Phytoplankton are single cell microalgae that exist primarily in the water column. Microalgae is the beginning of the marine food chain and the primary source of essential nutrients for most reef filter feeders. Filter feeders that rely directly on phytoplankton include bivalves like clams, scallops, mussels, and oysters, as well as many corals, feather dusters, sponges, ascidians and zooplankton like rotifers and copepods.
It was once thought that most reef animals do not require microalgae, however, we have learned in recent years that nearly all reef animals that can filter feed do rely on plankton for an essential portion of their diet. This is even true for animals that can use zooxanthellae and photosynthesis for nourishment. With hindsight this is obvious as zooxanthellae only contribute carbohydrates to the host animal, requiring the animal to look elsewhere for lipids, protein, vitamins and other nutrients.
In nature filter feeders rely on a mix of diatoms, golden-brown, green, yellow-green, blue-green and red microalgae for a nutritionally complete diet. These life forms may look similar, but they are so ancient that they have incredible biological and nutritional diversity. Biologically speaking, there is vastly more difference between a green algae and a golden-brown flagellate than between dandelion and a fir tree.