MadSci Network: General Biology Query:

I'm not sure what you mean by "moving out through their skin," but I think I can explain how sharks excrete nitrogenous wastes (i.e., urinate) and how this can give shark meat an ammonia smell and taste.

Like other fishes, sharks excrete nitrogenous wastes over their gills. The teleosts, or bony fishes, excrete nitrogenous wastes in the form of ammonia. Ammonia is a very toxic compound, and excretion of ammonia requires passing large amounts of water over the gills. Marine teleosts replace this water lost to excretion by drinking a lot of seawater and using their gills to remove excess salts.

Sharks, on the other hand, excrete nitrogenous wastes as urea, which is a less toxic compound. A complex biochemical pathway called the ornithine-urea cycle converts ammonia to urea, which can be stored more safely in the blood. In a live shark, ammonia doesn't accumulate because it is quickly converted to urea. When the shark dies, the urea deteriorates back into ammonia, which is why shark meat often tastes and smells of ammonia. Apparently you can remove much of this ammonia by soaking the flesh in freswater or lemon juice before cooking it.

I hope this answers your question!

Allison J. Gong

MAD Scientist

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