The genus Millepora Linne 1758 is a common, although generally subordinate, component of Caribbean and western Atlantic hermatypic coral communities. It is one of the few extant heavily calcified hydrozoans and has been compared with the extinct stromatoporoids. It shows a wide range of morphologic forms which have been interpreted as either ecophenes or biospecies.

The distribution of morphologic variants of Millepora was studied on a fringing reef and submerged barrier reef off the western coast of Barbados, West Indies. At depths between 0 and 15 m 4 morphologic forms of Millepora can be distinguished: (1) encrusting (taking the form of the substrate), (2) boxwork (erect, short-curved coalescing plates with irregular nodose surfaces), (3) bladed (smooth discrete plates), (4) branching (erect, smooth, digitate, irregularly dendroid to fan shaped). These forms are moderately distinct. Some specimens are hard to categorize but they do not represent a continuum of variation. Forms 2, 3, and 4 appear to correspond with the species M. squarrosa, M. complanata, and M. alcicornis, respectively. At depths exceeding 10 m all the forms are present in the small area studied. With decreasing depth as the shore is approached the branching, bladed, boxwork, and encrusting forms disappear in that order.

This distribution appears to reflect the relative strengths of skeletons of the different forms: those which are more compact and stronger extend farther into shallow water. This suggests that local water turbulence is the main factor governing the distribution of forms. Distinct forms in the same environment support the view that major variation in the form of Millepora is controlled genetically rather than environmentally.

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