Plankton includes plants and animals that float along at the mercy of the sea's tides and currents. Their name comes from the Greek meaning "drifter" or "wanderer." There are two types of plankton: tiny plants--called phytoplankton, and weak-swimming animals--called zooplankton. Some are babies that will grow into strong-swimming, non-planktonic adults. Others will remain plankton for their entire lives. All jellyfish, and the Ocean sunfish are such feeble swimmers that they too are included as plankton. Most of the plankton in the ocean are plants.
Phytoplankton produce their own food by lassoing the energy of the sun in a process called photosynthesis. So for sunlight to reach them, they need to be near the top layer of the ocean. So must zooplankton, which feed on the phytoplankton. Plankton have evolved many different ways to keep afloat. Spikes, like those on a radiolarian, help to distribute its weight over a large surface area and slowing its sinking. Many organisms, such as copepods and diatoms, produce oil to keep them afloat. The Portuguese man-o-war uses an air-filled sac to stay afloat.