Stygobiont polychaetes: Notes on the morphology and the origins of groundwater Namanereis (Annelida: Nereididae: Namanereidinae), with a description of two new species

More than half of the species in NamanereisChamberlin, 1919 (Nereididae: Namanereidinae), are adapted to life in subterranean waters. We document the taxonomy, distribution, and adaptations of the groundwater-inhabiting species in this genus for the first time, starting from the description of two new stygobitic species. The first, Namanereis pilbarensis sp. nov. from water-table carbonate deposits in the Pilbara region of north-west Australia, is representing the first record of a stygobitic polychaete from Australia, and the second, Namanereis socotrensis sp. nov., from karstic groundwater on Socotra Island (Yemen) is the second stygobitic Namanereis species from Socotra. Troglomorphic adaptations observed include the absence (or reduction) of eyes and cuticular pigment, and cirriform appendages of the head, parapodia, and pygidium that are all considerably longer than in their marine counterparts. The chaetae and jaws differ in some groundwater species but not others, so the troglomorphic nature of these features is less certain. Remarkably, the two species of the Socotra Archipelago (Namanereis gesaeFiege & Van Damme, 2002, from Abd al Kuri and the new species) seem to derive from different ancestors, respectively single terminal tooth and bifid tooth-jawed lineages. Based on the jaw morphologies, we suggest that the groundwater polychaetes of this genus might not have entered groundwater from freshwater/anchialine habitats during a single colonization event, as previously suggested, but at different times. Different geographical origins of two groups of species (Gondwanan and Tethyan) are suggested based on recent distribution patterns.