The flavors of ocean and freshwater creatures are very different. Because ocean fish breathe and swallow salty water, they had to develop a way of maintaining their body fluids at the right concentration of dissolved substances. Water in the open ocean is about 3% salt by weight, while the optimum level of dissolved minerals inside animal cells, sodium chloride included, is less than 1%. Most ocean creatures balance the saltiness of seawater by filling their cells with amino acids and their relatives the amines. The amino acid glycine is sweet; glutamic acid in the form of monosodium glutamate is savory and mouthfilling. Shellfish are especially rich in these and other tasty amino acids. Finfish contain some, but also rely on a largely tasteless amine called TMAO (trimethylamine oxide). And sharks, skates, and rays use a different substance: slightly salty and bitter urea, which is what animals generally turn protein waste into in order to excrete it. The problem with TMAO and urea is that once the fish are killed, bacteria and fish enzymes convert the former into stinky TMA (trimethylamine) and the latter into kitchen-cleanser ammonia. They’re thus responsible for the powerfully bad smell of old fish.