Comparison of Symbiodinium dinoflagellate flora in sea slug populations of the Pteraeolidia ianthina complex

Symbiotic dinoflagellates of the genus Symbiodinium (zooxanthellae) harbored by a nudibranch mollusk, Pteraeolidia ianthina complex, were investigated from host individuals collected at 10 locations in the tropic to temperate waters of the northwestern Pacific. The P. ianthina complex is widely distributed throughout the Indo-Pacific Ocean and, in the northwestern Pacific, contains 2 genetically highly diverged cryptic species with variable genetic structuring. Information on such genetic composition and structuring of the host is useful for assessing whether the relationship between the host and symbiont flora is determined by the genotype of host organisms or by environmental factors. The diversity of the harbored Symbiodinium was analyzed using the sequence differences of internal transcribed spacer region 2 in the nuclear ribosomal gene, chloroplast ribosomal gene large subunit 23S Domain V, and mitochondrial cytochrome b gene. Symbionts of clades A, C, and D were detected in this nudibranch host. Among them, 6 types were identified (1 clade A, 3 clade C, and 2 clade D types); 2 of them are suggested to be novel. The occurrence of each Symbiodinium type differed among the geographical locations, and hence the co-occurrence pattern (i.e. composition) of the symbionts also varied geographically. The geographical pattern of symbiont composition was correlated with the genetic structuring of the host as well as climatic environmental factors. Therefore, these results suggest that both environmental conditions and the host mollusk's genetics affect the symbiont composition.