Molecular genetic and taxonomic studies of the swamp eel (Monopterus albus Zuiew 1793)

Abstract

The swamp eel Monopterus albus is restricted to fresh water and readily adapts to rice field environments. The swamp eel is popular as food, is readily transported live and has achieved a wide distribution in tropical and subtropical Asia. The principal aim of this study is to investigate the taxonomy and phylogeny of M. albus using molecular genetic data, by analysing genetic diversity throughout its distribution, with a special focus on populations from Indonesia. Evidence for significant cryptic speciation within M. albus was found through DNA barcoding using the COI mitochondrial DNA fragment and a population genetic analysis using five microsatellite markers. These data reveal two distinct Indonesian forms of M. albus, which are significantly divergent from each other in sympatry and from other forms from more northly Asian countries. The first form is indigenous to Indonesia occurring on the islands of Java and several adjacent islands to the east and the second form is wide spread in Indonesia and in other Southeast Asian countries to the north, but may have had its distribution modified through human-mediated introductions. Integrated analyses of nucleotide sequences from multiple studies and gene regions indicate that M. albus is highly diverse genetically and comprises at least five cryptic species. Phylogenetic analyses indicate that the southern forms are more recently evolved from older lineages established in northern Asia (e.g. China and Japan). Morphological variation in the two forms of Indonesian Monopterus indicate significant size-related variation and phenotypic plasticity. Characters were found that correlate with the molecular data thereby supporting the presence of two cryptic species of Monopterus in Indonesia and providing a basis for their identification in the field. These findings contribute to the understanding of M. albus taxonomy through the discovery of cryptic species in what is best referred to as the M. albus species complex. Further, phylogenetic analyses provide a framework for understanding the evolutionary diversification and biogeography of this important group of freshwater fishes from eastern and Southeast Asia.