Diversity and distribution of Tanaidacea (Crustacea: Peracarida) in the abyssal polymetallic nodule fields of the eastern Clarion-Clipperton Fracture Zone

Tanaidaceans comprise a significant macrofaunal component of the abyssal plains in the Clarion-Clipperton Fracture Zone (CCFZ), Pacific Ocean. A large amount of tanaid material were examined from the eastern end of CCFZ, specifically the OMS (Singapore) and UK-I (United Kingdom) exploration areas and one of the nearest Area of Particular Environmental Interest APEI-6. Some 1800 individuals were collected from 89 stations at 4016–4336 m depth primarily using the box corer, epibenthic sled and multiple corer during three research cruises conducted between 2013 and 2020. Based on morphological characters, a total of 98 species and morphospecies belonging to 12 families were identified from these specimens, of which 85 % are undescribed. Only seven of the 35 described tanaid species previously recorded from CCFZ were rediscovered in this study. The OMS and UK-I areas recorded 85 and 71 species, respectively, and these high species richness as compared to values reported in other exploration areas could be due to greater sampling effort. Despite the close geographical proximity of OMS and UK-I, only 59 % of recorded species were common between the areas. Differences were observed in tanaid density, species richness and species composition between the northern and southern stations in the combined areas of OMS and UK-I. Interestingly, the relatively distant APEI-6 recorded 11 species, all of which were also present in OMS and UK-I. This study did not detect any relationship between tanaid community measures and abiotic factors. Another important outcome of this study is that the addition of the 250 μm mesh to the usual 300 μm mesh increased the total number of individuals retained on the sieves by 25–32 %, but not the total number of species. Even though the 0–2 cm sediment layer contained 1.4–3.0 times and 2.1–8.3 times more individuals than the 2–5 cm and 5–10 layers, respectively, no species was found exclusively at the top layer.