Alga (IMAGE) Penn State Caption Corals contain dense populations of round, golden-brown micro-algae commonly referred to as zooxanthellae. A typical coral will have one-to-several-million symbiont cells in an area of tissue the size of a thumbnail. Credit Todd C. LaJeunesse Usage Restrictions non commercial use only License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.