Any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (carbon dioxide, ammonium) will lead to expulsion.

Reports of 'clusters' of cases had raised questions about environmental triggers, such as electromagnetic fields around power lines.

They are 'expressed' in the skin and can induce inflammation in response to infection or environmental triggers.

But they are thought to be switched on by as yet unidentified environmental triggers.

This uncertainty about environmental triggers has itself become a trigger for wild speculation.