Ontogeny of the intermandibular and hyoid musculature in the suckermouth armoured catfish Ancistrus cf. triradiatus (Loricariidae, Siluriformes)

Loricariidae or suckermouth armoured catfishes are the most speciose catfish family, displaying morphological specializations toward the attachment onto substrates with their sucker mouth, and the scraping of algae and other food items off these substrates. The intermandibular and hyoid musculature differs from the general siluriform situation. This detailed study on several developmental stages of a loricariid representative aims to provide insight in the ontogenetic origin of these muscles, as well as on their morphology and homology. Serial sections and 3D-reconstructions are used to visualize the early muscle configurations. The intermandibularis anterior muscle develops two parts, inserting on the lower jaw but also on the lower lip tissue. A similar differentiation into a dentary and a labial part occurs in the intermandibularis posterior (usually erroneously referred to as protractor hyoidei in loricariids). The protractor hyoidei has a compound nature in teleosts, but in loricariids no interhyoideus portion fuses to the posterior intermandibularis portion. Several arguments, including the absence of a myocomma and a double innervation, indicate the absence of an interhyoideus portion. A double innervation has been found in the hyohyoideus inferior. The posteriormost muscles in the hyoid region are relatively small during early ontogeny: the sternohyoideus halves fuse relatively late; the hyohyoidei adductores develop latest of all ventral head muscles. A remarkable shift in orientation characterizes the hyohyoideus abductor.