How does a tadpole know when to metamorphose? A theory linking environmental and hormonal cues.

Tadpoles are unusual among free-living amphibians in having an atonic, non-acid secreting, underdeveloped stomach. Morphologically the typical tadpole foregut is most similar to the flaccid, non-acid secreting stomach of adult female of the gastric-brooding frog, Rheobatrachus, during brooding. In Rheobatrachus the brooding condition is induced by prostaglandin E2 secreted from the mouths of brooded larvae. I propose that typical, free-living tadpoles also excrete prostaglandins of the E family in their oral mucus and that these compounds are naturally swallowed with food particles by the tadpoles. According to this hypothesis, when food is abundant larvae swallow a large amount of mucus and, consequently, a lot of hormone, which retards differentiation of the adult, acid secreting, peristaltic stomach. However, when food is less abundant less food and mucus is swallowed. In this situation less prostaglandin passes down the alimentary tract and the gut proceeds to differentiate. If this theory is correct it provides a direct link between an environmental factor--the availability of food--and an endocrinological factor affecting metamorphosis. The theory is consistent with our current understanding of the endocrinology of metamorphosis, as well as the evolution of direct-development in anurans.