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.
Diatoms are the most common type of phytoplankton. They are single-celled yellow algae whose cell walls contain a lot of silica, glass-like substance. The actual diatom fits inside this cell wall, with one half of the wall fitting over the second half, like a lid. The name diatom actually means "cut in two" in Greek. There are many different kinds of diatoms, and they come in a variety of shapes--disk shaped, needle shaped, or linked together in chains.
Dinoflagellates are like both plants and animals: they can move themselves through the water using two flagella in grooves along their body; yet they can also produce their own food like plants. Two species of dinoflagellates, Gonyaulax and Gymnodinium, are the cause of the dangerous red tide. When their populations get large, the reddish-colored dinoflagellates not only make the surrounding water appear to be tinted red, they can produce a variety of toxic effects, including fish mortality and paralytic shellfish poisoning.
Radiolarians, tiny one-celled animals related to ameobas, live in glass-like shells and sometimes have long spines that radiate from holes in their shells. These projections are called "false feet," or pseudopodia, which they use to move. Radiolarians are part of the group sarcodina (meaning "creeping flesh"), whose members move by expanding and contracting projections of their jelly-like bodies.
Forams are also members of the sarcodina group. They start life with a one-chambered shell, and add compartments as they grow. Their pseudopodia project out through holes in the shell. They feed on diatoms and other protozoans by secreting digestive juices into their food to dissolve it.
Zooplankton can also be categorized according to size: nannoplankton are unicellular animals that feed on phytoplankton and are in turn eaten by other zooplankton (5/1000 mm to 60/1000 mm); microplankton (60/1000 mm to one mm) are composed primarily of eggs and larvae, usually of invertebrates; macroplankton (over one mm) often contain large numbers of copepods, along with amphipods, cumaceans and arrow worms; and megaplankton include mainly the large jellyfishes and their relatives the Portuguese man-o-war and the By-the-wind sailor, which move at the mercy of the currents.
"Swimming" zoea larvae of a crab. (~1.7 MG)
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Because phytoplankton make their own food using the energy of the sun (in a process called photosynthesis), they are called "producers." That places them at the very base of the food web. Many animals eat plankton directly, or feed on animals that eat plankton. Animals that eat plants or other animals are called "consumers," and bacteria that break down dead plants and animals are called "decomposers."