Development of the muscles associated with the mandibular and hyoid arches in the Siberian sturgeon, Acipenser baerii (Acipenseriformes: Acipenseridae)

Abstract

The skeleton of the jaws and neurocranium of sturgeons (Acipenseridae) are connected only through the hyoid arch. This arrangement allows considerable protrusion and retraction of the jaws and is highly specialized among ray-finned fishes (Actinopterygii). To better understand the unique morphology and the evolution of the jaw apparatus in Acipenseridae, we investigated the development of the muscles of the mandibular and hyoid arches of the Siberian sturgeon, Acipenser baerii. We used a combination of antibody staining and formalin-induced fluorescence of tissues imaged with confocal microscopy and subsequent three-dimensional reconstruction. These data were analyzed to address the identity of previously controversial and newly discovered muscle portions. Our results indicate that the anlagen of the muscles in A. baerii develop similarly to those of other actinopterygians, although they differ by not differentiating into distinct muscles. This is exemplified by the subpartitioning of the m. adductor mandibulae as well as the massive m. protractor hyomandibulae, for which we found a previously undescribed portion in each. The importance of paedomorphosis for the evolution of Acipenseriformes has been discussed before and our results indicate that the muscles of the mandibular and the hyoid may be another example for heterochronic evolution.

Evolution of the facial musculature in basal ray-finned fishes

“…Adductor mandibulae, segmentum mandibulo-facialis: symphysial portion of adductor mandibulae, ams [ 14 , 23 ].…”

“…Adductor mandibulae, segmentum facialis, pars malaris: superficial fibers or intermediate portion of symphysial portion of adductor mandibulae [ 23 ].…”

“…Constrictor mandibularis dorsalis: constrictor dorsalis ([ 17 ] (embryo)); constrictor I dorsalis ([ 14 ] (embryo)); constrictor mandibularis dorsalis ([ 23 ](embryo)); levator maxillae superioris ([ 21 ] (embryo)); protractor hyomandibulae ([ 15 ], [ 23 ] (adult)); protractor hyomandibularis ([ 14 ] (adult), [ 21 ] (adult), [ 22 , 24 ]); protractor of hyomandibula ([ 17 ] (adult)).…”

“…Constrictor mandibularis dorsalis, lateral section: superficial portion of protractor hyomandibulae [ 23 ].…”

The morphology, development, and evolution of the head of fishes: Foundational studies for a renaissance of comparative morphology

“…3‐D models of developmental stages, reconstructed after laser‐scanning microscopy of antibody stained specimens. (a) Siberian sturgeon Acipenser baerii at developmental stage 43 (17 mm TL), muscles shown in tones of red, cartilage shown in tones of blue, nerves shown in tones of yellow; from Warth, Hilton, Naumann, Olsson, and Konstantinidis ( Figure a). (b) Clawed frog Xenopus laevis at developmental stage 47/48, nerves shown in various colours, brain shown in yellow, eye shown in white, cartilage shown in transparent blue; from Naumann and Olsson (: Figure a)…”