of homologies can be advanced for adductor mandibulae divisions in most of the prior studies of that system. That said, broader investigations of the adductor mandibulae across the Cypriniformes are clearly necessary in order both to test our proposed synonymy in the more derived members of the order and to evaluate the apparently enormous amount of informative phylogenetic information present in this morphological complex.

The most comprehensive and detailed survey of the cranial musculature of the Cypriniformes to date is that of Takahasi (1925). The adductor mandibulae morphology of the Cyprinidae reported in that study is similar in most aspects to those of the taxa examined herein. Although the cobitoids reported in that study exhibit relatively modified forms of the adductor mandibulae compared with those of cyprinids, the section names applied by Takahasi (1925) seemingly consistently reflect their homology across the entirety of the Cypriniformes . Based on overall obvious topological correspondences between the muscle divisions in the Cypriniformes reported in Takahasi (1925), we confidently infer that his A 1 (or maxillaris) corresponds to the ectorictalis, the A 2 to the endorictalis, the A′ 3 to the malaris, and the A″ 3 to the stegalis.

The most ventrolaterally located facial division corresponding to the ectorictalis inserts primarily on the lateral surface of the maxilla across all the Cypriniformes ( Vetter, 1878; Takahasi, 1925; Edwards, 1926; Eaton, 1935; Van Dobben, 1935; Girgis, 1952; Munshi, 1960; Weisel, 1960; Alexander, 1966; Ballintijn, van den Burg & Egberink, 1972; Wu & Shen, 2004; Hernandez et al., 2005; Diogo & Doadrio, 2008; Diogo et al., 2008a, b; Staab & Hernandez, 2010; Staab, Ferry & Hernandez, 2012). In some cyprinids, the ectorictalis is undivided ( Fig. 15 View Figure 15 ), but in most other Cypriniformes this section is partially, or totally, subdivided into a dorsal ectorictalis superioris and a ventral ectorictalis inferioris ( Fig. 16 View Figure 16 ). These two subdivisions may undergo diverse modifications across the Cypriniformes ; however, in all known instances the insertion of the ectorictalis superioris on the maxilla is situated medial to the ectorictalis inferioris ( Vetter, 1878; Takahasi, 1925; Edwards, 1926; Van Dobben, 1935; Weisel, 1960; Alexander, 1966; Ballintijn et al., 1972; Wu & Shen, 2004; Staab et al., 2012). In many cypriniforms, the origin of the ectorictalis inferioris is expanded anteriorly so as to involve the lower jaw in addition to the quadrate ( Fig. 16B View Figure 16 ). In further advanced morphologies, this section loses its origin from the quadrate ( Vetter, 1878; Takahasi, 1925; Edwards, 1926). Some cobitoids have the insertional tendon of the ectorictalis inferioris dorsally shifted and attaching to the kinethmoid (= ‘rostral cartilage’ of Takahasi, 1925) or the ‘connective tissue located between dorsal midline of the premaxillae’ and ‘the base of the first barbel’ ( Kim & Kim, 2007). The ectorictalis superioris, in turn, tends to greatly expand dorsally so as to almost entirely cover the inner sections of the segmentum facialis ( Takahasi, 1925; Edwards, 1926; Eaton, 1935; Weisel, 1960; Kim & Kim, 2007). Amongst some derived cobitoids, the ectorictalis superioris differentiates into several subsections, some of which may attach to the second pre-ethmoid ( Takahasi, 1925; Kim & Kim, 2007). Precise homologies of all these sections remain to a degree unclear, but the dorsal-most of these subsections is particularly interesting because it apparently forms the so-called preorbitalis or praeorbitalis, a muscle that inserts on the lateral ethmoid of some cobitoids ( Takahasi, 1925; Kim & Kim, 2007). Such a homology hypothesis is supported by the fact that some taxa have a tendon running from the lateral ethmoid to the dorsomedial portion of the ectorictalis superioris ( Takahasi, 1925; Kim & Kim, 2007).

