Freud and his early lab work with..
Eels!
From the fantabulous THE GOSPEL OF THE EELS by Patrik Svensson (see images below).
NB: Freud was ultimately unsuccessful in this line of research, so he moved on to other areas, ironically giving rise to the murky, unverifiable, and gender-and-sexuality-driven field of psychoanalysis.
In other words, his inability to verify eel gender, according to the author, affected Freud’s disposition, driving his pursuit for what lays hidden and obscure.
It’s a poetic assessment, and not without merit. It certainly makes for a good story.