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The nitrogen content in urine is proportional to the total food protein in the person's diet, and the phosphorus content is proportional to the sum of total food protein and vegetal food protein. [ 17 ] : 5 Urine's eight main ionic species (> 0.1 meq L−1) are cations Na , K , NH 4 , Ca , and the anions , Cl , SO 4 , PO 4 , and HCO 3 . [ 18 ]
The excretion of urea is called ureotelism. Land animals, mainly amphibians and mammals, convert ammonia into urea, a process which occurs in the liver and kidney. These animals are called ureotelic. [3] Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion.
For example, the liver transforms ammonia (which is poisonous) into urea in fish, amphibians and mammals, and into uric acid in birds and reptiles. Urea is filtered by the kidney into urine or through the gills in fish and tadpoles. Uric acid is paste-like and expelled as a semi-solid waste (the "white" in bird excrements).
Waste feed may also provide additional nutrients; either by direct consumption or via decomposition into individual nutrients. In some projects, the waste nutrients are also gathered and reused in the food given to the fish in cultivation. This can happen by processing the seaweed grown into food. [30]
Fish waste is organic and composed of nutrients necessary in all components of aquatic food webs. In-ocean aquaculture often produces much higher than normal fish waste concentrations. The waste collects on the ocean bottom, damaging or eliminating bottom-dwelling life. [ 99 ]
The pronephros is a functioning kidney of the embryo in bony fish and amphibian larvae, [7] but in mammals it is most often considered rudimentary and not functional. [18] In some lungfish and bony fishes, the pronephros can remain functional in adults, including often simultaneously with the mesonephros. [ 7 ]
Cleaner fish maintain a balance between eating ectoparasites and mucus or tissue because of the respective nutritional benefits, sometimes despite the risk to the client. [4] For example, the Caribbean cleaning goby ( Elacatinus evelynae ) will eat scales and mucus from the host during times of ectoparasite scarcity to supplement its diet.
Cleaning symbiosis is a relationship between a pair of animals of different species, involving the removal and subsequent ingestion of ectoparasites, diseased and injured tissue, and unwanted food items from the surface of the host organism (the client) by the cleaning organism (the cleaner). [5]