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 Goni astrea aspera, G. retiformis, Favites abdita, Coeloseris mayeri, Gonio pora pandoraensis, and Galaxea fascicularis subjected to increased sea water temperatures and possibly increased irradiance in the held, reve aled a reduction in zooxanthellae density between 50 and 90% (dependin g on species) compared with the zooxanthellae complement of normally-c oloured corals. The study revealed a number of cellular mechanisms whi ch would result in reduced zooxanthellae densities in bleached corals. These included degeneration of zooxanthellae in situ, release of zoox anthellae from mesenterial filaments and, for two species in an advanc ed state of bleaching, release of algae within host cells which became detached from the endoderm. The existence of a number of cellular mec hanisms of zooxanthellae loss in naturally-bleached tissues contrasts with previous descriptions of a single predominant mechanism of zooxan thellae release in laboratory simulations, and highlights the importan ce of complementing laboratory studies with relevant field observation s.