In most, if not all cypriniforms, the ventral facial section positioned immediately medial to the ectorictalis, i.e. the endorictalis, inserts musculously on the coronoid process of the lower jaw ( Vetter, 1878; Takahasi, 1925; Girgis, 1952; Munshi, 1960; Alexander, 1966; Ballintijn et al., 1972; Hernandez et al., 2005; Diogo & Doadrio, 2008; Diogo et al., 2008a, b; Staab & Hernandez, 2010). At their insertion, the medial fibres of the endorictalis in some cypriniform taxa pass adjacent to, and are contiguous with, the segmentum mandibularis ( Fig. 15B View Figure 15 ; Takahasi, 1925). The malaris and stegalis are associated with the intersegmental aponeurosis and are, some- times, largely continuous with one another thereby forming a compound stego-malaris ( Takahasi, 1925). The dorsal portion of the stegalis may pass either medial (e.g. Raiamas ) or anterior (e.g. Danio and Rasbora ) to the levator arcus palatini. In the latter configuration, an unequivocal distinction of the stegalis from the malaris is sometimes impossible (A. Datovo pers. observ.; Takahasi, 1925). The endorictalis also may be continuous with the immediately dorsomedially situated malaris, thereby forming a compound endorictomalaris (e.g. Carassius ; Girgis, 1952; Munshi, 1960) or endoricto-stego-malaris ( Takahasi, 1925; Edwards, 1926; Weisel, 1960; Kim & Kim, 2007). In most, if not all, of these cases, each section apparently still retains its primitive insertion site: the endorictalis attaches to the coronoid process of the lower jaw whereas the fibres of the malaris/stego-malaris converge to the intersegmental aponeurosis.

The ramus mandibularis trigeminus nerve in the Cypriniformes may transit either between the ectorictalis and endorictalis [ Iksookimia , Misgurnus , Niwaella multifasciata (= Kichulchoia multifasciata ); Kim & Kim 2007], through the centre of the endorictalis ( Carassius , Danio , and Rasbora ; Fig. 15 View Figure 15 ), between the rictalis and malaris ( Raiamas ), or between the malaris and stegalis ( Carassius ; Wu & Shen, 2004).

The segmentum mandibularis is absent in some taxa in the Cyprinidae and apparently across all of the Cobitoidea ( Takahasi, 1925).

Howes (1978) examined the adductor mandibulae of Raiamas bola (= Barilius bola ). Comparison of his results with our observations on the congener Raiamas senegalensis leads us to conclude that his A1a in R. bola corresponds to the ectorictalis inferioris and his A1b to the ectorictalis superioris. The identities of the deeper muscle sections are somewhat uncertain, but the A2 of Howes (1978) possibly corresponds to the endoricto - malaris and his A3 to the stegalis. Owing to insufficient information, it is impossible to confidently synonymize the divisions of the adductor mandibulae of most of the other cyprinids surveyed by Howes (1978). For the same reason, the later analyses by that author involving cypriniforms ( Howes, 1982, 1984) are also not included in the following synonymy.

Synonymy

Pars ectorictalis

A0: Diogo & Doadrio (2008): Danio ; Diogo et al. (2008a): Danio ; Diogo et al. (2008b): Danio .

A 1: Ballintijn et al. (1972): Cyprinus ; Eaton (1935): Abramis , Catostomus ; Takahasi (1925): Acheilognathus , Carassius , Cobitis , Cyprinus , Hymenophysa curta (= Parabotia curta ), Ischikauia , Lefua , Leucogobio güntheri (= Gnathopogon elongatus ), Misgurnus , Opsariichthys , Orthrias (= Barbatula ), Pseudogobio , Pseudorasbora , Sarcocheilichthys , Richardsonium hakuensis (= Tribolodon hakonensis ), Zacco ; Vetter (1878): Barbus , Cyprinus .