8500 meters deep may be the limit for marine bony fish

Trimethylamine oxide (TMAO) helps stabilize proteins in fish at high pressures. It has been found that  TMAO concentration in fish that live in wider depth range increases with increasing depth. In deep sea where pressure go upto 1000 times of normal atmospheric pressures, the TMAO levels might be limiting factor.

Work published in PNAS predicts 8000-8500 meters might be the deepest limit that we can expect a bony fish to survive. Based on TMAO levels in fish at different depths and also the muscle osmolalities, it was found that at depths of about 8200 meters the fish would become isoosmotic instead of the normal hyperosmotic conditions. This would limit the bony fish survival to 8200 meters.

It may be possible that there might be some unknown species with very different kind of adaptation to high pressures which are able to live deeper than this. There are other family of organisms which are able to survive at depths of 11000 meters but any bony fish are yet to be discovered.

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