Since plants make their own food and release oxygen as a byproduct, all the other living things in the ocean depend on them directly or indirectly for food or oxygen. Diatoms are the most common type of phytoplankton. The actual diatom fits inside this cell wall, with one half of the wall fitting over the second half, like a lid. 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.
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. That places them at the very base of the food web. Many animals eat plankton directly, or feed on animals that eat plankton.