Complex sentence: Because fishermen never caught anything they recognized as eel young, the life cycle of the eel was a mystery for a very long period of scientific history, there are 6500 publications about eels but still much of its life history is enigmatic.
Split into simpler sentences: Because fishermen never caught anything they recognized as eel young, the life cycle of the eel was a mystery for a very long period of scientific history. There are 6500 publications about eels, but still much of its life history is enigmatic.