Despite the many scientific advances over the last 100 years, a question which has not been conclusively answered is: what is the full reproductive cycle of eels in the wild? Anguilla anguilla eels develop their sexual organs late into their life cycles and although it is hypothesised that these freshwater eels migrate thousands of kilometers to reproduce in the Sargasso Sea, in the North Atlantic Ocean, this reproduction has never been seen and eggs have never been found.
(https://go.nature.com/2WEq4I1, https://econ.st/3BOQmqv)