External ontogenetic changes of larval structures in Elachistocleis bicolor (Anura: Microhylidae: Gastrophryninae)

Although the external larval morphology of many species of Microhylidae has been characterized, ontogenetic studies in the family are scarce and partial. In the present study, we describe the development of the oral funnel, external gills, external nares, operculum, spiracle, and adhesive organs of Elachistocleis bicolor with the aim to contribute to future ontogenetic, evolutionary, and phylogenetic studies. The development of E. bicolor is similar to that of other species with free-swimming tadpoles as Microhyla ornata, Dermatonotus muelleri and Uperodon globulosus. The ontogeny of the oral apparatus shows only the early stages of the pattern most common in anurans. After the invagination of the stomodeum, the anterior and posterior labia are defined, but the keratinized structures are not formed. The conical adhesive glands are similar to those of Physalaemus and Dendropsophus tadpoles, but in E. bicolor, these develop in contact with the posterior labium. The external nares remain closed during most of the larval phase. During ontogeny, the external gills are simultaneously covered by a transverse operculum fold, which later advances towards the posteromedial ventral region, forming the spiracle, while in Type IV larvae the operculum folds close from right to left, covering the right gill first. The distinct larval characters of Microhylidae appear between the last embryonic stages and the first larval stages. Therefore, the consideration of these stages can be of great value in future studies of anuran development in phylogenetic and evolutionary contexts. Ontogenetic studies in microhylid tadpoles are fundamental to identify heterochronic processes, to interpret the variability of larval structures, and to establish or reinforce hypotheses about the origin of larval structures.