Essentially, bleaching has been quantified through measurementof zooxanthellaenumbers and the physiologicalstatusof the zooxanthellae.Theactualmechanismof zooxanthellae loss was first described for corals maintained in the laboratory and exposed to extreme temperature shocks (Steen and Muscatine 1987; Muscatine et al. 1991). Subsequently Gates et al. (1992) proposed, on the basis of laboratory experiments, that the primary bleachingmechanismintemperature-stressedtropicalanemones and corals involved host cell detachment whereby entire animal endodermal cells, their zooxanthellae, and accompanying vacuolar membranes were discharged into the coelenteron.Histological analysis of tissues from six coral species following a natural bleaching event in the field in Thailand in 1991