Increasing our knowledge on direct-developing frogs: The ontogeny of Ischnocnema henselii (Anura: Brachycephalidae)

Abstract

Due to the difficulty in finding newly fresh spawn in natural environments, few embryonic developmental studies have been carried out in anurans with direct development. Here we provide detailed data on the embryonic ontogeny of Ischnocnema henselii, and compare some morphological aspects related to its developmental mode within and outside of the brachycephaloid clade. Embryonic development in I. henselii is characterized by a unicuspid egg tooth (bicuspid in most other Brachycephaloidea), external gills present, open vent tube, and tail fins with dorsoventral orientation throughout the development. We also provide the first account of skin development in direct-developing frogs, revealing that the maturation of the integument has a typical dorsal to ventral sequence of changes as in most biphasic anurans. The early onset of thyroid development seems to be consistent with the hypothesis that the evolution of direct development in anuran amphibians involved precocious activation of the thyroid axis. A comparative analysis with the still few described embryonic ontogenies for direct-developing species reveals variation in the length of the embryonic period, rate of development, size at hatching and presence/absence of external morphological characters, which suggests heterochronic shifts in the rate of species-specific stage progression. All these details reinforce the idea of the high morphological variability among direct-developing frogs.