Resolving widespread and endemic dinoflagellates (Symbiodiniaceae) mutualistic with Indo‐Pacific octocorals reveals differences in specificity based on host phylogeny

Endosymbionts in the dinoflagellate family Symbiodiniaceae can form mutualisms with a diverse array of host invertebrates, constituting a widespread and ecologically important family. While those associated with reef-building corals (order Scleractinia) have received considerable research attention, the diversity and ecology of zooxanthellae from soft coral hosts (Octocorallia) have remained understudied and unappreciated. To address this lack of understanding, octocoral zooxanthellae were sampled across the Indo-Pacific and genetic, morphological, ecological, and geographic evidence were utilized to formally characterize five new species in the genus Cladocopium. Four species were associated with hosts in the family Sarcophytidae that horizontally acquire their endosymbionts. Of these new species, C. fabriciae sp. nov. and C. peratum sp. nov. are widespread across the Indo-Pacific whereas C. zanzibariense sp. nov. and C. belauense sp. nov. are known only from their type localities in Zanzibar and Palau, respectively. The fifth species, C. bilineaum sp. nov., occurs in the Pacific and Indian Oceans associated with hosts in the families Xeniidae, which can display either horizontal or vertical mode of transmission and Lemnaliidae, which horizontally transmit their endosymbionts. Because soft coral abundances are increasing with ocean warming across many geographic provinces and in various reef habitats, formal species descriptions of their endosymbionts should facilitate future physiological and ecological research toward a more comprehensive understanding of their natural history and contributions to coral reef ecosystem productivity.