What do the very largest ocean animals eat?

Almost the smallest things in the sea. Blue whales, basking sharks, whale sharks and manta rays - the four largest groups of animals in the ocean - are all filter feeders living on krill and plankton, and none of them can take anything much bigger.

It works because of what a food chain costs. Only about a tenth of the energy at one level makes it to the next, so a predator eating fish that ate smaller fish that ate plankton is living on a small fraction of what the plankton originally captured. Skipping straight to the bottom means the food is thin but astronomically abundant, and if you are large enough to process seawater by the tonne it is the most efficient meal available. A blue whale lunges into a swarm, takes in a volume of water greater than its own body, and strains out perhaps four tonnes of krill a day.

The sperm whale is the exception that proves the rule - the one giant that hunts individual prey, diving a kilometre down for squid in the dark and locating it with the loudest sound any animal makes. It is also the only one that had to evolve a brain the size of ours to do it.