From the incomparable Not Exactly Rocket Science blog at Discover Magazine:
[Hagfish] are disgusting feeders. They burrow deep into corpses and eat their way out, and can even absorb nutrients through their skin. And if they’re threatened or provoked, they produce slime – lots of slime, oozing from the hundreds of pores that line their bodies. The slime consists of large mucus proteins called mucins, linked together by longer protein threads. When it mixes with seawater, it massively expands, becoming almost a thousand times more dilute than other animal mucus...Much more at the link, where there is a second embedded video:
The hagfish in the videos are attacked by sharks, conger eels, wreckfishes and more. In less than half a second, the predator’s mouth and gills are filled with slime. It leaves, gagging and convulsing, slime hanging in long wisps from its head.You learn something every day.