In self defense, hagfish produce slime.
Picture from PHYS
When harassed, or when predators or other fish are trying to steal their food, a hagfish's glands secrete a stringy protein that expands into a transparent, sticky substance when it comes in contact with seawater.
To prevent choking on its own slime, a hagfish can “sneeze” out its slime-filled nostril, and tie its body into a knot to keep the slime from dripping onto its face.