Symbiosis process between Acropora larvae and Symbiodinium differs even among closely related Symbiodinium types

The symbiosis between corals and the dinoflagellate genus Symbiodinium is a fundamental pillar of coral reef ecosystems. Many of the coral species, including Acropora, acquire Symbiodinium from the environment during early ontogeny for symbiosis. Genetically diverse members of Symbiodinium have been identified from the environment, but naturally settled Acropora recruits (i.e. settlers or spats) usually harbor specific members of Symbiodinium. Thus, symbiosis between Acropora and Symbiodinium is not established haphazardly in the wild. Conversely, under laboratory conditions, Acropora larvae can acquire other Symbiodinium varieties. However, it remains unclear whether a stable symbiosis between these Symbiodinium that are never detected in natural Acropora recruits and corals is successfully established. Here, we artificially supplied A. tenuis larvae with 3 closely related Symbiodinium culture strains, namely, type A1 (common within natural Acropora recruits), type A2-relative (never detected within corals), and type A3 (often found within natural Acropora recruits). We then determined the percentage of Symbiodinium-infected larvae and the density of Symbiodinium cells infecting the larvae. For types A1 and A3, the percentage of Symbiodinium-infected larvae and the density of Symbiodinium cells did not decrease even after stopping the Symbiodinium supply. This result indicated that stable symbiosis was successfully established, even in the laboratory. However, the percentage of Symbiodinium-infected larvae and Symbiodinium cell density of type A2-relative declined after stopping the Symbiodinium supply. Thus, under laboratory conditions, A. tenuis larvae can acquire Symbiodinium that is never detected within natural corals; however, stable symbiosis between these Symbiodinium and corals was not successfully established.