A few weeks ago Greg Watkins-Colwell of our division went collecting at the beach of Morris Cove here in New Haven County with Jose Pereira of the National Marine Fisheries Service, along with two NMFS interns, Austin Hurst and Jason Bennett.
They seined up a glass eel! It was actually so glass-like and crystal clear that when Greg showed it to me in the bucket containing his catches, it took me a good few minutes to find it in the water! Here 'tis:
These transparent wonders are not a species, but rather they are the second stage in the life cycle of a number of eel species in the family Anguillidae. This particular specimen is an American eel, Anguilla rostrata. For the rest of the post I'll focus on the life cycle of this species for simplicity's sake.
American eels are catadromous, meaning that they spawn in the ocean, but they inhabit freshwater, estuaries, and brackish habitats for most of their lives. There are five stages in the life cycle of these eels, with the first stage being the leptocephalus larval stage. At this stage the eels are transparent like the glass eels, but they are smaller and more flattened. Here's a pic taken from Wikimedia commons:
The eels remain as leptocephali, floating about in the ocean where they were born, for about a year. They then metamorphose into glass eels, when they gain their adult shape (but not size) and are still transparent. At this point they migrate to estuaries on the Atlantic coast. Here's another pic of our recent glass eel acquisition:
The next stage is the juvenile or "elver" stage. At this point they gain pigment and size and generally begin to resemble adults. Also at this point some individuals will migrate pretty far inland. Here's a pic of this stage, taken again from Wikimedia commons:
The final inland stage is called the yellow eel stage. At this stage they can grow up to a meter long. Here's a pic of an American eel from our collection, YPM ICH 8080, at the yellow eel stage:
YPM ICH 8080 was collected in 1983 in Hesseky Brook in Litchfield County, CT. Here's a picture of its underside, which reveals quite clearly why they're called yellow eels!
The last stage of the life cycle is that of the silver eel, at which point the eels are sexually mature and migrate to the Sargasso Sea (in the heart of the "Bermuda Triangle") in the Atlantic Ocean to spawn. Here's a graphic that shows the location of the Sargasso Sea:
And here's a graphic depicting the final silver eel stage of the American eel, taken from The Inland Fishes of New York:
A final fun fact about glass eels: until 1893, (quite understandably!) glass eels were thought to be a distinct species of eel, Leptocephalus brevirostris, hence the name leptocephalus for the larval stage of these metamorphosing eels. Once a French zoologist raised a few leptocephali in a tank, however, the matter was clarified.