Aquatic Biotas of Sundaland Are Fragmented But Not Refugial

Tropical insular systems have long attracted biologists, stimulating some important controversies in ecology and evolution. Eustatic fluctuations during the Pleistocene have been invoked to explain species dispersal and proliferation in these fragmented systems by controlling the extent of landmasses and their temporary connections. In ancient archipelagos, the Pleistocene represents only a small slice of their history, so long-standing configurations might better explain insular diversity patterns. With a geological history of ca. 30 million years, the Sunda Shelf is old. Upon entering the Pleistocene, islands of the Sunda Shelf repeatedly separated and merged; however, recent reappraisals of its paleoenvironments and evolutionary dynamics have questioned their biogeographic significance. Based on the molecular inventory of 6 common freshwater fish families, we explored population fragmentation and demographic history of the most common species using mitochondrial DNA sequences. Species delimitation methods, applied to 1062 sequences belonging to 37 species from 188 sites, detected 95 Molecular Operational Taxonomic Units (MOTUs). Among the 9 most widespread species, the number of MOTUs ranged from 1 to 11, and correlated with time to the most recent common ancestor. Extended Bayesian Skyline Plots applied to mitogenomes and cytochrome c oxidase I sequences detected no variation in past effective population size within MOTUs, while hierarchical Approximate Bayesian Computation provided no evidence of congruent changes in effective population sizes. Fragmentation of an ancestral range is the most likely explanation for the rampant cryptic diversity observed, but demographic inferences do not support MOTUs as being refugial from an evolutionary perspective.