Have you been looking, without any luck, for eel testicles? Not sure how to sex an eel? Well don't worry...you're not alone. People from Aristotle onward tried & failed. Sigmund Freud once spent a summer in Italy dissecting 400 eels looking for their testes, without success. /1 
Turns out, eel sexual determination is pretty wild. It's largely environmentally determined, & only becomes set years into their life cycle. For most of their lives eels are juveniles, & their sexual organs only develop fully just before they head back to sea to mate & die. /2
This led to the long-held belief that eels reproduced asexually. Aristotle thought they sprang from the mud, writing that eels had neither sperm (milt) nor eggs. Pliny thought that they grew out of flakes of dead skin other eels had rubbed off on rocks. /3
This sounds nuts, but keep in mind that we've *still* never seen eels reproduce. We believe they all spawn in the Sargasso sea, but we think that because that's where there's the highest concentration of eel larvae, called leptocephali. And we've only known that since 1913. /4
The idea that eels were asexual had implications for the medieval diet. There were about 150 days a year when you weren't supposed to eat flesh (carnis) 'cause it's the product of sex (a carnal act). Fish are ok, & eels...well, Aristotle says eels don't even have sex! Perfect. /5
So eels became an important dietary staple in medieval England, & esp. in monasteries, which tended to be scrupulous about fish-days. This is partly why eel-rents lasted longer than other in-kind taxations; many of the landlords were monks, & they still needed their eels. /fin
