Aphanopus carbo, Lowe, 1839

Schepper, N. De, Wassenbergh, S. Van & Adriaens, D., 2008, Morphology of the jaw system in trichiurids: trade-offs between mouth closing and biting performance, Zoological Journal of the Linnean Society 152, pp. 717-736 : 721-730

APHANOPUS CARBO

Adductor mandibulae complex

The adductor mandibulae complex is a large muscle complex of the cheek region that covers the lateral surface of the suspensorium, below and behind the eye ( Fig. 2A). Three main parts can be recognized: the adductor mandibulae A 1 -A 2, separated from the A 3 ( Fig. 3A) and the AW ( Fig. 4A).

1. The superficial part of the adductor mandibulae complex is referred to as A 1 -A 2 since no clear-cut subdivisions (into the A 1 and A 2) are observed ( Fig. 2A). Though based on the nomenclature of Winterbottom (1974) the presence of a tendinous connection (T Mx) with the maxilla, lachrymal and primordial ligament suggests the presence of the A1. The bulk of the fibres of this complex may represent the A2 of the adductor mandibulae complex, according to the nomenclature of Winterbottom (1974) as they are latero-ventrally situated and insert indirectly onto the Meckelian fossa by means of the tendon T A1 - A2 ( Fig. 4A). This tendon additionally merges with the tendon of AW. Most of the fibres of the A1-A2 complex are antero-ventrally directed. Some fibres bordering the posterior edge of the orbit are more dorso-ventrally orientated. These fibres originate musculously from the lateral surface of the frontal and the antero-lateral surface of the sphenotic, and cover the levator arcus palatini and the anterior part of the dilatator operculi. The remaining fibres originate musculously from the lateral ridge of the elongate ventral arm of the hyomandibula ( Fig. 3A), the anterior border of the preopercle, the antero-ventral part of the lateral surface of the quadrate and the antero-dorsal part of the metapterygoid.

2. The second part of the adductor mandibulae complex, the A3, is the most medial part ( Fig. 3A). The origin of the A3 includes the ventral two-

thirds of the crescentic anterior edge of the preopercle, the ventro-posterior part of the lateral surface of the metapterygoid, the lateral surface of the symplectic and the ventro-posterior part of the lateral surface of the quadrate. The tendon of the A3 inserts between the medial surface of the dentary and the lateral surface of the AW, in the Meckelian fossa ( Fig. 4A).

3. The third part and anterior expansion of the adductor mandibulae complex, AW, consists of many short fibres which are antero-dorsally to antero-ventrally directed ( Fig. 4A). The insertion site includes the medial surface of the dentary and is expanded by the presence of a longitudinal bony ridge. These fibres are connected to a broad and strong tendon T AW, which fuses with the tendon of A1-A2 ( Fig. 4A). Additionally, the T AW splits postero-ventrally into two tendons: T AW- Q attaches to the medial surface of the quadrate, near its caudal border, while T AW- IOp attaches to the medio-lateral surface of the interopercle, near its rostral border.

Levator arcus palatini

The levator arcus palatini is a triangular muscle, with postero-ventrally orientated fibres ( Fig. 3A). This muscle stretches from its origin, being the frontal, pterosphenoid and sphenotic to a rather large insertion surface, i.e. the dorsal two-thirds of the dorso-lateral surface of the metapterygoid and the dorso-anterior edge of the hyomandibula. The insertion is accomplished by means of a superficial, flat tendon. The posterior fibres are incompletely separated from the anterior fibres of the dilatator operculi.

Adductor arcus palatini

This muscle is situated in the roof of the buccal cavity and extends between the skull and the suspensorium, forming the posterior and postero-ventral margin of the orbit ( Fig. 3A). The postero-dorsal half of the adductor arcus palatini is covered by the levator arcus palatini. The rostral half of the muscle is relatively thin and becomes gradually thicker posteriorly. The fibres are latero-ventrally directed and originate from the parasphenoid and prootic. They insert musculously as a narrow strip on the medial surface of the metapterygoid and antero-medial edge of the hyomandibula.

Levator operculi

The levator operculi runs between the lateral skull wall and the opercle ( Fig. 3A). Its anterior fibres are confluent with the posterior fibres of the dilatator operculi. The levator operculi arises from the neurocranium at the level of the pterotic and epiotic by means of a tendon sheet along the dorsal edge of the levator operculi over its total length ( Fig. 3A). The latero-ventrally orientated fibres insert musculously on the dorso-medial surface of the opercle and on the medial surface of the opercle at the level of a medial ridge.

