Plankton, which comes from the Greek word for "drifters", makes up the bulk of the animals and plants in the ocean. These tiny creatures, only seen with a magnifying glass or microscope, drift with the prevailing currents. Plankton occur everywhere and form the basis of the marine food chain. They are fed upon by small fish and invertebrates (including filter-feeding animals such as clams), who in turn are fed upon by larger predatory animals such as adult fish who then serve as food for sea birds, marine mammals, and humans.

There are two types of plankton, tiny animals called Zooplankton and tiny plants called Phytoplankton. The microscope photograph shows both types of plankton: the sausage shaped creature with large antennae is an animal called a Copepod and the smaller brown circles with lines are plants called Dinoflagellates.

Copepods are a group of small (1/16 in. long) crustaceans that can be found in both saltwater and freshwater.  They are major components of the plankton community and are significant prey items.  They use their antennae to swim through the water while feeding on phytoplankton.  The black spot between the antennae is a light-sensing organ allowing them to rise at night to feed and sink during the day to avoid predators.  The armored body contains the stomach and other organs and ends in a spiked tail.

Many Dinoflagellates contain photosynthetic pigments and rely on sunshine for nourishment.  However they are animal-like in that they have small flagella that they can use to swim.  Some species produce a neurotoxin that effects other marine life as well as humans who eat contaminated shellfish .  Blooms of some species can turn the water a reddish color called a "red tide", but such blooms do not necessarily produce a toxin.  To be safe consult your local health department for beach closures (whether a "red tide" is present or not) prior to collecting shellfish for consumption.