Bleaching and its Extent Bleaching is often referred to as the whitening of corals. However, bleaching has now been observed in just about all marine organisms that host zooxanthellae. The invertebrates host these photosynthetic algae within their tissues. Bleaching usually occurs when environmental stress causes the host species to suffer a loss of zooxanthellae. However, it can also occur when the host retains the zooxanthella but the alga expels its brownish-green pigmentation.
Psychedelic colours can also result during partial bleaching.
Although hosts commonly turn white when fully bleached, there are some host species that turn pink, yellow, purple, blue or iridescent green when partially bleached. Of the invertebrate hosts, it is the corals and giant clams that appear to rely most heavily on zooxanthellae for the production of energy for metabolic processes. More than 90 per cent of their energy requirements, for some coral and giant clam species, are provided through the process of photosynthesis. We don't know what initiates bleaching. Most people think it is the animal host, but at this stage the data are insufficient,' Dr Fabricius said.