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.

对于栖息于封闭环境中的生物而言，粪便污染是一项亟需重视的重要问题。两栖动物幼体以氨作为代谢废物的排泄形式。无尾类（Anuran）幼体通常生活在富含洁净水源的池塘或溪流中，因此其排泄的氨会被充分稀释，基本不会产生毒害作用。但部分物种栖息于狭小且无水流的水体系统中，这类环境极易受到粪便污染的影响。

我们在此报告：栖息于植物储水环境（phytotelmata）的台湾树蛙（Kurixalus eiffingeri）幼体，通过在肠道内储存粪便直至变态发育，以此规避氨带来的毒性损伤，从而解决了这一生存难题。该种幼体以亲代产下的营养卵为食，但我们的实验观察显示，它们直至变态发育前都不会排泄粪便。

我们据此提出假说：这种行为的演化意义在于规避铵污染带来的危害。其一，饲养实验结果表明，台湾树蛙幼体在饲养水体中几乎不会产生氨；其二，高浓度氨暴露实验显示，台湾树蛙的氨耐受能力显著优于其他蛙类。

综合以上结果，我们的研究数据证实，台湾树蛙演化出了一套独特的氨排泄与耐受卫生策略，以适应封闭水生栖息环境。本研究也是首个证实两栖动物取食幼体完全不进行排泄的相关学术报道。