This video says about itself:
Eel Suffers Toxic Shock From Brine Pool – Blue Planet II
This is an exclusive clip from the Deep episode of Blue Planet II. At the bottom of the ocean there are brine pools which have a salinity level three to eight times the usual sea level. When a Cutthroat eel gets too close it goes into toxic shock and twists and ties itself into knots, and is lucky to make it out alive.