Timing and process of entry of zooxanthellae into oocytes of hermatypic corals

We studied the timing and mechanism of entry of zooxanthellae into oocytes in four species of scleractinian corals that spawn eggs containing these symbiotic algae. Gonads of Montipora venosa, M. digitata, Pocillopora verrucosa, and P. eydouxi were sampled for 7-10 days before spawning and examined with light and electron microscopes. In all four species, oocytes were enveloped by a thin mesogleal layer and an outer follicle layer. The number and distribution of zooxanthellae in spawned eggs differed between Montipora and Pocillopora. Zooxanthellae were first observed in the follicle cells 5-7 days before spawning. A few days before spawning, the cellular processes that had intervened between oocytes and mesoglea disappeared. Zooxanthellae that were expelled from the follicle cells passed through temporary gaps in the mesoglea and probably entered the oocytes by phagocytosis. Our results suggest that follicle cells mediate the transfer of zooxanthellae from the parent to oocytes.