A few weeks ago, I commented in a lecture on vertebrates (animals living in nekton), on how marine animals (various fishes for example) may help keep their (near) buoyancy by exchanging heavier ions for lighter ones. I deduced this may be the case, as it was with plankton.
On page 108 of my reference (Nybakken, W. J., Bertness, D. M. 2005. Marine biology: An ecological approach. Pearson - Benjamin Cummings, San Francisco, CA) there is an interesting finding:
Another mechanism for ensuring neutral buoyancy is the replacement of the heavy chemical ions in the body fluids with lighter ones. We observed this mechanism in plankton as well.
And here comes the part where I was wrong.
The only nektonic animals in which the mechanism occurs are the squids, some of which have body cavities and tissues in which heavy sodium ions are replaced with lighter ammonium ions.
And here is the probably reason why:
Although common among plankton, this mechanism is rare among nekton because, for it to be effective, the amount of ammonium-dominated fluid must be large, and large fluid filled spaces give the animal a rotund appearance, markedly decreasing its ability to move rapidly.
I could perhaps note that large amounts of NH4+ could be too toxic for given organism, however this could probably be disputed, since some animals retain high levels of urea and/or uric acid to regulate osmosis.