Symbionts (2 of 3) (image) Penn State Caption Corals are able to build large reef structures in many of the worlds shallow tropical environments because they contain symbiotic micro-algae called Symbiodinium (and often referred to as zooxanthellae). Distinct symbiont species which are found in different corals look nearly identical. Credit Todd C. LeJeunesse Usage Restrictions None 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.