Demographic history of Diadema antillarum, a keystone herbivore on Caribbean reefs

The sea urchin Diadema antillarum was the most important herbivore on Carib bean reefs until 1983, when mass mortality reduced its populations by more than 97%. Knowledge of its past demography is essential to reconstruct reef ecology as it was before human impact, which has been implicated as having caused high pre-mortality Diadema abundance. To determine the history of i ts population size, we sequenced the ATPase 6 and 8 region of mitochondrial DNA from populations in the Caribbean and in the eastern Atlantic (which w as not affected by the mass mortality), as well as from the eastern Pacific D. mexicanum. The Caribbean population harbours an order of magnitude more molecular diversity than those of the eastern Pacific or the eastern Atlan tic and, despite the recent mass mortality, its DNA sequences bear the gene tic signature of a previous population expansion. By estimating mutation ra tes front divergence between D. antillarum and D. mexicanum, that were sepa rated at a known time by the Isthmus of Panama, and by using estimates of e ffective population size derived from mismatch distributions and a maximum likelihood coalescence algorithm, we date the expansion as having occurred no more recently than 100 000 years before the present. Thus, Diadema was a bundant in the Caribbean long before humans could have affected ecological processes; the genetic data contain no evidence of a recent, anthropogenica lly caused, population increase.