tl;dr: Fish rely on an osmolyte called trimethylamine N-oxide (TMAO) to stabilize the proteins used to build and maintain their cells. The concentration of TMAO

tl;dr: Fish rely on an osmolyte called trimethylamine N-oxide (TMAO) to stabilize the proteins used to build and maintain their cells. The concentration of TMAO seems to be extremely reliably correlated with depth and we know that at a certain concentration TMAO makes proteins too stable to work correctly. Other organisms like crustaceans are able to occupy deeper parts but they use 5 different osmolytes for this purpose whereas fish have only ever been observed using this one.