Zooxanthellae are dinoflagellates that have taken up residence inside a coral polyp's cells. They have a mutualistic relationship with the coral, using photosynthesis to convert the energy from sunlight into nutrients the coral can use. In exchange, the coral provides protection for the zooxanthellae (Padilla-Gamiño et al, 2012).
Fig 1 Anatomy of a coral polyp showing the location of zooxanthellae (Image source Encyclopedia Britannica)
But how did coral acquire these dinoflagellates in the first place? There are several different mechanisms behind this and depend on whether the coral reproduces asexually or sexually. In the case of an asexually reproducing coral, zooxanthellae transmission takes place through coral budding or fragmentation which form a new coral. The zooxanthellae residing in the donor tissue of clonal coral automatically relocate, thereby colonizing the new coral (Muller-Parker et al, 2015).
In sexually reproducing coral, zooxanthellae are either acquired through direct/vertical or indirect/horizontal transfer (Muller-Parker et al, 2015, Padilla-Gamiño et al, 2012). In direct or vertical transfer, the mother coral polyp releases the eggs with zooxanthellae inside, either being fertilized outside the mother coral or developing as larvae within it. But most coral eggs do not have zooxanthellae in them; the eggs have to obtain the zooxanthellae through phagocytosis from the coral polyp's gastrovascular cavity or be infiltrated by the zooxanthellae-containing cytoplasmic extensions of the coral polyp's gastrodermal cells (Muller-Parker et al, 2015). For the coral larvae that was borne from eggs without zooxanthellae, they can uptake their parent's zooxanthellae before their release into the surrounding seawater. But if they do not have this opportunity, they have to absorb them from the environment. This is called indirect or horizontal transfer. The concentration of free-swimming (motile) zooxanthellae over a reef is normally low but sometimes they show preference to newly settled coral. Chemotaxis is the mode of locomotion of such a zooxanthellae; much like diffusion of molecules from a region of large concentration to a region of lower concentration, motile zooxanthellae can show positive chemotaxis in the direction of corals with zero or lower concentrations of zooxanthellae (Muller-Parker et al, 2015). Additionally, corals can obtain zooxanthellae indirectly through the ingestion of fecal matter excreted by corallivores (animals that eat coral) and of animals who have eaten prey with zooxanthellae in their cells (prey such as jellyfish and sea anemones).
Fig 2 The basic relationship between zooxanthellae and coral (upper left) and indirect/horizontal transfer of zooxanthellae (Symbiodinium sp.) from the seawater to the coral (Image source Univ. of Wisconsin)
Over the course of their lives, corals are able to obtain multiple different species of zooxanthellae. During a bleaching event the zooxanthellae may be expelled from the coral, and if the coral survives, its tissues can be re-populated by a different species of zooxanthellae (Muller-Parker et al, 2015).
Padilla-Gamiño, J.L., Pochon, X., Bird, C., Concepcion, G.T., and R.D. Gates, 2012. From Parent to Gamete: Vertical Transmission of Symbiodinium (Dinophyceae) ITS2 Sequence Assemblages in the Reef Building Coral Montipora capitata. PLoS ONE 7 doi.org/10.1371/journal.pone.0038440.
Muller-Parker, G., D’elia, C.F. and Cook, C.B., 2015. Interactions between corals and their symbiotic algae. In Coral Reefs in the Anthropocene (pp. 99-116). Springer Netherlands.