Adductor operculi

The adductor operculi is laterally covered by the anterior part of the levator operculi ( Fig. 3A). The fibres arise musculously from the ventro-lateral surface of the pterotic and partially from the prootic. They insert on the medial surface of the opercle. The fibres run postero-ventrally and are laterally inclined.

Dilatator operculi

The dilatator operculi is subdivided into two parts: a dorsal and a ventral part. The dorsal part is long and slender and situated in the dilatator fossa ( Fig. 3A). Its origin is extensive and includes the frontal, pterosphenoid, sphenotic, pterotic, hyomandibula and supratemporal. The anterior fibres form a bipennate muscle with a long tendon, which is clearly visible, inserting on the lateral plate-like process on the articular head of the opercle. Some posterior fibres of the dorsal part insert musculously on the lateral plate-like process of the articular head of the opercle as well. These fibres merge ventrally with the dorsal fibres of the second part. This second, ventral part of the dilatator operculi arises from the posterior edge of the hyomandibula and the medial surface of the preopercle. These fibres converge postero-dorsally to insert musculously on the latero-ventral side of the same lateral plate-like process on the articular head of the opercle.

Intermandibularis

The fibres of the intermandibularis stretches out transversally between the two halves of the dentary and lie caudal to the dental symphyse ( Figs 4A, 5A).

Protractor hyoidei

The protractor hyoidei interconnects the hyoid with the dentary ( Fig. 5A). The protractor hyoidei comprises two parts, an inferior a- and a smaller superior b- part. The two halves of the PHb form two distinct bundles, separated by a deep, longitudinal groove containing the basihyal. The left and right halves of the PHa are ventro-medially fused. The PHb is ventrally fused to the PHa. The anterior tendon of the PHb is anteriorly fused with that of PHa, forming the anterior common tendon T PH A. This tendon inserts onto the dentary, just behind the intermandibularis. Posteriorly, the protractor hyoidei originates from the lateral surface of the anterior ceratohyal. The posterior tendons of the left and right PHb run backwards to attach on the dorso-lateral surface of the anterior ceratohyal. The left and right halves of the PHa share a common posterior tendon. This common tendon splits posteriorly and each branch runs towards the ventro-lateral surface of, respectively, the left and right anterior ceratohyal. The posterior tendons of PHa and PHb are connected by a tendinous sheet, which covers the antero-lateral surface of the anterior ceratohyal.

Hyohyoideus abductor

The hyohyoideus abductor connects the dorsal hypohyal to the first branchiostegal ray of each side ( Fig. 6A). The anterior fibres of the hyohyoideus abductor originate through a broad, flat tendon from the ventro-lateral surface of the ventral hypohyal. The two robust urohyal–hypohyal ligaments are partially covered by these tendons. The posterior fibres of the hyohyoideus abductor of each side insert through a flat tendon on the anterior surface of the first branchiostegal ray of the opposite side. Both tendons of the left and right side thus cross each other in the midline.

Hyohyoidei adductores

Hyohyoidei adductores lie between the successive branchiostegal rays ( Fig. 6A). Lateral fibres pass between the successive rays while the medial fibres form a continuous sheet on the medial side of the branchiostegal rays and continue behind the posterodorsalmost branchiostegal ray to attach to the medial surface of the opercle.

Sternohyoideus

Two myocommata divide the sternohyoideus into three myomeres ( Fig. 7A). The two halves of the sternohyoideus are separated in the midline by an aponeurosis. The postero-dorsal fibres of the sternohyoideus originate from the lateral, anterior and medial surface of the cleithrum. The origin onto the cleithrum is postero-dorsally extended, dorsal to the origin of the pharyngoclavicularis internus. The pharyngoclavicularis externus and internus cover the sternohyoideal fibres, both originating from the lateral surface of the cleithrum. The postero-ventral fibres of the sternohyoideus are continuous with fibres of the hypaxialis and are laterally covered by a fascia. The anterior fibres of the muscle insert musculously and tendinously through the sternohyoideal tendon on the lateral surface of the urohyal. The antero-dorsally directed sternobranchial tendon is merged with the aponeurosis between the two halves of the sternohyoideus. Only the tip of this tendon lies superficially, and runs forward to attach to the ventral processus of the third hypobranchial element, after fusion with the tendon of the opposite side.

