Plankton are tiny, typically microscopic, usually free-floating organisms that live in the water column of large bodies of freshwater and saltwater. Unlike Krill, plankton consists of a wide range of different organisms, including tiny bacteria, archaea, algae, protozoans, and tiny free-floating animals. Although most plankton are tiny, the term also covers relatively large organisms such as jellyfish. Some animals (such as various crustaceans) are also classed as plankton in their larval stage, although they eventually grow out of it as they mature.
Phytoplankton specifically are the most basic type of plankton, and rely on photosynthesis like plants. They are usually found near the surface, and are a very important source of oxygen.
Phytoplankton, zooplankton, bacterioplankton and mycoplankton form the base of the aquatic food chain. They are an especially important food source for tiny fish larvae, as well as countless other animals.
In Jurassicraft, Plankton are used to feed the Coelacanth (and eventually also other (typically small) aquatic creatures). In order to obtain Plankton, right-click on water with a Fine Net until a Plankton item goes to your inventory. Krill can also be obtained in the same way.
Once you have a Plankton item, right-click with it on the water surface (like a lily pad) to place a Plankton Swarm. The swarm will automatically dispense new Plankton items every 5 minutes or so. It is recommended that you place several of these in a pool or tank containing Coelacanths, to keep them reliably fed.
Left-click on the swarm to break it and obtain the Plankton item back into your inventory.
In future updates, Plankton will also be involved in the creation of ancient corals.