tl;dr Eels have a long lifecycle with several stages. They do not develop sexual organs until late in their life, when they migrate back to the Saragossa Sea. This meant earlier autopsies of eels revealed no sexual organs, even though scientists could provoke them with hormone therapy. So, a team lead by Jose Azevedo tagged female eels in the Azores in 2018, and tracked them via satellite [0].

[0] https://www.nature.com/articles/s41598-022-19248-8

I sometimes think about the selection pressures that lead to complex life cycles, like fig wasps. I find myself thinking about it naively, like one existed and the other grew into the niche. But, realistically, everything is changing (slowly) all the time. I just notice it for, say, influenza because their cycle time is so short.