Which sea creatures were once used as currency in Medieval England?
In Medieval England from the 5th - 15th century CE, coins were rare, so it was a common practice to trade goods, like grain and eggs, as payment. There were other, even stranger, currencies used to pay the bills. One of the most unusual forms of payment used in England was eels. At the time, they made up 20-25% of the fish in England's rivers.
The Catholic Church had a strict ban on eating meat during Lent (the annual 40-day period of religious fasting and penitence leading up to Easter). Catholics were expected to practice abstinence from sex during Lent. As meat was thought to excite people's libidos, it was necessary to abstain from meat as well. Seafood (especially eels) was not believed to encourage carnal desires, thus making eels a popular alternative protein source.
The relative abundance of eels also contributed to their popularity. An additional advantage was the fact that eels would stay well-preserved well past the fall, when they were harvested. One of the most common ways to preserve and transport eels at the time was by skewering them on a stick, then drying or smoking them. When the eels traveled downstream for their yearly autumn migration, peasants would trap, kill, and cure the eels so they'd be ready for spring rent payments.
It wasn't until the 1500s, when other forms of meat became more readily available, that using eels as payment finally fell out of fashion.