Did you know eels are problematic? They are, in a punny and scientific way, slippery.
I can truthfully say I've never been particularly interested in eels as a species. I knew nothing about them, and I didn't think my knowledge or lack thereof, would become a problem. But eels- they keep coming up in my research so I thought I'd sketch out here why they were so interesting to early 20th century biologists.
The mature form of the eel lives in particularly hard to reach places. They prefer to hang out near the ocean or estuary floor (some live in the ocean and some in fresh water), buried under mud or in rock crevices. This posed a huge problem for collectors of marine organisms in the early 20th century. Most collecting happened from boats- people threw nets or dragged the ocean floor- and examined what they could from these methods. Dredging involved basically dragging a metal object along the ocean floor behind a boat; some things looked like rakes or grappling hooks and others like big shovels. You can imagine what came up- slow-moving creatures, sponges, corals, sea stars, and sometimes a squid wrapped itself around the dredge. But eels, they're quick and slick and slippery and bringing mature forms to the service wasn't that easy.
The majority of eel forms that were collected during this period were from the surface of the water: eggs and leptocephalii. The leptocephalus, also known as the "slim head", is the larval form of the eel.
So far, these eels are sounding difficult, but not totally problematic. And actually a little boring. But here's where it gets good: Holy goodness these organisms end up all over the place!
Take, for example, the American eel. The American eel live in rivers on the Northeastern Coast of the United States in their mature form. But they are catadromous, meaning they migrate to salt water to spawn. Can you even imagine where they spawn? The Sargasso Sea. So, they swim from Eastern Estuaries into the Atlantic and lay millions of floating eggs and then, they die. The Sargasso Sea is big, and it's a gyre formation, meaning that things that float could end up anywhere. Down near the Bermuda Triangle, up near the coast of Newfoundland: there's much floating to be had- like a really long trip on the lazy river at Typhoon Lagoon.
|The gyre of the Sargasso Sea.|
|Eels don't have feet, but imagine just floating around and around maturing in the Atlantic Ocean.|
So eel eggs (and leptocephalii) are just out there floating around. And when early 20th century marine scientists find them in their nets, they have to try to figure out what mature form they might become. This is super hard for three reasons:
1. you can't match up eggs and mature forms based on collection location: as we just saw- there are just crazy eel eggs floating all over the place. Why would you think that something you found near South America had anything to do with a mature form that burrows in the mud in estuaries of the American northeast?
2. eel eggs and larval forms actually look pretty similar to each other- there's actually a time where nearly every eel species is just called leptocephalii because no one can tell them apart.
3. All this migrating during development meant that eel forms really needed something special at each part of their separate life stages and these needs were very difficult to transfer to the laboratory. So raising eels from egg to mature form wasn't really possible during this period. And, if you can't do that, it's hard to figure out what type of eel you're looking at.
|The migration of the American eel leptocephalii during its life cycle.|
Marie Poland Fish*, one of the most kick ass ichthyologists of the 20th century (and not just because her last name is Fish), tried figuring out what type of eel eggs she'd netted in the Sargasso Sea during the voyage of The Arcturus in 1925. Fish transferred the eggs to an aquarium on board ship and watched their development every day to try to ascertain what type of eel the eggs came from. She thought that they were American eel eggs, but couldn't rear them past the leptocephalic stage.
So, I've been looking at the mystery that is eel taxonomy and development at the turn of the twentieth century. And I suppose I assumed it was a problem that was cleared up sometime between Fish's observations and now. But it turns out that we still don't know that much about eel development. What are they doing out there in the Atlantic? What do they even eat?
While the pattern of migration has been worked out for the American eel, the life cycle of the Conger eel, who also spawns in the Sargasso Sea, is largely a mystery.
Interesting. An interesting eel mystery.
*I'm sure I'll get around to writing a blog about Marie Poland Fish because she may just be a new hero of mine. She made her name, not because of her work with eel embryology, but because of her later work with the Navy on marine sound detection. She basically helped the Navy calibrate their sonars to differentiate between fish and other sources of marine noise. Super super cool. Also, her last name is Fish and she works on fish. Still kills me!