Résumé
Major genetic breaks between the Indian and Pacific oceans have been reported for marine fishes and invertebrates. The genetic structure and history of Decapterus russelli in the Indo-Malay archipelago were investigated using the cytochrome b gene sequence as mitochondrial marker and two length-polymorphic introns as nuclear markers. The existence of two major mitochondrial lineages separated by 2.2% average nucleotide divergence, and their heterogeneous geographical distributions, were confimed. This indicated past geographic isolation, possibly caused by Pleistocene drop in sea level. The presence in sympatry of the two mitochondrial lineages was thought to result from secondary contact. Recent population bottleneck and subsequent rapid population expansion were indicated by low genetic diversities and strongly negative Tajima's D-values. This supports the hypothesis that the habitat available to D. russelli in the Pleistocene was restricted. Taking into account both mitochondrial and nuclear-DNA data, three geographically distinct populations were identified: one sampled in the Makassar Strait and in the Sulawesi Sea, one in the Arafura Sea, and the third from the entire western region of the Indo-Malay archipelago. Considering the high hydrological connectivity of this region of the Indo-Pacific, homing behaviour and, perhaps, the association of spawning with retention zones, are believed to be able to maintain such a structure in a species with pelagic life-history.