Hagfish are a type of non-vertabrate chordate--not a true fish, but not a true invertebrate. Locally called meokjangeo (먹장어), or "slime eel," they are only eaten only in Korean cuisine--mostly in Korea, but sometimes by Korean expatriates in Japan and California.
Hagfish are chewy, with a softer spinal chord that runs through their back, and have a mild taste, with an unpleasant aftertaste. Though unpalatable to foreigners, they are popular in Korea, where they are usually eaten by men as an aphrodisiac. For that purpose they are considered by Korean men to be interchangable with eels, an unrelated animal with a similarly phallic shape but remarkably different taste and texture.
An adventurous eater seeking to try one will have no trouble finding them in Korea, where seafood is displayed in large glass tanks. But the travel-limited culinary xenophile might also be able to obtain them directly from fishermen in California, and prepare then as described, using a hot plate instead of the traditional Korean barbecue set up.
To prepare them, they are sliced down the middle to remove the digestive tract, then marinated in a sauce used for Korean barbecue. Traditionally, the raw fish are then placed on a heated plate at the center of the table, where they are cooked and served like galbi, using scissors to slice the hagfish up. The cooked fish are moved to the side of the dish, with lettuce and gochujang, no amount of which can mask the animal's distinct taste. The head, containing the skull is left on the fish, and if a foreigner dines with Koreans, the honor will be offered to the foreigner.
Anecdotal evidence suggests that they are also popular eaten raw.
The hagfish produce large amounts of mucus as a protective measure, and can be made to produce more by placing them in a bucket and agitating the animal until it is filled. The resulting slime can then be used as a substitute for egg whites.