The presence of the eel in this novella might seem minor, but it has a double meaning.
In medieval Italy, the consumption of fish by members of the aristocracy was much higher than previously thought.In medieval Florence, as well as in other Italian cities, the exchange of fish (and in the Farnese family eels in particular) among noble friends and family was not uncommon. A fish was an excellent gift to keep relationships fruitful and led to a continuous cycle of communication, obligation and affection. It strengthened people’s political influence (See Luiten, "Friends and Family, Fruit and Fish”, 342-57.) Because the church held so many fasting days, people couldn’t eat meat for a large part of the year . Fish, however, was allowed, making it a popular source of protein.Luiten, "Friends and Family, Fruit and Fish”, 342-57. Of all fish, eels were very popular in medieval Florence. Rivers and lakes were full of eels, and what’s more, eels can stay alive outside the water for up to six days. It was therefore a frequently transported fish. Luiten, Loek. "Friends and Family, Fruit and Fish: The Gift in Quattrocento Farnese Cultural Politics." Renaissance Studies 33, no. 3 (2018): 342-57.
On a symbolic level, the eel stands for Ciacco and Biondello being slippery. Eels are notorious for being difficult to grab. In many languages, being ‘slippery as an eel’ means being ingenious and difficult to catch, often with a negative connotation.See Collins dictionary Ciacco and Biondello are both slippery as an eel: they try to profit from richer people by using their wit, and by doing so they live above their means. What’s more, they try to prank each other by inventing ruses, which can lead to negative consequences like a harsh beating. Until they reconcile, they live in constant resentment to one another. The fact that the eel plays a role in the prank on Ciacco has a symbolic meaning: it is a symbol for Biondello’s ingenuity.