Yes, some crustaceans do eat phytoplankton. Phytoplankton are microscopic plants that float in the water column and serve as a primary food source for many marine organisms. Crustaceans such as copepods, krill, and some types of shrimp feed on phytoplankton as part of their diet. They either filter the water to capture the phytoplankton or actively graze on surfaces where the phytoplankton are found.
