New insights in the groundwater species diversity of the isopod genus Metastenasellus in Benin and Cameroon

Among obligate groundwater organisms (stygobiontes), the Stenasellidae is a diverse family of isopods (Crustacea) that is mainly found in tropical Africa. However, our knowledge of the diversity and endemism of stygobiotic crustaceans found in Africa remains limited due to a lack of studies and a deficit in taxonomic expertise. Within the Stenasellidae, the genus Metastenasellus Magniez, 1966 is currently composed of nine species with a trans-Saharan distribution, but the recent descriptions of two new species in Cameroon suggest that its diversity is largely underestimated. Here, we further explore the diversity of Metastenasellus by sequencing the DNA barcode fragment of 57 Beninese and 27 Cameroonian specimens from 24 and 29 localities respectively. A data set of 84 new and 8 GenBank sequences was analysed with four species delimitation methods (ABGD, PTP, GMYC and KoT). All methods agreed on a minimum of 23 putative species, 11 from Benin and 12 from Cameroon. Ongoing morphological scrutiny of the male pleiopods of these putative species confirm that they correspond to distinct species. The coexistence of two putative species in some localities is further evidence that they represent separate evolutionary lineages. The geographical distribution of the 23 putative species suggests a restricted distribution and a low dispersal capacity of Metastenasellus species, as already observed for macrostygobionts in Europe. Given the number of potential species currently observed per sampled station, it is to be expected that many new species could be discovered by increasing the sampling effort. There also appears to be a decoupling between watersheds and species distribution, suggesting that watersheds are a rough approximation of water table boundaries or that the current distribution of Metastenasellus is more the result of historical factors than of dispersal constraints related to the current configuration of the catchment areas.

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