many bony fishes ,acquatic amphibians and acquatic insects are ammonotelic in nature while some marine fishes are ureotelic why not all marine creatures excrete only ammonia or urea?

Marine fishes are ureotelic because they face te problem of osmosis. In seawater the concentration of dissolved substances are usually lower in the body of the organism than in the seawater environment. Hence, water tends to diffuse out whereas salts tend to diffuse in. Marine fishes excrete excess salts through the gills. They excrete very little urine. They drink large amount of water to replace water loss. They cannot excrete urea because urea requires large amount of water to get dissolved which is not present in marine fishes.
Where bony fishes, aquatic amphibians live in freshwater. In freshwater the concentrations of dissolved substances are usually higher in the body of the organism than they are in the environment. Water therefore, tends to diffuse in the organism. Therefore freshwater fishes must get rid of excess of excess water. This they do in the form of ammonia. Since, ammonia requires large amount of water to get dissolved, hence fresh water fishes are ammonotelic. 

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