While the exact mechanism for coral bleaching is still unknown, there are two principle hypotheses for why zooxanthellae and cnidarian cells dissociate from one another. Bleaching may be an adaptive mechanism of corals to attempt to locate new species of zooxanthellae that are more tolerant to extreme conditions, as some symbiotic algae are more adaptive to fluctuations in temperature (Buddemeiser & Fautin 1993). According to this theory, the zooxanthellae are released into the gut of the coral polyp and expelled through the mouth (see figure 7). This process has been monitored in the lab but has never been observed in nature (Brown & Odgen 1993).

Agaricia tenuifolia

An alternative hypothesis is that stressed corals give the zooxanthellae fewer nutrients for photosynthesis, so the algae essentially leave the polyp and exist independently. The algae may do this by producing reduced oxygen intermediates, essentially toxic oxygen, under stress, which cause cellular damage to the polyps and forces the coral to release the zooxanthellae in these sub-par conditions. Even though corals need zooxanthellae for survival, the coral will release the zooxanthellae rather than suffer the effects of the toxins. This has been shown in the Caribbean coral, Agaricia tenuifolia, under temperature-induced stress, where the coral exhibited a decrease in photosynthetic ability and subsequent bleaching. Furthermore, if stressed corals are exposed to antioxidants, which negate the effects of reactive oxygen, they show improved photosynthetic ability on par with corals living at ideal temperatures (Lesser 1997).