Eels Alive!

What It Covers

Stuff You Should Know hosts Josh and Chuck explore the biology, history, and conservation status of eels, covering over 800 species across 20 families, their mysterious reproductive cycle centered on the Sargasso Sea, their role as medieval currency, and the 90% population decline since the 1970s.

Key Questions Answered

• •Eel Classification: Only members of order Anguilliformes qualify as true eels — electric eels are actually knifefish, closer to catfish. The order spans 20 families, 111 genera, and over 800 species, ranging from 4 inches to 12 feet long. All share elongated worm-like bodies, lack pelvic fins, and move via wave-like undulation.
• •Eel Blood Toxicity: Raw eel blood contains toxins harmful to humans, which is why unagi is always served cooked in Japanese cuisine. The cooking process neutralizes these compounds. Moray eel bites, while non-venomous, introduce hemagglutinin and cryotoxin into wounds, causing red blood cell clumping and destruction, deep punctures, and high infection risk.
• •Medieval Eel Economy: Dried eels functioned as currency in medieval England, with approximately 500,000 used annually to pay rent and debts. Eels were bundled in standardized denominations — a "stick" equaled 10 eels, a "bind" equaled 25. Their nutritional profile (high protein, vitamins B12, A, and D) made them a staple during 120+ Catholic fasting days per year.
• •Reproductive Mystery Solved: European and American freshwater eels migrate thousands of miles to breed in the Sargasso Sea — confirmed by a 2018 research team led by Rosalind Wright. Danish marine biologist Ernst Schmidt first proposed this in 1912 after tracking larval size across Atlantic fishing nets. Larvae drift back to Europe on ocean currents, metamorphosing through glass eel, elver, yellow eel, and silver eel stages.
• •Population Collapse and Aquaculture Risk: American, European, and Japanese eel populations have dropped over 90% since the 1970s. European eels are now critically endangered. Aquaculture farms cannot breed eels in captivity due to complex larval needs, so they harvest wild glass eels instead — removing the very survivors of mass larval die-off, directly accelerating population collapse before reproduction occurs.