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Foraminiferans
These organisms live within small shells made of calcium carbonate. A chemical "soup," a protoplasm mix similar to the contents of human cells, flows though their shell holes. This substance forms a sticky, food-catching network. When a foraminiferan outgrows its shell, the old one is discarded and the animal grows a new shell by using calcium carbonate from nearby water.
Radiolarians
Radiolarians are similar to foraminiferans, but their skeletons are made of a harder material called silica. Bodies of radiolarians litter the sea floor, covering about three million square miles of tropical oceans. According to Radiolaria.org, "Individual radiolarians are normally in the size range of hundredths to tenths of millimeters, but some reach dimensions of a millimeter or more, large enough to be seen with the naked eye. Some species are amassed into colonies, which may reach sizes of centimeter and even meter scale."
Copepods
It is estimated that seven out of every 10 zooplankton species are copepods. They tread water by beating their limbs as often as 600 times per minute. This requires a large amount of energy, the equivalent of their own weight in food each day. Copepods are related to crustaceans such as lobsters, shrimp and barnacles. Some large ocean inhabitants rely on them for food, including the basking shark and the whale shark.
Zooplankton Behavior
Zooplankton migrate vertically within the water column each day. During daylight hours, many zooplankton species confine themselves to a narrow vertical range. As the light fades, they drift upward until nightfall, when the entire zooplankton community is on the ocean surface. Here they feed on phytoplankton. Larger marine species take advantage of this opportunity to feed on the masses of zooplankton. As daylight nears, they drift down to their habitual levels in the water column.