Plankton are the floating or weakly swimming organisms that live in large bodies of water and serve as a food source for many aquatic animals. Plankton include both plants and animals, and can be found in fresh water, salt water, and brackish water environments. The term “plankton” comes from the Greek word meaning “drifter” or “wanderer”.
There are two main types of plankton: phytoplankton and zooplankton. Phytoplankton are microscopic plant-like organisms that float near the surface of the water and use sunlight to produce their own food through photosynthesis. Zooplankton are tiny animal-like creatures that either drift with the currents or swim slowly through the water. They feed on smaller planktonic organisms, as well as detritus (dead organic matter).
Some planktonic species are capable of producing bioluminescence, a bright light produced by a chemical reaction within the organism. This light is used to attract mates, startle predators, or warn other animals of danger. Bioluminescent plankton can often be seen at night in tropical waters where they form part of the so-called “milky sea” effect.
Most planktonic organisms are very small, but there are some exceptions. The giant squid (Architeuthis sp.), for example, spends its early life as a large zooplankter before eventually sinking to the ocean floor to live as an adult. Another exception is the whale shark (Rhincodon typus), which filter feeds on planktonic organisms throughout its lifetime. At up to 18m in length, it is by far the largest known extant fish species.