Morphology and Molecular Phylogeny of a New Species, Macrobrachium debaratae sp. nov. (Caridea, Palaemonidae) from Songkhram River, Northeast Thailand

Abstract

The recognized diversity of Macrobrachium prawns in Thailand has increased recently due to taxonomic investigation using integrative approaches, i.e. from morphology and DNA barcoding. The freshwater habitats in northeast Thailand such as riparian wetland ecosystems provide diverse ecological conditions that promote adaptive diversification among organisms. The varying condition of the freshwater bodies can support the existence of cryptic species among the aquatic animal fauna, including in prawns of the genus Macrobrachium. Several Macrobrachium species have previously been reported in association with the vast network of tributaries that form the Mekong River basin. Newly described species and unidentified morphospecies have particularly been reported from this area in previous systematic studies, which indicates that the current known diversity of the aquatic fauna is likely to be largely incomplete. In this study, a new species of the genus Macrobrachium from the Songkhram River, one of the Mekong tributaries, is described and named as Macrobrachium debaratae Siriwut, sp. nov. This new species contains several distinct morphological characters from its congeneric and co-existing species group, such as the rostral teeth formula, the size and the shape of second pereiopods, and the tooth present on the cutting edge of the fingers of the chelae of the second pereiopods. Genetic distance analysis among the known Macrobrachium species supported this morphological classification as a new species with an interspecific COI divergence of 13%. The COI phylogenetic tree indicated that M. debaratae Siriwut, sp. nov. was monophyletic and was placed close to M. sirindhorn, a member within the M. pilimanus species group. This study highlights the need for a detailed morphological inspection to examine the variability in the taxonomic characters of Macrobrachium, particularly in the M. pilimanus species group found in mainland Southeast Asia tributaries. Further taxonomic review based on intensive sampling is required to provide a more adequate understanding of the diversity of Macrobrachium in the Mekong basin.