OK

Most of you are at least somewhat familiar with the life cycle of Salmon in the Pacific Northwest. Born in rivers and streams far inland, the newly hatched young float downstream toward the ocean where they live out most of their lives in salt water.  Each year a mass migration occurs where adults battle their way upstream and over waterfalls back to their fresh water nursery grounds where they spawn, die and become bear food.  Fish that do this are called anadromous.

Here in my New England backyard lives another species of fish, the American Eel (Anguilla rostrata), which does the opposite. Known as catadromous fish, they are born in salt water, spend their lives in freshwater rivers and lakes and then return to the ocean to spawn.


American Eel

First, let's define exactly what an eel is since I've found there is a lot of confusion about this.  Eels aren't simply long, skinny fish. Many fish look like eels but do not belong to the order Anguilliformes. Members of this group of "true eels" have one main distinguishing characteristic which separates them from all other types of fishes: A continuous vertical fin.  In other words the top fin (the dorsal), the tail fin (the caudal) and the bottom fin (the ventral) are all connected to form a single long fin that starts on the back and ends at the belly.

The rock gunnel, below, may resemble an eel but the three fins are clearly separate.  In a true eel, like the one up top, there is no distinction as to where one fin ends and the other begins.  Very simple, right?  Good, because as we shall see, not much else about eels is.


The Rock Gunnel (Pholis gunnellus) is not a true eel

The American eel exhibits sexual dimorphism. That is, there is a very clear difference in size between males and females.  Males may only reach a length of eighteen inches whereas females can grow to be up to four feet long.  

They also do not share the same habitat throughout most of their lives, which I believe is unique among fish; at least I can't think of any other species where the habitat is different for each sex.  Males stick close to the ocean, inhabiting river mouths and brackish water estuaries. Females on the other hand may travel many miles inland, following rivers, streams and even roadside ditches far into the interior, although because of dams they are now unable to go as far inland as they once did. Still, some are known to travel as far west as Ohio.

These female eels are very resourceful and can survive hours out of water, crossing over land or marsh to reach the next body of water.  This behavior makes it infuriating to keep them in captivity since they seem to be able to escape from nearly any type of tank or enclosure. (Although only fully mature eels can be sexed, I have taken to the assumption that if it hasn't escaped from my tank display it's a male.)

In the 4th century BCE, Greek know-it-all Aristotle first attempted to describe the natural history of this fish. Actually he studied the closely related European version, Anguilla anguilla, and because he could never find any juvenile eels he believed they came from earthworms that took to the sea (wrong phylum there, Ari).  Up until the early 20th century the actual life cycle of this fish baffled naturalists.  That is until 1922 when a Danish biologist named Johannes Schmidt discovered both its spawning grounds as well as its incredibly odd larval stages (which were known about previously but thought to be completely separate species of animals).

Once the adults fully mature (males at around five years and females up to twenty) it's time for them to spawn. Females head back towards the ocean where they meet up with the waiting males.  They then embark on an epic migration all the way to the Sargasso Sea, a roughly million square mile area of the mid-Atlantic Ocean just southwest of the island of Bermuda.  During the journey the normally yellowish adults turn silver and stop eating, using all their energy reserves getting to the spawning grounds.  In fact, the eel's digestive system whithers away during the trip.  When they reach the Sargasso Sea each female lays as many as four million eggs, which are then fertilized by the males in what can only be described as a massive, slithering orgy.  Once they have finished mating all the parents literally die of exhaustion.

The eggs hatch and the tiny young rise to the water's surface.  This transparent larva is known as a "leptocephalus" and is shaped like a tiny transparent leaf (with two large black eyes) and they immediately start heading slowly toward the North American shoreline, a trip that may take them up to a year to complete.  Along the way they gradually transform into a worm-like animal (still transparent) known as a "glass eel".  By the time the young reach the shore they resemble a miniature version of the adult they will become (these baby eels are known as "elvers").  Once they arrive near shore they repeat the behavior of their parents; the males stay close to the coast and the females head deep into the interior of the continent.

 
Leptocephalus larva, left.  "Glass eels", right.

Before moving on, I should note the importance of transparency for the two larval stages.  In the open ocean there is no place to hide. The best defense from predators is simply not to be seen.  This defense loses its effectiveness once they reach the coast where the ability to countershade, camouflage or hide among rocks and weeds becomes a more useful protective technique.

Moving on. Meanwhile, across the Atlantic, the European eel displays the exact same behaviors, including traveling to the same spot in the Sargasso Sea at the same time as their American cousins. Although A. rostrata and A. anguilla are nearly identical they don't interbreed. When the eggs hatch the American larvae travel back to America while the European larvae migrate back to Europe. No one knows how the young know which continent to return to.

As I said, the two species are nearly indistinguishable. Placed side by side you would be unable to tell which was which. But they differ mainly in two ways: First is in the number of vertebrae in their backbone.  And second, the American eels have a shorter larval stage.  Why?  Because the larvae of both species are helped back to their respective coasts by the Gulf Stream current, which follows the North American shoreline south to north (map). When the American larvae reach the northern states they leave the current and head west towards the coast. However the European young remain in the current and still have two more years of traveling as transparent larvae before reaching their home continent.  

Fun Fact: The area around the Sargasso Sea is sometimes referred to as the "Horse Latitudes".  Because of the lack of wind in this area, Spanish warships bound for the New World would at times be stranded for weeks and in order to conserve water the crew were forced to toss all of their horses overboard.

And take the poll. Next Friday I'll write on whichever subject gets the most votes.

Other diaries in this series can be found here.

Originally posted to Mark H on Fri Jul 28, 2006 at 04:46 PM PDT.

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