Ip Yuen K, Chew Shit F
Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore.
Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore.
Acta Histochem. 2018 Oct;120(7):680-690. doi: 10.1016/j.acthis.2018.08.013. Epub 2018 Sep 2.
During water-land transition, ancient fishes acquired the ability to breathe air, but air-breathing engendered problems in nitrogenous waste excretion. Nitrogen is a fundamental component of amino acids, proteins, and nucleic acids, and the degradation of these nitrogen-containing compounds releases ammonia. Ammonia is toxic and must be removed. Fishes in water excrete ammonia as the major nitrogenous waste through gills, but gills of air-breathing fishes are modified for air-breathing or largely replaced by air-breathing organs. Notably, fishes emerged from water can no longer excrete ammonia effectively because of a lack of water to flush the gills. Hence, ancient fishes that participated in water-land transition must have developed means to deal with ammonia toxicity. Extant air-breathing fishes, particularly amphibious ones, can serve as models to examine adaptations which might have facilitated the emergence of ancient fishes from water. Some of these fishes can actively emerge from water and display complex behaviors on land, while a few can burrow into mud and survive for years during drought. Many of them are equipped with mechanisms to ameliorate ammonia toxicity during emersion. In this review, the mechanisms adopted by air-breathing fishes to deal with ammonia toxicity during emersion were organized into seven disparate strategies. In addition, eight extant air-breathing fishes with distinctive terrestrial behaviors and peculiar natural habitats were selected to describe in detail how these seven strategies could be adopted in disparate combinations to ameliorate ammonia toxicity during emersion.
在从水生向陆生转变的过程中，古代鱼类获得了呼吸空气的能力，但呼吸空气给含氮废物的排泄带来了问题。氮是氨基酸、蛋白质和核酸的基本组成部分，这些含氮化合物的降解会释放出氨。氨是有毒的，必须被清除。水生鱼类通过鳃将氨作为主要含氮废物排出，但呼吸空气的鱼类的鳃已为呼吸空气而发生改变，或在很大程度上被呼吸空气的器官所取代。值得注意的是，从水中出来的鱼类由于缺乏用于冲洗鳃的水，无法再有效地排出氨。因此，参与水生向陆生转变的古代鱼类必定已经发展出应对氨毒性的方法。现存的呼吸空气的鱼类，特别是两栖鱼类，可以作为研究可能促进古代鱼类从水中出现的适应性的模型。其中一些鱼类可以主动从水中出来并在陆地上表现出复杂的行为，而少数鱼类可以钻进泥里并在干旱期间存活数年。它们中的许多都具备在离开水期间减轻氨毒性的机制。在这篇综述中，将呼吸空气的鱼类在离开水期间应对氨毒性所采用的机制归纳为七种不同的策略。此外，还挑选了八种具有独特陆地行为和特殊自然栖息地的现存呼吸空气的鱼类，详细描述这七种策略如何以不同的组合方式被采用，以减轻离开水期间的氨毒性。
