The eel comes from mud. It is so slippery that the tighter it is held the sooner it slips away. Drinking wine in which eels are killed gives one a distaste for wine. Eels are neither male nor female and produce no eggs; instead they are generated from mud or stagnant water.

There are four kinds of eels described in medieval manuscripts and their sources: the eel (called anguilla), the lamprey and the moray (both called muraena), and the conger (congris). Not all sources distinguish between them.

Thomas of Cantimpré (quoting Aristotle) says it is dangerous to take moray eels as food unless they are first boiled in wine for a long time, and carefully seasoned with aromatic species, and especially with pepper, because they abound in poisonous humors, and therefore do not yield to easy cooking.