1. They live as deep as 1700m!!!!
2. They have four hearts. The brachial heart serves as the main vessel and three others serve as accessory pumps.
3. These hearts don't require constant oxygen in order to continue to pump which is an advantageous adaptation for the fact that....
4. They feed on decomposing organisms and are often found at the sites of whale falls and other large organisms. Burying themselves inside a dead carcass often causes hypoxic conditions that they can thrive in thanks to...
5. Their extensive capillary system within the skin that provides oxygen to the body while in hypoxic or anoxic conditions.
6. When under stress from predators, Hagfish expel a thick, slimy mucous from hundreds of glands along their body. This slime clogs the gils of predators and suffocates them, in addition to making them very hard to hang on to.
7. They avoid getting suffocated by their own slime by sneezing (yes sneezing) and tying themselves in knots that shield their gils and mouth from the slime.
8. They have incredibly slow, anaerobic metabolism and as a result can go months without food, which is good because population density is very low in the deep sea and they often rely on organism falls for a meal
9. Fossils that date back 300 million years ago suggest that the Hagfish hasn't changed structure much
10. They're boneless but have a brain!! They lack a spine but instead have a notochord, a structure that is later replaced by a vertebral column in all chordates but Hagfish and Lamprey once maturity is reached.
Footage of a whale fall (all of the squirming eel-like creatures are Hagfish eating their way into the whale)