Giant clam
Although larval clams are planktonic, they become sessile in adulthood. The creature's mantle tissues act as a home for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which the adult clams get most of their food. By day, the clam opens its shell and extends its mantle tissue so that the algae get the sunlight they need to photosynthesize.