Very interesting indeed! It’s fascinating how many adaptations there are that aren’t as obvious as anatomical ones.

Although I would take the validity of this excerpt with a grain of salt. The evolution of different kinds of fish is really messy (as they are a paraphyletic group anyways). I couldn’t find any info on how any fish or sharks evolved in freshwater conditions for example (just that their urea content is indeed higher). If you have any info on that, I would be glad to read it :)

The only interesting bit I did find was this pbs eons episode on how armored fish evolved to probably store minerals like calcium and potassium and how other vertebrates today use their endoskeleton to store those same minerals.