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

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).

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A new genus and species of Nanaloricidae (Loricifera), Phoeniciloricus simplidigitatus, is described inhabiting fine sand covered by a layer of volcanic ash at a water depth of 1,813 m in the New Ireland Basin near the Kilinailau Trench (north of Papua New Guinea). The described specimen is a postlarva enclosed in a larval exuvium. This is the first report of a species belonging to the Nanaloricidae from the deep sea. This occurrence is surprising, because Nanaloricidae are typical inhabitants of coarse sands in the intertidal or littoral zone. Preference for these shallow water habitats is reflected in many morphological features which characterize the Nanaloricidae, and are not normally found in Loricifera inhabiting fine-grained, clayish, deep-sea bottoms. The postlarva of the new species is characterized by a long narrow mouth tube, an urn-shaped lorica divided into ten plates, and 13 small lorica spikes. Distinguishing features of the Higgins-larva include short spinose toes lacking mucros but having small and slightly enlarged bases, short scalids on the introvert, many thoracic plates arranged in 6-8 rows, numerous small papillate flosculi in the collar and caudal regions, and three pairs of filiform, short locomotory appendages on the ventral side. Some features of the new species, especially of the Higgins-larva, are discussed as adaptations to the deep-sea environment.

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Glochinema bathyperuvensis sp. n., the first record of Epsilonematidae from bathyal muddy sediments of the oxygen minimum zone (OMZ) off Callao, Peru (eastern Pacific Ocean) is described. The new species belongs to the subfamily Glochinematinae and is characterized by the possession of more than 8 pairs of subdorsal thorns asymmetrically arranged and a single dorsal one in the pharyngeal region, as well as by the presence of ten to thirteen blade-like cuticular protrusions forming a latero-dorsal palisade around the posterior head region and anterior cervical region. In addition, G. bathyperuvensis is characterized by its large size, more than 240 cuticular annules, provided with an extremely dense „hairy“ body ornamentation and, four rows of ambulatory setae: two inner subventral ones with 8 to 9 setae in males, and 5 to 6 setae in females, and two outer subventral rows composed each of 9-12 ambulatory setae followed by a large number of modified somatic setae. The dense hairy body spines, the large number of modified somatic setae, its dominant occurrence exclusive in oxygen-depleted, organic-rich soupy surface sediments indicate an adaptation to the stressed OMZ habitat conditions.

A newly discovered species of Pliciloricus from the deep sea of the Angola Basin (Namibia, Atlantic) seems to have two types of postlarvae in its life cycle. Type I is a simplified but clearly identifiable postlarval stage, the other type II consists only of a thin layer of cuticle as the remnant of a postlarva. Both types contain adults that have moulted from them. The simplified type I postlarva has a fully developed lorica with an ornamentation identical to that of the adult, but other body regions are reduced with only a few structures left. The discovery of this clearly identifiable postlarva is important, because it supports the conclusion that Pliciloricus-species originally moults from postlarval stages as well as other taxa of Loricifera. Furthermore, it could be concluded that the simple cuticle layer surrounding most adults found during their metamorphosis is the remnant of a postlarval stage. The life cycle of the new species seems to include two phases. After to the bisexual is a unisexual phase, represented by a simplified parthenogenetic adult stage which lacks most parts of the adult morphology. The adults of P. diva sp. n. differ from other species in having among others (1) a mouth cone with four strong cuticular bars plus eight primary oral ridges; (2) leaf-like clavoscalids which are very broad basally and narrow distally, and have more than 22 transverse cross walls; (3) a strongly sclerotized double-organ consisting of four rami; (4) large spinoscalids of second row shorter than clavoscalids, (5) short type B spinoscalids of fourth row with claw-tips and with a double row of five teeth as well as distally with a double row of minute denticles; (6) an anterior margin of the lorica with bicuspid protrusions and specific crescent-shaped ornamentations; (7) a midventral plica with five bar-like transversal strengthened ridges. Distinguishing features of the Higgins-larva are (1) short clavoscalids with broad second segments; (3) a collar with seven flosculi located in small pits; posterolateral setae being short but strong and pod-like. The study also revealed new information about the double-organ of the adult and the buccal structures of the Higgins-larva.