Pseudoliparis swirei was described in 2017 from specimens trapped between roughly 6,900 and 8,000 metres. In 2022 a snailfish was filmed at 8,336 metres in the Izu-Ogasawara Trench off Japan, and a specimen was caught at 8,022 metres, both records for the group.
The limit appears to be chemical rather than mechanical. Deep-sea fishes accumulate trimethylamine N-oxide, a compound that stabilises proteins against pressure, and its concentration rises with depth. Extrapolation of that trend suggests that at somewhere around 8,200 to 8,400 metres the required internal concentration would leave the fish's tissues in osmotic imbalance with seawater, which places a physiological ceiling on how deep any fish can live.
The convergence of that prediction with the observed record is striking, but not proof. The trenches are sparsely sampled, and the deepest snailfish records come from a very small number of expeditions with baited landers and traps.