> [Freud] had to return to check his results later in the year, he chanced upon a pair of furrowed organs, hidden away in the abdominal cavity, which he identified as the much-sought-after testes of a male. With the two sexes now clearly distinguished, definitive evidence of sexual reproduction had been found; and, after more than 2,000 years, Aristotle’s theory of spontaneous generation could at last be laid to rest.
We don't know for certain because we've never seen them reproduce. But we do know that their digestive system dissolves as they gain their reproductive organs, meaning that they lose the ability to eat once they have gonads so it seems a fairly robust assumption.
And the female octopus. So intelligent, but after they reproduce they stop eating and die. (Apparently they sometimes eat their mate beforehand, which I didn’t know until now). It seems like such a waste. I count myself lucky that humans don’t follow this pattern!
https://en.wikipedia.org/wiki/Eel_life_history