Onset of symbiosis in planula larvae of scleractinian corals

The flexibility of symbiosis establishment between the corals Platygyra acuta and Acropora valida with the dinoflagellate Symbiodinium was examined. Planula larvae of these species were provided with homologous and heterologous sources of Symbiodinium C1 and Symbiodinium D8-12. These larvae were able to acquire symbionts prior to settlement and form symbiosis with both symbiont types from all sources, revealing flexibility of symbiosis establishment at this early stage of development. No significant difference in infection percentages was found among different sources of Symbiodinium C1, suggesting that host source of the symbiont types was unimportant. Symbiodinium D8-12 resulted in significantly higher infection rates in both species. While the advantage of hosting this heterologous symbiont type in coral larvae is unclear, this may simply be a result of the infectious nature of the clade D symbiont, rather than a selection choice of the host larvae. Settlement behavior of planula larvae of P. acuta did not depend upon their status of being symbiotic or aposymbiotic. Longer term post-settlement monitoring of the growth and survival of coral larvae may provide more insight on the benefits provided by earlier uptake of symbionts and also the hosting of clade D symbiont during pre-settlement stage of the larvae.