Gosner stage


Gosner stage refers to a generalized system describing stages of embryonal and larval development in anurans. The Gosner system includes 46 numbered stages, from fertilized embryo to the completion of metamorphosis. It was introduced by Kenneth Gosner in 1960. The system is widely used in herpetology to describe tadpoles. Gosner stages are based on certain morphological landmarks that allow comparing development in different species that may greatly differ in age or size.

Developmental stages

The Gosner system includes 46 stages, from fertilized embryo to the completion of metamorphosis. In the original publication, only two major developmental stages were distinguished, embryonic or prefeeding stages and larval stages. McDiarmid and Altig distinguish four major developmental stages, as detailed in the table below.
Developmental categoryGosner stagesDescription
Embryo1 to 20 Intracapsular. Development through cleavage, gastrulation, and appearance of neural tube and eventually gills and tail.
Hatchling21 to 24Transition from relatively immobile embryo to an active, feeding tadpole. Specimens at these stages may sometimes be referred to as "larvae".
Tadpole25 to 41Longest part of larval period that is marked by growth and development of limbs. The generic term "larvae" often refers to these stages.
Metamorph42 to 46Loss of larval characters and development of adult structures; typically associated with the transition from the aquatic to the terrestrial habitats

In ecological literature, the term "hatchling" may refer to stages 24–26. However, for direct-developing species, hatching occurs in the end of the development.

Limitations

As a generalized system, Gosner stages may not be adequate for describing development of some anuran tadpoles. For example, in the torrent-dwelling tadpoles of Ansonia longidigita and Meristogenys orphnocnemis, the usual Gosner stages become inappropriate beyond the stage 41 because the tadpoles retain their oral disc longer than the system depicts. This is interpreted as an adaptation that allows the tadpoles to cling to the rocks until the tail is almost fully resorbed. In Limnonectes limborgi, the tadpoles are free-living but non-feeding and retain their yolk sac until stage 37, at least. Direct-developing frogs hatch directly as froglets, without free-living larval stage.