Hidden but not lost: the larval cranial anatomy of the Majorcan midwife toad (Alytes muletensis)

Alytes muletensis is a frog endemic to the Balearic island of Majorca (or Mallorca). Its tadpole lives in semi-permanent plunge pools which remain after mountain streams have dried up. Little is known about the morphology of the different members of the Alytidae and the relationships of its members are still unclear. Alytidae are among the most basal anurans, and improved knowledge of their larval morphology can contribute to our understanding of the morphological evolution of anurans. Herein, I describe the external morphology as well as skeletal and muscular features of a tadpole of A. muletensis at Gosner stage 25 by using standard histology, clearing and staining and 3D-reconstruction based on serial sections. The tadpole displays different discoglossoid traits such as the presence of two portions of the M. levator mandibulae externus, the separation of hypobranchial elements by the basibranchial, and the absence of the M. constrictor branchialis IV, among others. Unusual is the absence of the M. constrictor branchialis I as well as the absence of the M. intermandibularis. Remarkable is the presence of the admandibular cartilage as an additional skeletal element of the lower jaw.