For almost as long as we’ve known about eels we’ve known them to be carnivorous and cannibalistic. Some of us knew them to be a good meal. What nobody knew for an awfully long time was how they reproduced. We couldn’t even find the female eel’s ovaries for a remarkably lengthy period, and it took even longer to find the male eel’s testes.
It was eventually discovered that the tiny translucent creatures in rivers that were commonly seen were actually eel larvae. The eel larvae embed themselves in the mud and grow rapidly. Once big they apparently disappeared. They must having been swimming off to a destination, but said destination was unknown for many years. It was seemingly around the Bermuda Triangle area. This was were they developed their sex organs. With these useful parts of their biology formed they would then travel back to rivers, have children, and promptly die. Explaining why it was just so difficult to find developed eels in the first place!