Omnivore — Piers Anthony

These are the species which lived on the mysterious planet Nacre. Three scientists came to Nacre to discover, to explore, to record. Utterly defenseless, they trekked through a grotesque jungle of multiform mushrooms and dense spore-clouds, hoping to unlock the secret of this strange world. The stunning climax of their mission was just the beginning of a complex drama in which their survival -- and return to earth-could spell the extinction of humanity!