UROPTERYGIINAE, Fowler, 1925

Johnson, G. David, 2019, Revisions of Anatomical Descriptions of the Pharyngeal Jaw Apparatus in Moray Eels of the Family Muraenidae (Teleostei: Anguilliformes), Copeia 107 (2), pp. 341-357 : 351

UROPTERYGIINAE

Figures 8–10

‘‘There is no dorsal retractor that spans the posterior portion of the epibranchial and the vertebral column, suggesting that retraction of the jaws is accomplished primarily by the esophagus and the pharyngocleitheralis [sic].’’ (M&W: 614)

M&W seem to have misinterpreted Nelson’s terminology for the dorsal retractor (DR), stating that it is only present in muraenine eels. As Nelson reported, ‘‘Dorsal and ventral paired retractor muscles are present in all of the eels examined. In most forms they are only partly distinct subdivisions of the inner longitudinal muscle layer of the anterior esophagus. In eels of the subfamily Muraeninae , they acquire an attachment to the vertebral column.’’ Thus, Nelson’s dorsal retractor (comprising the longitudinal fibers of the sphincter oesophagi) is present in all eels (see also Springer and Johnson, 2015: 598). The distinction is that in muraenines these fibers emerge from the outer circular fibers of the sphincter oesophagi posteriorly to attach to the vertebral column, and in uropterygiines they do not.

‘‘As in the muraenine condition, there is a single internal branchial levator, levator internus 4, that originates from the posterior part of the parasphenoid and inserts on the dorsal side of the fourth pharyngobranchial just anterior to the Pb4/Eb4 joint.’’ (M&W: 613)

As described above, there is no fourth internal levator in any actinopterygian. This muscle is LI2, which, unlike in most muraenines, is represented by a single bundle in uropterygiines.

As in muraenines, uropterygiines have two internal levators. LI1 (overlooked by M&W) originates in the epaxialis and inserts on the anterior one-third of the dorsal surface of UPT. LI2 is represented by a single bundle (there are two in muraenines) that originates on the occipito/otic region of the braincase and inserts on the anterior end of UPT. I concur with Nelson’s (fig. 9) interpretation of the insertion of these two muscles but not with his labeling of LI2 as the ‘‘protractor medialis’’ (his MP).

M&W did not recognize a major difference between muraenines and uropterygiines because they described muraenines as having LE4, an external levator that inserts on Eb4. Because that muscle actually inserts on UPT in muraenines, it is an internal levator. Ironically, uropterygiines have the muscle (LE4) M&W incorrectly attributed to muraenines. Nelson (table 2, 359, fig. 9) described and illustrated this muscle correctly, though he called it protractor posterior (his PP) rather than LE4.

The most conspicuous unrecognized difference between uropterygiines and muraenines is the presence of an additional muscle. Posterior to the gill-arch skeleton, a large oblong to trapezoidal muscle (first identified herein and described as hypaxial retractor, HR) originates within the hypaxialis in the vicinity of the cleithrum and inserts by a short tendon on the fourth epibranchial just above its articulation with the fourth ceratobranchial ( Figs. 9 , 10 ).

Figures

Fig. 8. Uropterygius alboguttatus, USNM 338657, 350 mm. (A) Left lateral view of left fourth gill arch and associated musculature, removed from body, circular fibers of SO removed to expose longitudinal fibers of DR, hypaxial retractor removed. (B) Medial view of A. Note insertion of single branch of LI2 on anterior end of UPT and posterior insertion point of LE4 near Eb4-Cb4 articulation. AD, adductor dorsalis; Cb, ceratobranchial; DR, dorsal retractor; Eb, epibranchial; LE levator externus; LI, levator internus; LPT, lower pharyngeal toothplate; UPT, upper pharyngeal toothplate.

Fig. 9. Uropterygius alboguttatus, USNM 338657, 310 mm. Left lateral view of left fourth gill arch and associated musculature in situ, posterior end of pharyngocleithralis and anterior end of hypaxialis excised at red lines to expose hypaxial retractor; LE4 passes medial to the insertion of HR to insert more posteriorly (see also Fig. 10). AD, adductor dorsalis; Cb, ceratobranchial; Eb, epibranchial; Ep, epaxialis; HR, hypaxial retractor; Hy, hypaxialis; LE levator externus; LI, levator internus; Pcl, pharyngocleithralis; SO/DR, sphincter oesophagi/dorsal retractor; Sph, subpharyngealis.

Fig. 10. Uropterygius alboguttatus, USNM 338657, 340 mm. Left lateral view of left fourth gill arch and associated musculature, removed from body. Note insertion of hypaxial retractor on Eb4. AD, adductor dorsalis; Cb, ceratobranchial; DR, dorsal retractor; Eb, epibranchial; HR, hypaxial retractor; LE levator externus; LI, levator internus; LPT, lower pharyngeal toothplate; SO/DR, sphincter oesophagi/dorsal retractor; UPT, upper pharyngeal toothplate.