Initiation of symbiosis between the soft coral Heteroxenia fuscescens and its zooxanthellae

Symbiotic dinoflagellates of the genus Symbiodinium, commonly referred to as zooxanthellae, inhabit a large portion of the world's corals and are essential for the existence and well being of tropical reef ecosystems. The initiation of this symbiosis was studied using the Red-Sea soft coral Heteroxenia fuscescens. Results show that the motile algal cells are attracted to chemical substances emanating from the mouths of juvenile (zooxanthellae-free) polyps but not of adult (zooxanthellae-bearing) ones. The oral acquisition of symbionts is immediate; within 15 min of encountering the polyp, 15.6 ± 5.2 cells gain access into it. Once initial acquisition is concluded, the net number of algae within the polyp does not change significantly over the next 3 h, while algae continue to penetrate through and be expelled from the polyp mouth.