The revenant: rediscovery of <i>Margaritifera homsensis</i> from Orontes drainage with remarks on its taxonomic status and conservation (Bivalvia: Margaritiferidae)

Since <i>Margaritifera marocana</i> (Pallary, 1918) and <i>M. laosensis</i> (Lea, 1863) were rediscovered, <i>M. homsensis</i> (Lea, 1865) remains the only pearl mussel species known solely based on old shell samples from natural history museums. This is also the last pearl mussel species, which is absent in a phylogeny of the family. Here, we aimed to provide an integrative revision of the taxonomic status of <i>M. homsensis</i> from the Orontes Basin. Using a newly collected specimen from the River Karasu, Hatay Province, southern Turkey, five gene partitions were sequenced, the cytochrome c oxidase subunit I (COI), large ribosomal subunit rRNA (16S), large ribosomal subunit rDNA (28S) and its D3 expansion segment (D3), and small ribosomal subunit rDNA (18S). The multi-gene phylogeny indicates that <i>M. homsensis</i> is a sister taxon of <i>M. auricularia</i>, but both these species are closely related to <i>M. marocana</i> by nuclear genes. The main conchological features, i.e<i>.</i>, the shell shape, teeth morphology, and mantle attachment scars, as well as Fourier shell shape analysis have not shown principal differences between <i>M. homsensis</i> and <i>M. auricularia</i>. Based on these data, we concluded that <i>M. homsensis</i> is a valid species that is most closely related to <i>M. auricularia</i>. Special conservation efforts for a population of <i>M. homsensis</i> discovered in Turkey, including the formation of a nature reserve, might contribute to the conservation of the species. Finally, an extensive search for surviving populations in Orontes drainage (southern Turkey, Lebanon, and Syria) and the Nahr-el-Kabir River (Lebanon and Syria) remains necessary to develop a transboundary conservation strategy for this unique taxon.