Phytotelmata-dwelling frog larvae show no defecation behavior: a unique adaptation to a closed aquatic environment

For organisms living in closed environments, fecal pollution is a significant concern. Amphibian larvae excrete their wastes as ammonia. Anuran larvae usually live in ponds or streams that contain excess clean water; therefore, the excremental ammonia is diluted and unlikely to be harmful. However, some species inhabit tiny, non-flowing water systems vulnerable to excremental pollution. We here report that the phytotelmata-dwelling larvae of Taiwanese tree frog Kurixalus eiffingeri resolve this problem by retaining feces in their gut until metamorphosis to avoid the toxic damage from ammonia. K. eiffingeri larvae feed on trophic eggs produced by mothers, but our experimental observation showed that they never produce feces until metamorphosis. We hypothesized that this behavior is functional to avoid ammonium pollution. Firstly, the rearing experiment showed that K. eiffingeri larvae produce almost no ammonia in the living water. Secondly, experimental exposure to high ammonia levels showed that K. eiffingeri had higher ammonia tolerance than other frogs. Taken together, our data show that K. eiffingeri evolved a unique sanitation strategy in ammonia excretion and tolerance that is adaptive in closed aquatic habitats. This is the first study to show the lack of excretion in the feeding larvae of amphibians.