Phylogeography of the trumpetfishes (Aulostomus): Ring species complex on a global scale

The distribution of circumtropical marine species is limited by continental boundaries, cold temperate conditions, and oceanic expanses, but some of t hese barriers are permeable over evolutionary time scales. Sister taxa that evolved in separate ocean basins can come back into contact, and the conse quences of this renewed sympatry may be a key to understanding evolutionary processes in marine organisms. The circumtropical trumpetfishes (Au lostom us) include a West Atlantic species (Pi. maculatus), an Indian-Pacific spec ies (A. chinensis), and an East Atlantic species (A. strigosus) that may be the product of a recent invasion from the Indian Ocean. To resolve pattern s of divergence and speciation, we surveyed 480 bp of mitochondrial DNA cyt ochrome b in 196 individuals from 16 locations. Based on a conventional mol ecular clock of 2% sequence divergence per million years, the deepest parti tions in a neighbor-joining tree (d = 0.063-0.082) are consistent with sepa ration of West Atlantic and Indian-Pacific species by the Isthmus of Panama , 3-4 million years ago. By the same criteria, trumpetfish in the East Atla ntic were isolated from the Indian Ocean about 2.5 million years ago (d = 0 .044-0.054), coincident with the advent of glacial cycles and cold-water up welling around South Africa. Continental barriers between tropical oceans h ave only rarely been surmounted by trumpetfishes, but oceanic barriers do n ot appear to be substantial, as indicated by weak population partitioning ( phi (ST) = 0.093) in A. chinensis across the Indian and Pacific Oceans. Fin ally, morphological and mitochondrial DNA data indicate hybridization of A. strigosus and A. maculatus in Brazil. After 3-4 million years and a globe- spanning series of vicariant and dispersal events, trumpetfish lineages hav e come back into contact in the southwest Atlantic and appear to be merging . This ring species phenomenon may occur in a broad array of marine organis ms, with clear implications for the production and maintenance of biodivers ity in marine ecosystems.