Large animals have big appetites. Exactly how big depends on how fast you burn energy. At the bottom of the scale are the invertebrates. Jellyfish can grow to be huge, but their body is about 95 per cent water and they move very slowly. Eating just 0.04 per cent of their body weight is enough to sustain them. Blue whales, at the other end of the scale, have a warmblooded body to support as well as a complex brain. But the hungriest creatures in the ocean are the killer whales. Their extremely active, predatory lifestyle means they need 3.7 per cent of their body weight each day to survive; when your body weighs six tons, that’s a lot of fish!