Thread by @greenleejw

Did you know that eels migrate thousands of miles, or that their bodies change dramatically during their lives? Do you want to know?

That's right, friends...it's eel life cycle time!

Before we start, let's remember that I'm a historian, not a scientist. But I'll do my best. /1

All the eels in Europe and N. America are born (we think) in the Sargasso Sea, in the middle of the Atlantic. We've never actually seen eels mate, but this is where the smallest larvae have been found.

Eel larvae are transparent, leaf-shaped things. They look like this! /2

They can't really swim...they mostly float on the currents. They catch eventually a ride on the Gulf Stream and make their way towards land. For European eels, this means a 3,000+ mile journey.

This can take up to three years. /3

By the time they get to land they look less like leaves and more like eels. At this stage they're transparent, & called glass eels. You can see their vertebrae, their blood, and their lunch.

They hit land in the early spring, & wait in estuaries for warming river water. /4

They start filling in with color, getting darker and bigger.
At this stage they're called elvers. Once the water gets warm enough they start upstream, looking for a new home.

At this point their sex isn't set yet, & it's not entirely clear what determines as eel's sex. /5

Sex may, in fact, be environmentally determined. Generally the eels that wind up further upstream are females. They are bigger than the males, as well.

Once it finds a place it likes, an eel will often set up housekeeping and live there for a very long time. /6

Eels spend most of the rest of their lives in their new home, staying 10-20 years (sometimes longer!). They're called Yellow Eels at this stage, developing a brown & yellow coloring.

They remain juveniles for most of that time, & their sexual organs remain underdeveloped. /7

Eventually an eel will decide it's time to migrate back to sea to mate & die. At this point their color changes again, becoming darker to blend into to deep waters of the ocean. Now they are called silver eels.

Come the fall they leave their homes, & head back down stream. /8

When they get down to the sea they pause a bit, and their bodies morph one more time. Their eyes adjust to work in the dark of the depths, their become better able to handle greater water pressure

And their digestive system basically dissolves into their reproductive system. /9

For the rest of their lives -- for the whole trip back to the Sargasso Sea -- eels don't eat. This makes them hard to catch, since they won't take bait, & helps explain why, for a very long time, no one was sure if they had reproductive organs. /10

https://twitter.com/greenleejw/status/1205524393490833408

They eave the continent, & head out to sea. We know that they navigate using magnetic bearings & we've tracked them on their journey almost to the Sargasso. But then we lose them. We don't know how, or when, they spawn & die.

But they do...and the whole thing starts over. /fin

I should add, too, that if you're interested in folks who do actual conservation and science work with eels, then you should totally follow (among others):

@GlavenEel
@pollycouldrick
@EelTown_org
@SEGandrewK
@harrietalvis