The key role of the sea urchin Diadema aff. antillarum in controlling macroalgae assemblages throughout the Canary Islands (eastern subtropical Atlantic): an spatio-temporal approach. Diadema aff. antillarum performs a key role in organizing and structuring rocky macroalgae assemblages in the Canary Islands. Densities of D. aff. antillarum higher than 2 individuals m(-2) are found to drastically reduce non-crustose macroalgal cover to below 30% and wave exposure appears as a major factor determining sea urchin density, which decreases with exposure level. Substrates containing >20% sand limit urchin to under 1 individual m(-2) but high relief rocky habitats show higher density. Moreover, several anthropogenic factors (number of islanders and tourists per coastal perimeter, and number of operational fishing boats) were positively correlated with urchin abundance. A trend of increasing urchin density through time was found, although well structured marine systems found at Mar de Las Calmas Marine Protected Area and at the no-take area of La Graciosa Marine Protected Area do not seem to follow this general trend.