Unlike their land relatives, many marine organisms do not belong to either the plant or animal kingdom. They’re both at the same time! These hybrid cells are the result of symbiosis between algae and animals both being single-celled protists. Like all plants, algae produce organic matter using sunlight.
Through photosynthesis, they supply food for their animal host. The host, in turn, nurses its algae by providing optimal conditions: nutrients, exposure to sunlight as well as protection from predators.
Symbiosis is widespread among unicellular organisms, but is important for multi-cellular creatures too: Certain jellyfish or velellas form alliances with algae. Harmonious cooperation helps them survive in the plankton jungle!