Gymnotus Linnaeus, 1758

Peixoto, Luiz Antônio Wanderley & de Pinna, Mário, 2022, Patterns of diversification and phylogenetic structure in the dorsolateral head musculature of Neotropical electric eels (Ostariophysi: Gymnotiformes), with a myological synonymy, Neotropical Ichthyology (e 210009) 20 (1), pp. 1-117 : 28-29

Gymnotus Linnaeus, 1758

Adductor mandibulae. The stego-malaris originates in the hyomandibula, metapterygoid, quadrate, parasphenoid, frontal and sphenotic. The subsections become gradually differentiated towards their insertion, where the dorsolateral fibers, corresponding to the presumed malaris, converge on a thick mandibular tendon inserted into the mesial face of the coronoid process. The middle fibers, presumably corresponding to the stegalis, diverge in the meckelian tendon and are inserted on the dorsal margin of the coronomeckelian bone. Some lateral fibers near the insertion site of the stego-malaris are inserted on the buccopalatal membrane, which in turn is associated with the posterior margin of the maxilla.

Stego-malaris is composed mainly of fibers. However, in Gymnotus gr. carapo the more lateral fibers of this sub-section, corresponding to the presumed malaris, have tendinous ossifications forming intermuscular bones. The mesial fibers, corresponding to the presumed stegalis, are laterally positioned in relation to the posterolateral region of the basal portion of the endopterygoid, not completely overlapping it, and are lateral relative to the adductor arcus palatini, totally overlapping it.

The rictalis originates in the preopercle, quadrate and hyomandibula, with lateral fibers surpassing the anterior margin of the preopercular fossa and reaching the posterior margin of that bone. This subsection is inserted mainly on the coronoid process, but with some more lateral fibers associated with the buccopalatal membrane. The rictalis is normally composed of fibers only, except in Gymnotus gr. carapo , in which some tendons ossify and form intermuscular bones. The ramus mandibularis trigeminus nerve is mesial to all sub-sections of the adductor mandibulae (in one specimen of G. coropinae , the nerve trespasses the rictalis).

Levator arcus palatini. The levator arcus palatini has the shape of an inverted triangle; with its origin twice as wide as its insertion. This muscle originates in the ventral margin of the frontal and sphenotic and is inserted on the hyomandibula. The levator arcus palatini is a non-sectioned muscle, except at the insertion, where four sub-sets of fibers are identifiable. Each of them has distinct modes at its insertion point in relation to the malaris: the anterolateral fibers, posterolateral and posteromesial is disposed laterally in relation to the malaris and dorsally to the rictalis; and the anteromesial fibers are arranged mesially in relation to the malaris. This muscle is partially mesial to the dilatator operculi, which exceeds the medial portion of the levator arcus palatini ( Fig. 1 ).

Dilatator operculi. The dilatator operculi is roughly conical. It originates in the posterior margin of the sphenotic, pterotic, frontal and dorsal portion of the hyomandibula, and inserts on the dorsal and anterodorsal portion of the dorsal process of the opercle. In Gymnotus cylindricus its insertion includes the dorsoposterior portion of the dorsal process of the opercle. The dilatator operculi is lateral to the levator arcus palatini, overlapping approximately 2/3 of its mid-posterior portion.

Levator operculi. The levator operculi anterior originates in the ventral margin of the pterotic and is inserted in a ridge on the dorsal margin of the opercle, posterior to its anterodorsal process. The levator operculi posterior originates in the postotic canal segment corresponding to the supracleithrum and posteroventral margin of the pterotic, inserting along a ridge on the posterolateral margin of the opercle, with fibers extending beyond the dorsal margin of the bone.

Figures

FIGURE 1 | Dorsolateral head muscles of Gymnotus cylindricus (Gymnotidae), USNM 134701, 178.5 mm LEA. A. Lateral view; B. dorsal view. Anatomical abbreviations in Tab. 1. Scale bars = 4 mm.