Onset of algal endosymbiont specificity varies among closely related species of Acropora corals during early ontogeny.

Juveniles of a number of corals with horizontal transmission of dinoflagellate endosymbionts naturally acquire and maintain Symbiodinium types that differ from those found in adult populations. However, the duration of this early period of symbiont flexibility and successional changes leading to dominance by the characteristic adult (homologous) type are unknown. To document natural succession of Symbiodinium types within juvenile corals, we monitored Symbiodinium communities in juveniles of Acropora tenuis and Acropora millepora for 3.5 years. Juveniles originating from one of three reef populations, characterized by differing adult coral-Symbiodinium associations, were raised in a common environment. In four out of five cases, juveniles became dominated initially by a nonhomologous adult type. Changes in Symbiodinium communities associated with A. tenuis juveniles led to the establishment of the adult homologous association at approximately 3.5 years of age. These changes were not linked to the onset of reproductive maturity, but may be linked to micro-environmental changes associated with vertical growth of juvenile corals. We hypothesize that fine-tuning of specificity mechanisms takes place during ontogeny in A. tenuis, leading to the eventual establishment of the adult homologous association. However, Symbiodinium communities in A. millepora juveniles did not change significantly over the 3.5 years, potentially reflecting (i) lack of specificity, (ii) more than a 3.5-year delay in the onset of specificity, or (iii) lack of availability of the adult Symbiodinium type. This study demonstrates that juvenile corals may survive for extended periods of time with nonhomologous Symbiodinium types and that closely related species of Acropora differ in the timing of the onset of specificity for algal symbionts.