There may be a depth limit where no fish can live

The deepest fish ever recorded is a snailfish, filmed by NOAA scientists at 8,336 metres (27,000 feet) below the surface. Yet no fish has ever been found in the deepest quarter of the ocean — the stretch from roughly 8,400 metres down to nearly 11,000 metres. There appears to be a floor for fishy life, and the reason is chemistry, not just crushing pressure.

The key is a molecule called TMAO (trimethylamine N-oxide). It acts as a “chemical chaperone,” shielding a fish’s proteins from being deformed by extreme pressure. Deep-sea fish carry far more of it: levels climb from about 40 mmol/kg near the surface to 261 mmol/kg at 4,850 metres, with hadal snailfish higher still.

Pile in too much TMAO and proteins become so rigid they can no longer flex and function.

Extrapolating that rising trend, researchers calculate the molecule maxes out at about 8,200 metres. Beyond that, the very chemistry that lets fish survive the depths turns against them — a likely biochemical limit for all fish.