Ontogenetic Fusion of the Third and Fourth Pharyngobranchial in Barbeled Dragonfishes (Stomiidae, Teleostei) with a Revision of the Identity of the Single Posterior Upper Pharyngeal Toothplate

Barbeled dragonfishes (Stomiidae) comprise about 281 species in 27 genera, most of them highly specialized mesopelagic predators. Fusion of pharyngobranchiais three and four has been reported in several genera, but without supporting ontogenetic evidence. We studied cleared and double stained ontogenetic series of species in 22 of the 27 recognized stomiid genera and found that the third and fourth pharyngobranchials fuse ontogenetically in 17 genera. In contrast, in four genera (Neonesthes, Borostomias, Rhadinesthes, and Stomias), the fourth pharyngobranchial cartilage remains separate from the third in adults, as it generally does in teleosts. One genus, Astronesthes, shows an intermediate state in which these two elements fuse only partially. Additionally, our data indicate that the posteriormost upper pharyngeal toothplate in stomiids is the fourth (UP4) rather than the fifth (UPS), as previously proposed. The fourth upper pharyngeal toothplate is closely associated with the ventral surface of the fourth pharyngobranchial cartilage during early larval ontogeny. Therewith, we address a long-standing homology problem regarding the identity of UP4 and UP5 among basal neoteleosts and our results are discussed comparatively.