The role of colony morphology and substratum inclination in the success ofMillepora alcicornis on shallow coral reefs

Millepora species are conspicuous members of shallow coral reefs where they occupy a variety of substrata and produce morphologically complex skeleton s. This study focuses on the roles of growth on vertical and horizontal sur faces and the production of encrusting bases and branches (a "sheet-tree" m orphology) for the success of the Millepora alcicornis on coral reefs. The effects of inclination were investigated by comparing the size and growth r ates of M. alcicornis on vertical and horizontal surfaces at 3-5 m depth, i n St. John, US Virgin Islands. The consequences of morphological complexity were investigated by comparing polyp density, chlorophyll content and biom ass between encrusting bases and branches; the role of branches in asexual reproduction was also quantified. Colonies on vertical surfaces had larger encrusting bases, longer perimeters and lower densities of branches compare d to those on horizontal surfaces. Growth rates also varied significantly b etween surfaces, largely because colonies on horizontal surfaces shrank in area while those on vertical surfaces increased in area, albeit slowly. Bra nches were not specialized in comparison to encrusting bases in terms of th e density of dactylozooids and gastrozooids, chlorophyll content and biomas s, but they were effective asexual propagules. During one storm, 79% of the branches were removed from colonies of M. alcicornis, and 4% attached to t he substratum to produce new colonies at a density of approximate to 0.5 co lonies.m(-2). Anecdotal observations suggest that such storms rarely damage d encrusting bases on vertical surfaces, but often destroyed those on horiz ontal surfaces. Thus, the encrusting bases on vertical surfaces are likely to be large because of greater age rather than faster growth, while those o n horizontal surfaces are likely to be small because they are relatively yo ung and short lived. These findings suggest that the success of M. alcicorn is is a result, in part, of the beneficial consequences of their "sheet-tre e" morphology, that supports: (a) slow growth and resistance to wave damage of encrusting bases on vertical surfaces, and (b) the use of branches as a sexual propagules.