First description of algal mutualistic endosymbiosis in a black coral (Anthozoa: Antipatharia)

The class Anthozoa is the largest metazoan group forming mutualistic symbioses with microalgae. These algal symbionts (most commonly dino - flagellates of the genus Symbiodinium) are distributed across most anthozoan orders. Records of algal cells in antipatharian (black coral) tissues have been reported, but no detailed descriptions of a mutualistic endosymbiosis exist. Here we report on zooxanthellate specimens of an unidentified black coral species within the genus Cirrhipathes that were collected from reef slopes at depths of 15 to 38 m in the Indonesian Archipelago. Symbionts were abundant (~107 symbionts cm–2) and ultrastructural analysis revealed the presence of a distinct symbiosome surrounding the algae, as well as algal reproduction inside the gastrodermal layer. Molecular analysis revealed the algae to be closely related to the symbionts (Symbiodinium clade G) of clionid sponges. There was also evidence for additional symbionts in clade C at low abundance. Taken together, these findings (high abundance, taxonomic identity, presence of symbiosome, in situ reproduction, and depth distribution) strongly suggest that these algae are functioning as mutualists. This study confirms and describes the symbiosis between Symbiodinium and a black coral species of the genus Cirrhipathes, supports the pervasiveness of mutualisms among anthozoan taxa, and highlights the diversity and flexibility of these symbiotic associations in a poorly studied group.