Here is another article that says that low nutrient systems are possible causes for necrosis.
"Comment at this point: Starvation in animals results in a loss of body tissue and low fat stores, and this is observed to occur with basically normal body chemistry parameters such as C:N ratio. (The diagnosis of food-starvation is based purely on low body weight/length, there is no chemical test for it.) Impaired reproduction and weakened immunity are also well known side effects of undernutrition. Compare this information with the following excerpt from Martin Pecheux's comprehensive literature review on coral bleaching:
"œTissue of bleached corals shows general atrophy and necrosis. There is 30-50% less tissue per surface, with a normal C:N ratio (Szmant and Gassman, 1990)...one "œhealthy"-looking Pocillopora was in early stage of necrosis (loss of architecture, basophilic tinge in mesoglea), from which it was concluded that the problem is on the animal side, with maybe thereafter nutrient-starvation of zooxanthellae (Glynn et. al., 1985). Gonads are reduced and reproduction is generally impaired...Bleached corals had half normal lipid levels (Glynn et. al., 1985). Phenoloxidase, a biomarker of immune capability, was found to have lower activity in bleached and semi-bleached M. annularis (Scith, 1992). Secondary parasites were observed in a few cases (fungus, bacteria). There is no transmission of bleaching following iso-allo-and xenografts...During bleaching there is no visible calcification." (Taken from Pecheux, 1992)
There is good reason to suspect that the low lipid reserves and lowered immunity features precede the bleaching event rather than result from it, as one might suspect. Widespread outbreaks of infectious diseases and parasitic problems among (not bleached) corals have been another cause of serious concern to reef scientists, also starting only in recent years. And, as mentioned earlier, slowed growth of (not bleached) corals has actually been occurring in some "œpristine" areas for decades. Therefore it is reasonable to suspect that some underlying factor has caused these changes and thereby weakened coral communities as a whole, and predisposed many corals now to "œbleaching" under temperature stress. Food-deprivation is certainly consistent with the pattern of presenting symptoms (slowed growth, low lipid reserves, lowered immunity and reproduction, ultimate death). "
