The great variety of geological and hydrological conditions in the deep sea generates many different habitats. Some are only recently explored, although their true extent and geographical coverage are still not fully established. Both continental margins and mid-oceanic seafloors are much more complex ecologically, geologically, chemically and hydrodynamically than originally thought. As a result, fundamental patterns of species distribution first observed and explained in the context of relatively monotonous slopes and abyssal plains must now be re-evaluated in the light of this newly recognized habitat heterogeneity. Based on a global database of nematode genus composition, collected as part of the Census of Marine Life, we show that macrohabitat heterogeneity contributes significantly to total deep-sea nematode diversity on a global scale. Different deep-sea settings harbour specific nematode assemblages. Some of them, like coral rubble zones or nodule areas, are very diverse habitats. Factors such as increased substrate complexity in the case of nodules and corals seem to facilitate the co-existence of a large number of genera with different modes of life, ranging from sediment dwelling to epifaunal. Furthermore, strong biochemical gradients in the case of vents or seeps are responsible for the success of particular genera, which are not prominent in more typical soft sediments. Many

Nematode assemblages from mining track and adjacent undisturbed sites were compared. ► This 26-year-old track resulted from the mining of deep-sea polymetallic nodules. ► The nematode assemblage inhabiting the track showed lower diversity and density. ► The assemblage composition in the track also differed from that in the undisturbed sites.

The deep-sea nematode assemblages of the French mining claim area in the Clarion-Clipperton Nodule Province (the Northeastern Pacific Basin, 14°N, 130°W, depth 4,947-5,046 m) were studied at species, genus and family levels. Material was obtained using multicorers and box corers. In total, 2,174 individuals from 14 stations and 21 samples were examined. The nematode assemblages from the nodule-bearing (NB) part of the sampling area differed significantly from those at the nodule-free (NN) section. The average nematode density was 103 inds/10 cm 2 . The average nematode densities in the NB and NN parts were 69 inds/10 cm 2 and 137 inds/10 cm 2 respectively. Altogether, at least 325 putative species, 97 valid genera and 33 valid families were found. The real number of nematode species for the sampling area was estimated at about 510. The richest genera were Acantholaimus (33 putative species) and Thalassomonhystera (23 putative species). The proportions of juveniles, males and females among nematode individuals were about 44%, 23% and 33% respectively. About 2/3 of all nematodes were deposit-feeders and around 1/3 were epistrate-feeders. The nematode assemblage of the studied area may be considered to be a Theristus discolensisThalassomonhystera sp.3 assemblage with variation of the dominant species. The dominant species at the NN site were Theristus discolensis (12.2%) and Thalassomonhystera sp. 3 (5.3%). In contrast, at the NB site, Thalassomonhystera sp. 3 was most abundant (14.1%), followed by T. discolensis (6.2%). At genus level, Thalassomonhystera (17.9%), Theristus (13.0%) and Acantholaimus (12.8%) dominated at the NN site, whereas the abundance of Theristus was half of that (6.4%) at the NB site. The most dominant families at the NN and NB sites were Xyalidae (28.1% and 16.6%, respectively), Monhysteridae (17.9% and 25.3%) and Chromadoridae (16.2% and 22.5%).

All available information from literature sources dealing with deep-sea nematode species was analyzed, in order to obtain an overview of the state of knowledge in deep-sea nematode taxonomy and answer the question of how many valid nematode species are known from the deep sea so far. One hundred and twenty-seven taxonomic and ecological literature sources reported a total of 638 valid species belonging to 175 genera and 44 families, from 474 deep-sea stations at depths of 400-8,380 m. This number is less than 16% of all known marine nematode species, whereas the deep sea comprises about 91% of the ocean bottom. Of these species, 71% were initially described from the deep sea. Most of the valid species have been reported from the North Atlantic, including the Mediterranean. The rest of the World Ocean, including the Pacific, Indian, Arctic and Antarctic oceans, is considerably less studied. The largest numbers of valid species were reported from the continental slope and the abyssal plains, while information on valid species from trenches, deep-sea canyons, and seamounts is extremely scanty. Some deep-sea families are much more investigated than others in proportion to their relative species abundances in the deep sea, i.e., the percentage of valid species from these families among all valid deep-sea species is much higher than the real percentage of species from these families reported in faunistic studies (e.g., Desmoscolecidae, Comesomatidae, Sphaerolaimidae, Benthimermithidae, Leptosomatidae, and Draconematidae). On the other hand, the families Xyalidae, Oxystominidae, and Monhysteridae were recognized as the most "underinvestigated," as, in spite of their high species abundance in the deep sea, there are quite a few taxonomic studies on these taxa. Some deep-sea nematode species were reported from two or three oceans, and can be considered probable cosmopolitan species. Some number of probable eurybathic species were also found (the difference between minimum and maximum depth was from 1 km to more than 5 km).