Influence of wave exposure and habitat complexity in determining spatial variation of the sea urchin Diadema aff. antillarum (Echinoidea: Diadematidae) populations and macroalgal cover (Canary Islands - Eastern Atlantic Ocean)

Diadema aff. antillarum performs a key role in organizing and structuring rocky macroalgae assemblages in the Canary Islands; increased sea urchin population density can result in the formation of rocky grounds covered with crustose algae. In the Canary Islands this type of system alternates with non-crustose macroalgal systems. However, understanding of the process controlling formation, persistence and alternation between states is still poor. Moreover, knowledge of spatial and temporal variations of this key herbivore populations are scarce. Here an extensive study of D. aff. antillarum populations throughout the Canarian Archipelago were presented, with particular emphasis on two main themes: firstly the influence of sea urchin population on non-crustose macroalgal assemblages. Secondly, the spatial variation of adult sea urchin populations at different scales: both on a large scale (between islands and sites with different wave exposure) and on a medium-small scale (between depths and habitats with different characteristics), and including comments regarding human influence on the observed spatial pattern between the studied islands. The main results were: (1) densities of D. aff. antillarum higher than 4 ind.m-2 drastically reduce non-crustose macroalgal cover to under 30%; (2) Tenerife Island comprises the highest urchin densities while El Hierro has been found to have the lowest; (3) wave exposure is a factor determining sea urchin density: more highly exposed sites present lower urchin density and vice versa; (4) rocky grounds covered with crustose algae can be found at greater depth in more highly exposed sites and at shallower depths in lesser exposed sites; (5) when sand percentage over the rocky substrate exceeds 20%, urchin density is limited to under 1 ind.m-2 and (6) high relief rocky substrate present higher urchin densities and hence a lower non-crustose macroalgal cover. Rev. Biol. Trop. 56 (Suppl. 3): 229-254. Epub 2009 January 05.