We investigated the use of demographic characteristics to differentiate between closely related coral species that are difficult to distinguish morphologically and genetically. We looked at colony size and density in six morphospecies of the coral genus Madracis in Curaçao (Netherlands Antilles). Colonies of all species (n = 1585), including individuals as small as 0.01 cm2, were measured in 30 × 1 m belt transects over a reef slope at 5, 10, 20, 30, 50 and 60 m depth. This resulted in a detailed description of species population structures over depth. Temperature, light, sedimentation and water movement were recorded. Our survey showed Madracis to be one of the most abundant coral genera (mean of 9.2 colonies m-2) at the reef slope. The size frequency distributions were species specific and provide us with a set of characteristics, in addition to traditional morphological and genetic species classifications, to distinguish between coral taxa. Species density (n m-2) and mortality rates differed between morphospecies and within morphospecies among depths, suggesting that population density is under environmental control (selective environment). Population structure, in terms of size frequency distribution, remained stable over depth and appears to be largely dependent on differences in species-specific life-history strategies (developmental environment). The population structure of all Madracis species changed only at the upper and lower margin of their distribution. These changes were similar in all species: (1) Densities of populations decreased, resulting in 'saw-shaped' size distributions; (2) Overall variation in un-transformed colony size distributions, expressed as coefficient of variation (CV), decreased while the variation in log-transformed size-distributions, which is indicative of the time a colony is present on the reef, increased; (3) Decreasing skewness (g1) in untransformed size-distributions indicated that the proportion of smaller colonies decreased within the population; (4) The relative differences in the contribution of larger individuals to the total population surface increased, indicated by higher values for the Gini-coefficient (G); and (5) Mean colony sizes, after range standardization of size-data, became larger than 0.12. Life-history strategies partially overlap for morphospecies sharing the same colony morphology. Population turnover rates differed between species present at the same depth, ranging between >100 yrs for Madracis senaria to five yrs for Madracis decactis, which reflects species-specific susceptibility to environmentally induced mortality.