Mismatch between molecular (mtDNA) and morphological classification of Macrobrachium prawns from Southern Nigeria: Cryptic freshwater species and brackish water morphotypes

• First phylogeographic assessment of Macrobrachium in Nigeria
• Identification of a cryptic freshwater species with high potential for aquaculture
• Brackish water population do not form genetically distinct lineages (i.e. resources).
• Contrasting phenotypes represent different ecotypes of the same species.

With a wide distribution across brackish and freshwater habitats in West African coastal regions, the giant prawns, Macrobrachium vollenhovenii and Macrobrachium macrobrachion, are potential candidates for aquaculture in the region. Here, we present the first molecular investigation of the phylogeography and systematics of these prawns. Morphological analyses unambiguously classed individuals into two clusters corresponding with the recognized species. However, phylogenies based on 3 mitochondrial DNA regions (CO1, 16S rRNA,12S rRNA) consistently recovered two highly divergent clades. One clade comprised all individuals from two geographically distant upstream (freshwater) populations of M. vollenhovenii, the other all individuals from brackish water sites, comprised of both morphospecies. Within mtDNA clades, there was no apparent genetic differentiation between morphospecies or geographic location, which is most consistent with gene flow through human-mediated translocation. Our results indicate a cryptic Macrobrachium species which appears to be adapted to freshwater conditions and therefore highly suitable for freshwater aquaculture. Further investigations are required to determine whether the existence of two apparent morphospecies in brackish water results from intraspecific polymorphism, recent speciation or extensive hybridization.