observed that stony corals that were being overgrown by P. carib-aeorum, often exhibited areas of dead tissue several cms from the edge of the zoanthid colony. This could have represented a "zone of death" caused by the release of palytoxin, allowing overgrowth to occur unchallenged. When combined with the rapid growth of zoanthids, these chemicals give them a distinct advantage over other organisms when competing for space on the reef. Given these facts it is somewhat surprising that zonathids do not completely cover all available substrates on a reef. Although predation by organisms such as some polychaetes, certain fish, crabs and gastropods may be a factor, this alone is not enough to explain their lack of dominance. Some stony corals such as Agaricia and Siderastrea in the Caribbean offer little resistance to Palythoa caribaeorum overgrowth, but others such as Acropora and Montastrea appear to be more resistant (Suchanek and Green, 1981). At this time, the mechanisms that control zoanthid population growth are not yet fully understood.
The anti-oxidant mycosporine-glycine has also been found in the tissues of Palythoa tuberculosa (Dunlap and Yamamoto, 1995). This is an anti-oxidant that may allow these corals to break down excess oxygen produced under high levels of illumination and can also act as a UV-B light absorber. Palythinol and palythine were discovered by Dunlap et al. (1986) in P. tuberculosa and have also been found to be effective absorbers of UV-B light, especially in the 310-320 nm range, wavelengths damaging to living tissue.
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