Mediomastus is a common genus around the world, and Chinese specimens used to be identified as M. californiensis Hartman. In this study, 118 Mediomastus specimens collected from four localities along the southeast coast of China were examined. They belonged to an unknown species. We treat them as a new species, described herein. The new species is characterized as follows: (1) chaetigers 1–9 biannulate with chaetal fascicles at midsegment, thereafter multiannulate with chaetal fascicles posterior to midsegment; (2) parapodial ridges on posterior abdomen; (3) possession of bilimbate capillary chaetae on chaetigers 1–4; (4) thoracic hooks with long, straight shaft, hood length about 3–5 times width; (5) abdominal hooks shorter, with developed shoulder and distinct constriction, markedly different from those in thorax; (6) unique staining pattern with methyl green. The present study indicated that Mediomastus species inhabiting Chinese waters might be misidentified as M. californiensis, and that Mediomastus chinensis sp. nov. is widely distributed along the southeast coast of China.

Based on polychaete specimens collected from coastal areas of China in 1959, a new species of the family Orbiniidae, Leodamas sinensis sp. nov. is described in the present paper. The new species can be distinguished from its cogeners by the combination of following characters: branchiae present from chaetiger 5; each thoracic neuropodial postchaetal lobe possessing broad tori, with a digital to triangular-shaped papilla at its midlength; 4 or 5 vertical rows of uncini and one posterior row of accompany capillaries in thoracic neuropodia; acicula present in both abdominal notopodia and neuropodia, slightly or distinctly projecting. A key to all known species of the genus is provided.

Sabaco sinicus sp. nov. is described based on material collected from Chinese coast. It has been misidentified as Asychis gangeticus Fauvel, 1932 since Uschakov and Wu (1962). Here, we recognized it as a new species based on combination of the following characters: dark-brown pigmentation spots on nuchal grooves and first three chaetigers; glandular pattern on chaetigers 4–6; preanal achaetigerous segments absent; anal plaque trumpet-shaped with a disc-shaped dorsal lobe; notochaetae consisting of geniculate capillaries and fine capillaries with or without spinose spiral bands; neurochaetae presented as a single row of rostrate uncini. Asychis gangeticus differ from the new species in having no pigmentations on body and a triangular dorsal lobe of anal plaque. Distribution region of this species have moved northward significantly from sea area south of Changjiang Estuary since the 1950s.

A taxonomic study of Hyalinoecia species based on the materials deposited in the Marine Biological Museum of the Chinese Academy of Sciences (MBMCAS) yielded two species: Hyalinoecia tubicola (Müller, 1776) and H. papillata Imajima, 1999. Both species were collected from the sandy bottoms in the northern South China Sea. Hyalinoecia tubicola is widely distributed from the coast of Guangdong Province southwards to the Xisha Islands. Hyalinoecia papillata is reported for the first time in the South China Sea. It differs from H. tubicola in having a prolonged, tongue-like prechaetal lobe with papillae along the lateral margins in the first parapodium (vs. auricular prechaetal lobe with smooth margins). The morphological description and illustration of both species are given.

A new record of the ampharetid genus Auchenoplax, A. worsfoldi Jirkov and Leontovich, 2013, is reported and described from the East China Sea. The specimens agree well with the original description of Auchenoplax worsfoldi with two pairs of branchiae, two thoracic uncinigers with enlarged neuropodia and 12 abdominal uncinigerous segments. The present specimens have long and filiform branchiae extending to thoracic chaetiger 9, which was not included in the original description. This is the first record of this species in Chinese waters.

Two new species of free-living marine nematodes of the genera Hopperia and Nemanema are described from the sublittoral sediment of the East China Sea. Hopperia macramphida sp. nov. is characterized by having large amphidial fovea with five turns; arcuate spicules with enlarged proximal end and central internal lamella extending one third of spicule length from proximal end; gubernaculum with bent apophyses, six papilliform precloacal supplements; and conico-cylindrical tail with swollen tip and terminal setae. Nemanema minutum sp. nov. is characterized by relatively smaller body size in the genus; large oval amphidial fovea; conical tail; slender spicules with alae and hooked proximal end; ring-like gubernaculum; and papilliform precloacal supplements with three short setae. An updated dichotomous key is provided for the 17 species of Hopperia.

Actinonema falciforme sp. nov. was discovered from the continental-shelf sediments of the East China Sea. It is characterized by six outer labial setae and four cephalic setae in one circle, complex and heterogeneous cuticular ornamentation with lateral differentiation, large and conspicuous amphideal fovea which is transversely oval-shaped, a single anterior testis, two opposed reflexed ovaries, falciform telamon (lateral pieces of gubernaculum), and the lack of spicule and gubernaculum. Within the genus Actinonema, only A. longicaudatum and the new species possess telamon and lack spicule and gubernaculum. Actinonema falciforme sp. nov. can be easily differentiated from A. longicaudatum by its much shorter body length, the peculiar shape of the telamon and a shorter tail.

Two new species of free-living marine nematodes of the genus Wieseria are described from the sublittoral sediment of the Jiaozhou Bay in Qingdao. Wieseria sinica sp. nov. is characterized by having short labial and cephalic setae in a backward direction; oblong amphidial foveas with double contour; arcuate spicules with alea; ring-like guberculum; and clavated tail. Wieseria tenuisa sp. nov. is characterized by its very large “a” value (133.6), long labial setae in a forward direction; spicules with alea and hooked proximal end; rodlike gubernaculums; and relatively longer clavated tail (7.9 a.b.d. long). It differs from Wieseria sinica sp. nov. mainly by the longer labial setae, slightly straight spicules with hooked proximal end, and its thinner body. Amended dichotomous key to male species of Wieseria is provided.

Two new free-living marine nematode species from the mangrove wetlands of Fujian Province, China are identified and illustrated. Sabatieria conicoseta sp. nov. is characterized by its short conical somatic setae. There are 12–15 tubular pre-cloacal supplements and 12–15 μm long straight gubernacular apophyses. Dorylaimopsis papilla sp. nov. is characterized by spicules which are 1.5–1.8 a. b. d. long with media cuticularized strip along entire spicules, and have a cephalated proximal end with small hooked. There are 16–18 small papillate pre-cloacal supplements and 37–40 μm long dorso-caudal gubernacular apophyses.

Two new species of free living nematodes Tripyloides mangrovensis n. sp. and Tripyloides amoyanus n. sp. of genus Tripyloides de Man, 1886 are described from mangroves of the Tong’an Bay and Haimen Island in the Xiamen Bay, China. The two species are both characterized by six stout outer labial setae, male without preanal papillae, tail conico-cylindrical without swollen terminally. Tripyloides mangrovensis n. sp. can be additionally by its buccal cavity consisting of two parts, amphid circular, distinct proximal conical and distal slender cylindrical portions, 2/3 of the tail cylindrical. Tripyloides amoyanus n. sp. is distinguished by buccal cavity consisting of three parts with small tooth, amphid small, rounded, like a comma-shaped loop, 1/3 of the tail cylindrical, gubernaculum with four denticles.