From zooplankton to larger animals
The zooplankton are eaten by a huge variety of other animals. These range from small fish like sardines to giant manta rays, whale sharks, and even the largest animal on Earth, the blue whale. Some seabirds also feed exclusively on zooplankton.
The small fish are in turn eaten by bigger animals - squid, tuna, marlin, sharks, seabirds, dolphins, and toothed whales, to name a few.
The open ocean is vast, and large areas are devoid of life. Thus animals higher up the food chain need to travel huge distances to find food and places to breed.
Some fish predators are large, powerful, and fast, allowing them to cross the oceans in search of food. Sail fish are the fastest fish in the ocean, reaching speeds of 120km/h. Bluefin tuna are almost as fast, and can accelerate faster than a Porsche. In addition, unlike most fish bluefin tuna are warm-blooded, allowing them to survive in cooler waters.
Many whale species migrate thousands of kilometers each year between their warm breeding grounds and their rich Arctic and Antarctic feeding grounds. Marine turtles too make epic voyages across the oceans between their nesting beaches and feeding grounds.
Seabirds also travel huge distances. Albatross, for example, fly for thousands of kilometres without landing, scouring the oceans for fish, krill, and other food.
There is some shelter at the ocean surface. Driftwood, pieces of kelp, and other natural debris floating on the water provide valuable shelter where small fish can hide from predators. Man-made debris can similarly be used.
Islands of life
There is also some land in the open ocean: underwater mountains known as seamounts, which can come within metres of the ocean surface.
Formed by volcanoes or sinking islands, seamounts rise steeply from the ocean floor. Deep-water currents are forced up their sides, bringing nutrient-rich water to the surface around shallow seamounts. This allows plankton to thrive - supporting complex food chains and making these areas islands of life in the open ocean.
Visitors also drop by. Some fish species periodically gather at seamounts to feed and spawn, while marine turtles and whales stop at shallow seamounts for food and shelter during their long migrations.
Find out about deep-sea life on seamounts...