Answer: zooxanthellae

AnswerCrossword Clue
zooxanthellaeZOOXANTHELLA, a symbiotic dinoflagellate of the genus Symbiodinium that lives in the tissues of a number of marine invertebrates and protists
zooxanthellaeA yellowish-brown symbiotic dinoflagellate present in large numbers in the cytoplasm of many marine invertebrates