Phylogenetic aspects of tooth and jaw structure of the Medaka, Oryzias latipes, and other beloniform fishes

Tooth structure is described for adult male, female, and juvenile Oryzias latipes (Temminck & Schlegel), the Medaka. Adult males have enlarged, unicuspid teeth posteriorly on the premaxilla and dentary. Oral teeth are smaller and more numerous in females, in which no tooth is notably larger than the others. Juveniles have numerous small teeth from about 3 mm SL (standard length) onwards. By about 16 mm SL, males begin to develop the large posterior teeth, as well as other secondary sexual characters. Lower and upper pharyngeal teeth of both males and females are fine, and in numerous even rows. The large, posterior oral teeth of males are fully-ankylosed to the attachment bone, and, hence, are not depressible. In female Medaka, as in the Halfbeak Dermogenys pusillus van Hasselt, the oral teeth have a ring of unmineralized collagen at the base, and are not depressible. Pharyngeal teeth of Medaka have a ring of unmineralized collagen at the base, and a distinct wedge of collagen absent posteriorly, such that the pharyngeal teeth may be depressed. Bone in adult Medaka is acellular. Incompletely mineralized teeth, acellular bone, a protrusible upper oral jaw, and a mobile branchial apparatus with an interhyal bone, form a complex characteristic of advanced teleosts. The Medaka differs in several ways from the model advanced teleost: absence of an interhyal bone, ascending and articular processes of the premaxilla, and the rostral cartilage, as well as presence of cartilaginous symphyses between the dentaries and premaxillae, all contribute to the fixed or nonprotrusible jaws. Reduction in the premaxilla is a derived character within beloniform fishes for which an enlarged, beaked outer jaw is considered plesiomorphic.