An updated phylogeny of the family Stenasellidae (Crustacea, Isopoda), with a new species of Stenasellus from southern Vietnam

Our current understanding of the molecular systematics of Stenasellidae, a species-rich family of obligate groundwater isopods known from Africa, Asia, Europe and North-America, is based primarily on specimens of the genus Stenasellus Dollfus, 1897 collected in Europe. Here, we provide an updated phylogeny of the family comprising specimens of Stenasellus collected from multiple countries in Asia, including Iran, Turkmenistan, India, Cambodia, and Vietnam as well as specimens of Metastenasellus Magniez, 1979 collected in Africa.

First, we used COI - based molecular species delimitation methods to delimit putative species within the family Stenasellidae. Second, we built a phylogeny of the Stenasellidae comprising 48 putative species, for which two mitochondrial and two nuclear genes were available. Third, we describe Stenasellus angiangensis sp. nov. from the hyporheic zone of streams located in mountainous islands of the Mekong River Delta, the first representative of the family Stenasellidae described from Vietnam.

We show that the number of putative species delimited using various COI - based molecular species delimitation methods largely surpass the number of species described based on morphological criteria. The four-gene phylogeny reveals, for the first time, a robustly supported and broadly distributed Asian clade comprising Stenasellus species from Iran, India, Cambodia, and Vietnam, including Stenasellus angiangensis sp. nov. as the sister taxon to Stenasellus cambodianus Boutin & Magniez, 1985 from southern Cambodia.

Our phylogeny strongly questions the monophyly of the genus Stenasellus, even though the latter cannot be definitively rejected due to difficulties in resolving the phylogenetic relationships among the clades identified within stenasellids.