Leedsichthys was a giant fish that would have dwarfed every other animal in the sea, but it was a gentle giant that lived on the tiny shrimps, jellyfish and small fish that make up plankton. It would have swum slowly through the upper waters of the ocean, taking mouthfuls of plankton-rich water and sieving them through the giant mesh-plates at the back of its mouth. Its feeding habits were similar to the modern blue whale, which also survives on nothing but plankton.
They probably travelled large distances to find parts of the world where seasonal conditions caused plankton to form itself into a dense concentrated organic soup. Once a year, and probably after plankton feasts, Leedsichthys would have shed the giant filter plates from the back of its mouth, meaning it was unable to feed itself for several weeks, whilst the new ones grew back. Towards the end of this time it would have become weakened through hunger and vulnerable to attack.
The Jurassic seas in which Leedsichthys lived were a dangerous place and despite its size, it had no formal means of defending itself against predators such as Liopleurodon and Metriorhynchus. One attack would be unlikely to kill a full-grown Leedsichthys, but several predators could have inflicted fatal damage, leaving this defenceless giant to die slowly from its wounds.