Pharyngoclavicularis internus

This muscle lies medial and posterior to the pharyngoclavicularis externus ( Fig. 7A). It is tendinous at both its origin and its insertion. The posterior fibres attach tendinously to a small groove in the anterolateral margin of the cleithrum. The fibres run in dorso-rostral direction. The anterior tendon inserts on the antero-ventral region of the fifth ceratobranchial. Pharyngoclavicularis externus

This muscle originates from a groove on the anterolateral surface of the cleithrum and inserts beneath a dorsal ridge on the dorso-mesial surface of the fifth ceratobranchial ( Fig. 7A). The fibres run dorsally and slightly rostrally.

Epaxials

The epaxials originate from the dorsal surface of the neurocranium, above the posterior edge of the eye ( Fig. 2A). The origin includes the frontals, parietals, exoccipitals, supraoccipital and epiotics. The origin is not additionally extended by a supraoccipital ridge.

Supracarinalis anterior

The anterior fibres of the supracarinalis anterior insert musculously on the postero-dorsal region of the skull, forming a cord-like muscle bundle in the dorsal midline ( Fig. 2A). The posterior fibres originate by means of two postero-lateral tendons on the first pterygiophore of the dorsal fin. The fibres are anteroposteriorly directed.

MYOLOGICAL DIFFERENCES FOUND IN TRICHIURUS LEPTURUS

The cranial muscles in T. lepturus are very similar to those in A. carbo ( Figs 2 B-7B). Origin, insertion and configuration are similar in the levator arcus palatini, adductor arcus palatini, adductor operculi, dilatator operculi, intermandibularis, hyohyoidei abductores,

hyohyoidei adductores, pharyngoclavicularis internus and pharyngoclavicularis externus.

Adductor mandibulae complex

In T. lepturus the configuration of the adductor mandibulae complex is similar, though small differences are observed ( Figs 2B, 3B). Some fibres arising from the dorsal half of the lateral surface of the preopercle seem to be somewhat separate from the body of the muscle mass, but are strongly connected by a superficial sheet of faint tendons, which is absent in A. carbo . The tendon T A1-A2 splits, in contrast to that in A. carbo , into two smaller parts which separately insert onto the edge of the dentary ( Fig. 4B).

Levator operculi

The origin includes similar sites as in A. carbo though in T. lepturus the fibres arise musculously from the neurocranium ( Fig. 3A). The tendon is absent.

Protractor hyoidei

As in A. carbo , both halves of the PHa are ventromedially fused ( Fig. 5B). By contrast, in T. lepturus the fibres of the anterior tendons of the PHa halves are fused, forming one common ventral tendon which extends rostrally to insert on the dentary behind the intermandibularis. In contrast to A. carbo , the posterior tendons of the PHa remain separate in T. lepturus , whereas in A. carbo the posterior tendons of PHa are rostrally fused.

Sternohyoideus

The sternohyoideal tendon is shorter and situated in the anterior part of the muscle ( Fig. 7B). The origin of the sternohyoideus includes the medial, anterior and lateral surface of the cleithrum. The origin of the pharyngoclavicularis externus is similar though its lateral surface is covered by a thin sheet of fibres of the sternohyoideus, inserting onto the cleithrum as well. The postero-ventral fibres of the sternohyoideus, also continuous with the hypaxials, are not laterally covered by a fascia. The sternobranchial tendon has a more superficial position (visible on the lateral surface of the sternohyoideus) compared with that in A. carbo . At about two-thirds of the length of the muscle this tendon becomes superficial.

Epaxials

Origin, insertion and configuration are similar, although the origin is expanded due to the presence of the supraoccipital ridge ( Fig. 2B).

Supracarinalis anterior

The supracarinalis muscle is present in T. lepturus but due to the anterior displacement of the dorsal fin,

compared with A. carbo , this muscle is less anterocaudally extended ( Fig. 2B).

DYNAMICS OF MOUTH CLOSING

The simulations with the mouth-closing model of A. carbo and T. lepturus show the following general pattern. Initially, during the first 15 ms after the start of mouth closing, jaw muscle force is almost entirely used to accelerate the lower jaw ( Fig. 8). Shortly after this, when the lower jaw has nearly reached its maximal velocity (peak near 20 ms), drag becomes the most important factor of resistance to lower jaw rotation. During the final half of the mouthclosing phase (around 30–65 ms), the force generated by the jaw adductors is predominantly countering resistance caused by stretching of the jaw-opener muscles and the forces exerted on the lower jaw resulting from super-ambient pressure inside the mouth that typically appears near the end of mouthclosing ( Fig. 8). According to the model, A. carbo is able to close its mouth from a gape angle of 50° to 10° in 64.8 ms, while T. lepturus needs 74.2 ms to achieve this.

FUNCTION OF THE DIFFERENT JAW MUSCLES DURING