The open ocean is the sunlit top layer of the ocean beyond the continental shelves. The vast bulk of the sea, it covers over 360,000,000 square kilometres of the Earth's surface. It's referred to as the 'marine desert' because nutrients are lower here than in the shallow seas and life can be scarce. Open ocean, or pelagic, predators therefore have to travel fast and far to find food, hence many – such as marlin, tuna and dolphins – are swift swimmers.