This study addresses whether or not nematode assemblages inhabiting the soft sediment in abyssal nodule fields are unique and differ from those occurring in the nodule-free abyssal areas. This question is important for the evaluation of possible diversity losses caused by nodule mining operations. The analysis involved seven datasets at genus-level of nematode assemblages from three nodule-bearing abyssal areas in the Pacific (the Clarion-Clipperton Fracture Zone or CCFZ, and the Peru Basin) and the Indian Ocean (CIOB), and five datasets from nodule-free abyssal regions (the Atlantic Ocean). Only individuals identified to valid genera were used in the analysis. Overall, more than 20,000 nematode individuals from 90 samples were analysed. The analyses showed that the nematode communities in the nodule fields did differ from those in the nodule-free abyss. The nodule fields are characterized by a higher microhabitat heterogeneity, which resulted in a higher diversity of nematode communities inhabiting the nodule fields. However, one set of dominant taxa (Monhysteridae, Microlaimus, Acantholaimus, Theristus and Desmoscolex) was identical for both the nodule-bearing and nodule-free abyss, although some genera were strongly and significantly associated with either the nodule-free abyss (Microlaimus) or the nodule-bearing abyssal regions (Capsula). On the other hand, nematode assemblages at nodule-bearing and nodule-free sites within the nodule-bearing abyss proved to be similar at the genus level. This makes it possible for the nodule-free sites to serve as donors of colonizers for the mining-impacted nodule-bearing abyss. However, this suggestion has to be treated with caution, because the species-level differences have not been tested yet.
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