If you haven’t found the Jellyfish yet, check the map guide :).
I have been killing jellyfish for nearly an hour now, but they never drop a marine membrane. It's always either tentacles or seaweed. How low is the chance for them to drop a marine membrane?
Could be a very bad RNG seed, if such a thing happens :(. You should have about 21% chance per won battle. So like 1 per every 5 battles won.
Exiting and entering the game again will regenerate the random seed, which may help with this.
