Man what a beautiful bit if knowledge, reminds me a bit of the eel mystery. Thanks to the researchers doing this sort of work, and I hope their conservation efforts go well
To do your question justice would take a while but its quite an interesting story to google. The gist: We did not have confirmation of where NA/EU freshwater eels laid their eggs until 2022. No one has ever seen them reproduce. This mystery is also old: Aristotle wrote about it. They have been an enigma for a long period of human history despite being an important part of it!
Also, before he got famous for inventing diagnosing everyone with penis envy, Freud spent a summer trying and failing to find the eels' reproductive organs.