WHEN OCEANS MEET - A TELEOST SHOWS SECONDARY INTERGRADATION AT AN INDIAN-PACIFIC INTERFACE

The Indo-West Pacific is characterized by extraordinary marine species diversity. The evolutionary mechanisms responsible for generating thi s diversity remain puzzling, but are often linked to Pleistocene sea l evel fluctuations. The impact of these sea level changes on the popula tion genetic architecture of the estuarine fish Lates calcarifer are i nvestigated via a natural experiment in a region of the Indo-West Paci fic known to have undergone considerable change during the Pleistocene . L. calcarifer, a coastline-restricted catadromous teleost, provides an excellent model for studying the effects of sea level change as its habitat requirements potentially make it sensitive to the region's ph ysical history. Evidence was found for a large phylogenetic break (4% mtDNA control region; 0.47% ATPase 6 and 8) either side of the Torres Strait, which separates the Western Pacific and Indian Oceans, althoug h some mixing of the clades was evident. This suggests clinal secondar y introgression of the clades via contemporary gene flow. Further, pop ulations on Australia's east coast appear to have passed through a bot tleneck. This was linked to the historical drying of the Great Barrier Reef coastal lagoon, which resulted in a significant loss of habitat and forced retreat into isolated refugia. These results suggest that h istorical eustatic changes have left a significant imprint on the mole cular diversity within marine species as well as among them in the Ind o-West Pacific.