From the article > It rests on the particular molecular mechanism they use in their tissues to withstand crushing pressures. To go deeper would require fish to

From the article > It rests on the particular molecular mechanism they use in their tissues to withstand crushing pressures. To go deeper would require fish to evolve some other mechanism, the team tells the journal PNAS. The all-important molecule is a so-called osmolyte called trimethylamine N-oxide (TMAO). It is what gives fish their “fishy smell”. TMAO acts to stabilize the proteins fish use to build and maintain their cells. Without its presence, the proteins would be distorted by the high pressures found at depth and stop functioning.