I have read a story long ago, where some explorers of an alien planet take an animal and use it like a boat (with a description similar to a very large Victoria.
Eventually, they have serious problems with the boat-animal (it begins to shake vigorously), and end up discovering it was just giving birth during the ride.
Earlier in the tale, some supposedly tame flying animal comes attacking them, and they discover that it is actually being neurologically controlled by a parasite, in a very similar way the Navi control their flying dragons in the Avatar Film: some animals in that planet evolved to "control" one another by a sort of lock-and-key neuro-anatomical structure.
Anyone knows the name and author of that story? I believe it's in some of Asimov's compilation books, but I'm not sure.
EDIT: actually the large boat-plant was not giving birth... But the accepted answer contains a link to the full text for the curious :o)