Symbiotic Algae of Acoel Species in the Seto Inland Sea and Symbiont Selectivity in the Hosts.

is an acoel flatworm that inhabits the sandy beaches in the intertidal zone of the Seto Inland Sea. This species carries sp., a green unicellular chlorophyte, as a symbiont in its body, and depends on algal photosynthetic products to survive. However, the eggs of contain no symbiotic algae, and juvenile acquire symbionts from the surrounding environment through horizontal transfer after hatching, thereby establishing new symbiotic relationships in each generation. Other acoel species, spp., also inhabit the Seto Inland Sea shores and acquire symbiotic green algae via horizontal transfers. To characterize their symbionts, these acoels were collected from a wide area of the Seto Inland Sea and partial nucleotide sequences of the chloroplast ribulose diphosphate carboxylase large subunit (rbcL) of the symbiotic algae were determined and used for molecular phylogenetic analysis. Symbionts of both and spp. belonged to the genus but were phylogenetically distant, and both species established symbiotic relationships with different symbionts even when they were sympatric. To test whether each species selects specific algae in the environment for symbiosis, we established algal strains from and sp. symbionts and conducted uptake experiments on aposymbiotic juveniles of . The results suggest that symbiotic algae from could be taken up by juveniles, but were unable to establish a normal symbiotic relationship with the juveniles.