Five tropical air-breathing fishes, six different strategies to defend against ammonia toxicity on land.

Most tropical fishes are ammonotelic, producing ammonia and excreting it as NH3 by diffusion across the branchial epithelia. Hence, those air-breathing tropical fishes that survive on land briefly or for an extended period would have difficulties in excreting ammonia when out of water. Ammonia is toxic, but some of these air-breathing fishes adopt special biochemical adaptations to ameliorate the toxicity of endogenous ammonia accumulating in the body. The amphibious mudskipper Periophthalmodon schlosseri, which is very active on land, reduces ammonia production by suppressing amino acid catabolism (strategy 1) during aerial exposure. It can also undergo partial amino acid catabolism, leading to the accumulation of alanine (strategy 2) to support locomotory activities on land. In this case, alanine formation is not an ammonia detoxification process but reduces the production of endogenous ammonia. The snakehead Channa asiatica, which exhibits moderate activities on land although not truly amphibious, accumulates both alanine and glutamine in the muscle, with alanine accounting for 80% of the deficit in reduction in ammonia excretion during air exposure. Unlike P. schlosseri, C. asiatica apparently cannot reduce the rates of protein and amino acid catabolism and is incapable of utilizing partial amino acid catabolism to support locomotory activities on land. Unlike alanine formation, glutamine synthesis (strategy 3) represents an ammonia detoxification mechanism that, in effect, removes the accumulating ammonia. The four-eyed sleeper Bostrichyths sinensis, which remains motionless during aerial exposure, detoxifies endogenous ammonia to glutamine for storage. The slender African lungfish Protopterus dolloi, which can aestivate on land on a mucus cocoon, has an active ornithine-urea cycle and converts endogenous ammonia to urea (strategy 4) for both storage and subsequent excretion. Production of urea and glutamine are energetically expensive and appear to be adopted by fishes that remain relatively inactive on land. The Oriental weatherloach Misgurnus anguillicaudatus, which actively burrows into soft mud during drought, manipulates the pH of the body surface to facilitate NH3 volatilization (strategy 5) and develops high ammonia tolerance at the cellular and subcellular levels (strategy 6) during aerial exposure. Hence, with regard to excretory nitrogen metabolism, modern tropical air-breathing fishes exhibit a variety of strategies to survive on land, and they represent a spectrum of specimens through which we may examine various biochemical adaptations that would have facilitated the invasion of the terrestrial habitat by fishes during evolution.