Overview of Stuff You Should Know: Eels Alive!
This episode digs into the strange, fascinating world of eels: what they are, how they move, where they live, why their reproduction baffled scientists for centuries, and why they’ve been so important in human diets and economies. Along the way, Josh and Chuck cover eel varieties, migration patterns, the Sargasso Sea mystery, and the conservation crisis facing several eel species today.
What Makes an Eel an Eel
- True eels belong to the order Anguilliformes.
- There are 20 families, 111 genera, and more than 800 species.
- They range from tiny species to 12-foot moray eels.
- Shared traits:
- Long, worm-like bodies
- No pelvic fins
- Many lack pectoral fins
- Smooth, slippery skin covered in protective slime
- They swim by undulating their whole bodies in a wave-like motion, and can even swim backward by reversing the wave.
Types of Eels Covered
Moray Eels
- The best-known ocean-dwelling eels.
- Hide in coral and rocky crevices during the day.
- Have two sets of jaws, including a pharyngeal jaw that helps pull prey inward.
- Appear to “show their teeth” because they lack opercula and must ventilate their gills through the mouth.
- Their bites are dangerous because of deep puncture wounds, slime-related toxins, and high infection risk.
Conger Eels
- Large, deep-water eels.
- The European conger is highlighted as the heaviest, reaching around 242 pounds.
Garden Eels
- Small, charming eels that live in burrows in sandy bottoms.
- Often appear like a field of seagrass swaying in the current.
- They can cluster by the thousands.
Gulper / Pelican Eels
- Deep-sea eels with a huge, expandable jaw.
- Their mouth can open into a scoop-like shape to catch prey.
- Their appearance is described as especially alien and dramatic.
Ribbon Eels
- Known for their vivid colors and ribbon-like shape.
- Notable for sex change over their lifespan: they begin as males and later become females.
Electric Eels
- The episode notes that electric eels are not true eels.
- They’re closer to knifefish/catfish than actual anguilliform eels.
The Eel Life Cycle and Reproduction Mystery
- Eels are catadromous: they live much of their lives in freshwater but travel to the ocean to spawn.
- Their life stages include:
- Leptocephalus larva
- Glass eel: transparent juvenile
- Elver: young eel entering freshwater
- Yellow eel: growing adult phase
- Silver eel: mature reproductive stage
- For centuries, no one knew how eels reproduced because:
- No one saw them mating
- Their reproductive organs were hard to identify
- Their early life stages looked so different from adults
Key Scientific Milestones
- 1777: Carlo Mondini identified eel ovaries.
- Sigmund Freud later dissected many eels while searching for the testes.
- Scientists eventually realized the “willow-leaf” larvae were actually eel larvae.
- Ernst Johan Schmidt and later researchers traced the breeding grounds of European eels to the Sargasso Sea.
- A 2018 study using tagged eels helped confirm the Sargasso Sea as a spawning area.
Why the Sargasso Sea Matters
- The Sargasso Sea is a relatively still region of the Atlantic Ocean formed by circulating currents.
- It’s warm, saline, and rich in floating sargassum seaweed.
- It serves as a key breeding area for European eels.
- The episode also notes that other eel species spawn in different regions:
- Japanese eels near underwater mountains around the Mariana Ridge
- African longfin eels in the Indian Ocean
Eels and Human History
- Eels have long been a major food source in Europe, Japan, and North America.
- They were:
- Easy to catch in rivers and streams
- Nutrient-dense
- Easy to smoke, dry, salt, and transport
- In medieval Europe, eel was so valued it was sometimes used as currency.
- The episode mentions hundreds of thousands of dried eels used annually for rent and debts in England.
- Eel was also important during Christian fasting periods, when meat was prohibited but fish was allowed.
- In Japanese cuisine, unagi remains especially famous.
- The episode notes that eel is commonly cooked, unlike most sushi fish.
- It’s traditionally grilled with kabayaki-style sauce.
Conservation Concerns
- Several eel populations are in serious trouble:
- European eel: critically endangered
- American, Japanese, and New Zealand longfin eels: endangered
- Populations of American and European eels have dropped by more than 90% since the 1970s.
- Major threats include:
- Dams and hydropower turbines
- Overfishing
- Wetland loss
- Pollution
- A major issue is that eel aquaculture often relies on wild-caught glass eels, since breeding them in captivity is extremely difficult.
Listener Mail / Bonus Segment
- The episode closes with listener mail about a baking soda + coconut oil deodorant recipe.
- The suggested improvement:
- Add cornstarch to reduce irritation
- Use only enough coconut oil to form a paste
- The note emphasizes careful skin testing, since baking soda can irritate underarms.
Main Takeaways
- Eels are among the most unusual fish in the ocean, both anatomically and biologically.
- Their reproduction was one of biology’s great mysteries for centuries.
- They’ve played an outsized role in human food culture and trade.
- Many eel species are now under threat, largely because human infrastructure disrupts their life cycle.