Meet the Critters of the Charles: the American Eel

It's time to get to know some of the species that inhabit our beautiful waterway- even the icky ones. Yes, eels are slimy and gross and they eat each other without any visible remorse, but sometimes nature just be that way, so deal with it. You can be glad there aren't any really creepy animals in the Charles, like this sea spider below.

Make some noise for  Sericosura verenae  (image from Wikipedia)

American eels are the fourth most common fish on the Charles river (USGS), but the first time I heard about eels on the Charles was this summer. As I was canoeing around looking for water chestnuts, a young gentleman in a motor boat approached me and asked what I was doing. I told him I was pulling invasive species and he replied, "Y'ever see any eels in there?". I told him no, and he said "They like to hide in the weeds there", and then sped off. Yes indeed, the American eel, Anguilla rostrata, can be eaten like other fish, but you may want to refrain from this practice for reasons I will discuss later. 

The genus Anguilla contains sixteen freshwater eel species, but only two, the American eel and the European eel, breed in the Atlantic. As you might expect, these two species are very closely related and only diverged about 10 million years ago (Tsukamoto and Aoyama, 1998). Speciation from the ancestral freshwater eel may have begun only 20 million years ago, though this is a tentative estimate based on how many mutations have accumulated in eel mitochondrial DNA since then (Minegishi et al., 2005). This makes freshwater eels relative newcomers, especially compared to our new friends the sea spiders, which have been around since the Cambrian, 500 million years ago. 

The American eels that inhabit the Charles only stay there for part of their lives. Freshwater eels are catadromous, meaning that they return to the sea to breed. In the next few months, from all along the eastern coast of the US, American eels will swim down their streams and rivers to the Sargasso Sea (which is right smack dab in the middle of the Atlantic), and soon we will have a whole new generation of A. rostrata. After a year of drifting in ocean currents in a transitional state called a leptocephalus, the little guys become translucent glass eels, and begin finding their way to fresh water. Moving mostly at night, they follow the Gulf Stream up the east coast until they detect fresh water. Once they enter a river or estuary they begin to eat basically anything they can fit in their mouths (including their siblings). They gain color quickly and then grow until over many years until they are about 24 inches long (McCord). 

If you didn't think eels were weird enough already, American eels may practice environmental sex determination, meaning that the sex of these eels is determined long after the eels have spawned. This can be advantageous because in some situations (like when a stream is crowded with A. rostrata), it makes sense for most of the eels to be male with a few larger females (Krueger and Oliveira, 1998). 

I guess I should include a picture of the thing I'm talking about
(from  http://fish.dnr.cornell.edu)

Unfortunately there is evidence that populations of both the American and European eel are under stress (Wirth and Bernatchez, 2002). Glass eels are commonly used as fish bait, though this practice is now discouraged in some areas. Freshwater eels also need well-oxygenated water, and we all know that water chestnuts, present on the Charles, remove oxygen from the water as they decay. Furthermore, as predators, eels are vulnerable to bio-accumulation of environmental toxins. Heavy metals (like lead, cadmium and mercury) and polychlorinated biphynels are taken up by the eel's prey, and then can accumulate to dangerous levels in the tissues of the eel (McCord). 

Another potential problem for A. rostrata comes from (surprise!) climate change. We know that the Gulf Stream, which the eels use to get to freshwater, slowed during the last North American glaciation, which reached its maximum extent 21,000 years ago. There is evidence that this climactic event initiated a decline in Atlantic eel populations from which the eels have been unable to recover (Wirth and Bernatchez, 2002). Further perturbation of the Gulf Stream, by rising ocean temperatures or the melting of ice on Greenland could spell disaster for this Charles River native. 

Sources-

Aoyama, Jun, Mutsumi Nishida, Katsumi Tsukamoto. Molecular Phylogeny and Evolution of the Freshwater Eel, Genus Anguilla. Molecular Phylogenetics and Evolution. Volume 20, Issue 3, September 2001.
McCord, John. American Eel. South Carolina Department of Natural Resources.

Minegishi, Yuki, Jun Aoyama, Jun G. Inoue, Masaki Miya, Mutsumi Nishida, Katsumi Tsukamoto. Molecular phylogeny and evolution of the freshwater eels genus Anguilla based on the whole mitochondrial genome sequences. Molecular Phylogenetics and Evolution. Volume 34, Issue 1. January 2005.

Tsukamoto, Katsumi, and Jun Aoyama. Evolution of freshwater eels of the genus Anguilla: a probable scenario. Environmental Biology of Fishes. Volume 52, Numbers 1-3. 1998.

United States Fish and Wildlife Service. http://www.fws.gov/northeast/newsroom/eels.html

Weiskel, P.K., 2007, Understanding the Charles River, eastern Massachusetts—scientific information in support of environmental restoration: U.S. Geological Survey General Information Product—47, 12 p.

Wirth, Thierry, and Louis Bernatchez. Decline of North Atlantic Eels: A fatal synergy? Proc. R. Soc. Lond. B. Issue 270. 2003.