Mechanisms of bleaching deduced from histological studies of reef corals sampled during a natural bleaching event

Histological examination of bleached tissues from the reef corals Goniastrea aspera, G. retiformis, Favites abdita, Coeloseris mayeri, Goniopora pandoraensis, and Galaxea fascicularis subjected to increased seawater temperatures and possibly increased irradiance in the field, revealed a reduction in zooxanthellae density between 50 and 90% (depending on species) compared with the zooxanthellae complement of normally-coloured corals. The study revealed a number of cellular mechanisms which would result in reduced zooxanthellae densities in bleached corals. These included degeneration of zooxanthellae in situ, release of zooxanthellae from mesenterial filaments and, for two species in an advanced state of bleaching, release of algae within host cells which became detached from the endoderm. The existence of a number of cellular mechanisms of zooxanthellae loss in naturally-bleached tissues contrasts with previous descriptions of a single predominant mechanism of zooxanthellae release in laboratory simulations, and highlights the importance of complementing laboratory studies with relevant field observations.