Description of embryonic stages in Melanosuchus niger(Spix, 1825) (Crocodylia: Alligatoridae)

ABSTRACT

The determination of embryonic stages is very useful in comparative and evolutionary anatomy, experimentalresearch and ecological studies. Although tables of development stages already exist for some crocodilianspecies, no table has yet been published for Melanosuchus niger. Therefore, the aim of this work is to monitorthe development of this species of Alligatoridae and characterize its stages based on the main morphologicaltransformations the embryo undergoes during the incubation process. Under natural conditions, two sampleswere collected daily in the Lake Cuniã Extractive Reserve in Rondônia, Brazil, making a total of 186 eggs.The incubation period was 90 days at an ambient temperature varying from 25.2 to 31.7 °C. We established24 embryonic stages between the formation of the pharyngeal arch and hatching. Primordia for the sensoryorgans and somites were observed in the early stages. The buds of the pelvic and thoracic limbs emergedsimultaneously, also in the early stages. An analysis of these structures showed evidence of considerableevolutionary modifications when compared with embryos of different species. In stage 12 we observed subtlepigmentation in the region of the head and, in stage 25, the pigmentation and striped pattern were typical ofthe newly hatched specimen. The eyetooth was visible in stage 18, disappearing within the first few weeks afterhatching. The vitellus was completely absorbed in the penultimate stage. Direct comparisons of the table ofembryonic stages were made between M. niger and other crocodilian taxa, and the main differences identifiedwere chronological variations.