Ed Yong in The Atlantic:
At first glance, the hagfish—a sinuous, tubular animal with pink-grey skin and a paddle-shaped tail—looks very much like an eel. Naturalists can tell the two apart because hagfish, unlike other fish, lack backbones (and, also, jaws). For everyone else, there’s an even easier method. “Look at the hand holding the fish,” the marine biologist Andrew Thaler once noted. “Is it completely covered in slime? Then, it’s a hagfish.”
Hagfish produce slime the way humans produce opinions—readily, swiftly, defensively, and prodigiously. They slime when attacked or simply when stressed. On July 14, 2017, a truck full of hagfish overturned on an Oregon highway. The animals were destined for South Korea, where they are eaten as a delicacy, but instead, they were strewn across a stretch of Highway 101, covering the road (and at least one unfortunate car) in slime.
Typically, a hagfish will release less than a teaspoon of gunk from the 100 or so slime glands that line its flanks. And in less than half a second, that little amount will expand by 10,000 times—enough to fill a sizable bucket.