1) Marlin is the epic hero in Finding Nemo who goes on a quest to find his son Nemo after he is captured, while Frodo is the epic hero in Lord of the Rings who must destroy the One Ring.
2) Dory and Frodo's fellow hobbits are the sub-heroes who help Marlin and Frodo on their quests.
3) Marlin bravely faces dangers like sharks to find Nemo, while Frodo displays courage facing the ringwraiths and other enemies.