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Syringolaimus is the most abundant and diverse genus of the family Ironidae (Nematoda) found in the Campos Basin, off Rio de Janeiro, Brazil. In this article, four new species of this genus are described. S. annae sp. n. is characterized by a conical-cylindrical tail without a spinneret. S. magdae sp. n. possesses a pair of ejaculatory glands located in the precloacal region, and a spinneret, which comprises 21.2% of the total tail length. In S. smolae sp. n. the spicule is setiform. S. taniae sp. n. has the anterior region of the tail rounded, with its terminal portion cylindrical-filiform.

Three nematode species of the genus Syringolaimus (Enoplida, Ironidae) were found in the Angola Basin (south-east Atlantic Ocean) at depths of almost 5500 m. One of them is described as new for science and the other two were initially recorded from the deep sea off the Brazilian coast and are redescribed here. Syringolaimus elegans sp. n. differs from all other species of Syringolaimus by a slimmer body (a=99 vs. 30–73) and from all species except S. ingens (nom. nud.) by its greater body length (2718 μ m vs. 760–1750 μ m), longer buccal cavity (79 μ m vs. 28–58 μ m), and longer terminal pharyngeal bulb (70 μ m vs. 53 μ m). All the Syringolaimus species together make up from 0.75 to 2.82% of all nematode individuals from all samples. Diagnoses of Syringolaimus annae Lima et al ., 2009 and S. smolae Lima et al ., 2009 are updated. An annotated list of Syringolaimus species is provided; the species annae , elegans , filicaudatus , loofi , magdae , renaudae , smolae , striatocaudatus and taniae are considered as valid, while brevicaudatus , caspersi , lichenii , marisalbi , smarigdus and venustus as species inquirendae, and gladiatus , ingens, nitidus and venustus as nomina nuda . Diagnostic characters and problems of species identification are discussed.

The highest numbers of nematodes with a percentage ranging from 76.4% in Cung Hau to 77% in Ham Luong estuaries. There were 92 genera (71 genera in Ham Luong and 62 genera in Cung Hau) of nematodes recorded in both estuaries. The mean nematode densities varied between 90 ± 31 - 1524 ± 269 ind.10 cm² and 105 ± 79 - 1120 ± 534 ind.10 cm² in Ham Luong and Cung Hau estuary, respectively. The nematode communities were mainly composed of Dichromadora, Daptonema, Oncholaimellus, Syringolaimus, Tripyloides, Parodontophora, Theristus and Halalaimus belonging to several dominant families, such as, Oncholaimidae, Xyalidae, Chromadoridae, Oxystominidae, Axonolaimidae and Sphaerolaimidae. In both estuaries, nematode density decreased from inland towards the mouth before they increased again at the mouth stations.