Intra-colony variation in the growth of Acropora formosa: extension rates and skeletal structure of white (zooxanthellae-free) and brown-tipped branches

The presence or absence of zooxanthellae near the tip of Acropora formosa branches is correlated with apical skeletal structure and extension rates. White (zooxanthellae-free) tips are lightly calcified, possess thin, widely spaced skeletal elements and bear only a few, poorly developed radial corallites. Brown tips are heavily calcified, possess smaller axial corallites and larger, more numerous radial corallites. White tips exhibit a range of normally distributed extension rates. Brown tips do not extend, but radial growth and internal calcification continue. These processes progressively alter the appearance and density of brown tipped branches. In addition, the axial corallite of brown tips becomes progressively smaller and is eventually indistinguishable from adjacent radial corallites. Although brown and white tips can change from one form to the other, with a corresponding change in extension rate, it is hypothesized that in brown tips with degenerated axial corallites, a new axial corallite must develop before extension can resume. Brown tips predominate in the interior of arborescent colonies, where space for continued extension is limited. They may therefore represent a means of coordination of growth within a colony. Field and experimental evidence suggest that brown tips may develop in response to micro-environmental conditions. White, zooxanthellae-free zones are also characteristic of other branched and plate-forming species, which exhibit rapid extension in a localized region of the colony.