Sea-urchins are important, though often neglected grazers of coral-reef ecosystems. One of the most conspicuous of them is the long-spined sea urchin Diadema setosum, which may occur in aggregations of up to several tens of individuals. Long-spined sea urchins are considered as model species for studying molecular evolution in marine invertebrates.
For the first time, we analyzed DNA samples of D. setosum collected throughout the Indo-Malay archipelago. Using the mitochondrial (mt) DNA as genetic marker, we found that D. setosum consisted of geographically separate, genetically distinct groups, although most mtDNA haplotypes were widely shared across groups. Although the geographic partition thus uncovered partly coincides with the localization of past geographic barriers, secondary contact mediated by a highly dynamic oceanographic environment should have erased it. Therefore, we must admit that there is some form of reproductive isolation between groups, which may have been acquired as recently as the last glaciations, when the sea level was lower and D. setosum populations were much smaller and geographically restricted to a discrete number of refugia.