Massive gene ̄ow across the world's most potent marine biogeographic barrier

The `Eastern Paci¢c Barrier' (EPB), 5400 km of uninterrupted deep water between the central and eastern Paci¢c, constitutes the greatest marine obstacle to the dispersal of shallow-water organisms. However, some species are found on both sides of the EPB.These `transpaci¢c' species are considered by `dispersal' biogeographers as evidence of invasions through the barrier. `Vicariance' biogeographers, on the other hand, think that transpaci¢c species are morphologically conservative remnants of previously continuous distributions. We compared nucleotide sequences in a 642 bp region of mitochondrial DNA, and electrophoretically detected alleles in 17 enzymatic loci of central and eastern Paci¢c populations of Echinothrix diadema, an Indo-Paci¢c sea urchin recently reported from the eastern Paci¢c. Both types of molecules produced clear evidence of massive, recent gene £ow across the EPB. Thus, rather than being isolated relicts of Tethyan distributions, conspeci¢c populations from the eastern and central Paci¢c are genetically connected. Though the EPB is biogeographically important as a cause of speciation in many groups, it allows genetic connections in others, possibly through larval transport during El Nin¬ o events.