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that feed on plankton are in turn preyed on by larger fish, which may be eaten by still larger fish or even whales. However, some of the largest whales feed directly on the plankton by swimming slowly through the surface layers, filtering the water to obtain enough of the tiny creatures to sustain them. The most productive areas for plankton are places where cold currents well up from the ocean depths, carrying nutrients from the seabed to the surface sunlight. Here, billions of tiny creatures flourish and sus- tain small creatures that become food for seabirds, seals, and whales. Filter-feed- ing mollusks that thrive on plankton are dietary staples for sea mammals such as the walrus.

Several drifting creatures utilize winds blowing across the ocean’s surface. The deadly Portuguese man-of-war has an air- filled sail that extends above the surface to catch air currents. As its long, stinging ten- tacles stream out below in the water, its sail catches the wind and moves the animal over great distances, catching tiny organisms as it travels. In the Depths Light penetrates only a short distance below the ocean’s surface. At moderate depths, where some light filters through, countless species of large, open-ocean fish swim—some in shoals, some alone. They are streamlined and swift; with no place to hide, they must

The giant jellyfish, measuring up to 8 feet (2.5 meters) in length, drifts slowly and silently through deep water, ensnaring small fish and other creatures in its trailing tentacles.

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