A new genus and three new species of free‐living marine nematodes from the Yellow Sea, China

One new genus, Paramphimonhystrella gen. nov., and three new species of free‐living marine nematodes in this genus, P. elegans sp. nov., P. minor sp. nov., and P. sinica sp. nov., are described and illustrated from the Yellow Sea, China. Paramphimonhystrella gen. nov. differs from the similar genus Amphimonhystrella by the elongated buccal cavity, cervical setae and large caudal glands. Paramphimonhystrella elegans sp. nov. is characterized by ovoid amphids and spicules with a small hook at distal end and cylindrical part of tail shorter than conical part. Paramphimonhystrella sinica sp. nov. is characterized by circular amphids, wide buccal cavity, spicules with blunt tip, and cylindrical part of tail longer than conical part. Paramphimonhystrella minor sp. nov. differs from the other two species by its smaller body (shorter than 800 µm), circular amphids, and spicules with sharp tip. The type species of this new genus is P. elegans gen. nov. and sp. nov. Types are deposited in the College of Marine Life Sciences, Ocean University of China.


Introduction
Undisturbed sediments were collected from a grid of 23 sampling stations (from 32.5uN to 37uN, 121.5uE to 125uE) during the course of an ecological survey of the overwinter grounds of anchovy in the Yellow Sea, China in January 2003. Meiofaunal abundance varied from 553 to 1400 individuals per 10 cm 2 (929 ¡289 ind/10 cm 2 ), with 81-93% (809 ¡247 ind/10 cm 2 ) of the specimens being nematodes. Up to now, only 36 species of marine nematodes have been recorded from the Yellow Sea (Zhang and Platt 1983;Zhang and Ji 1994;Hope and Zhang 1995;Zhang 2004, 2005;Zhang 2005;Zhang and Huang 2005). In this investigation, a number of previously undescribed species of nematodes were found. These were studied in detail at the Ocean University of China.
In this paper we describe the new genus Paramphimonhystrella gen. nov. in the family Xyalidae, belonging to the order Monhysterida; three new species of this genus were found in the Yellow Sea, China.

Material and methods
Sampling, subsampling, sorting, and slide mounting were carried out as detailed by Zhang (2004, 2005) and Zhang (2005).
Morphometic data are presented using the modification of Filipjev's standard formula described by Platt (1973). The types are deposited in the College of Life Sciences, Ocean University of China. Measurements are in mm. Abbreviations are as follows: a.b.d., anal body diameter; b, body length/oesophagus length; c, body length/tail length; c.d., corresponding body diameter; Sc, spicule length as chord; V, vulva distance from the anterior end of body; V%, V/total body length.

Diagnosis and discussion
Paramphimonhystrella gen. nov. is close to Amphimonhystrella Timm, 1961, but differs from it by its elongated cylindrical or conical stoma; two groups of 8-10 cervical setae each, instead of four cervical setae each group; two of three caudal glands large and obvious. The body size is larger than in species of Amphimonhystrella. This new genus is also similar to Elzalia Gerlach, 1957, but differs from the latter by the elongated stoma, presence of cervical setae, and the structure of spicules. The principal difference between this new genus and Amphimonhystrella and Elzalia is the structure of the buccal cavity, which is regarded as characteristic of genera in the family Xyalidae. The buccal cavities of species in this new genus are elongated conical and differ distinctly from other genera in this family. Consequently, we propose Paramphimonhystrella as a new genus. The type species of this new genus is Paramphimonhystrella elegans sp. nov.
( Tables I, II

Etymology
This species is named after the beautiful slender body.

Diagnosis and discussion
Paramphimonhystrella elegans sp. nov. is characterized by ovoid amphids, the spicules with a small hook at distal end. It is close to Paramphimonhystrella minor sp. nov. But the principal differences between the two species are the body size, the shape of the amphids, the structure of the head, buccal cavity and spicules. In this species, the body length is more than 1200 mm; the amphids are ovoid. In P. minor sp. nov. the body is shorter than 800 mm; amphids are circular; spicules with sharp tip. It is similar to Paramphimonhystrella sinica sp. nov. in body size, but amphids are circular, and spicules have a blunt tip in the latter.

Etymology
This species is named for the small body.

Description
Body length 740-792 mm; maximum body diameter 13-16 mm. Cuticular striation not observed. Head attenuated. Buccal cavity deep, conical, and strongly cuticularized. Ten cephalic setae, about 2 mm long. Two groups of eight cervical setae each, first group at the level of the amphids, 3-4 mm long; the second group of setae 7-8 mm long. Amphids circular, diameter about 5 mm and 0.5-0.58 corresponding body diameter wide; about 12 mm from the head end. Tail 110-160 mm long, conico-cylindrical. Conical part about half of tail length; anal diameter 10-13 mm. Two large obvious caudal glands. Three terminal caudal setae, 2-3 mm long.
Female. A single anterior ovary; vulva at about 61-63% of body length.

Diagnosis and discussion
Paramphimonhystrella minor sp. nov. is characterized by its small body (shorter than 800 mm), circular amphids. It is close to Paramphimonhystrella elegans sp. nov. in the structure of the buccal cavity, but differs in body size and in the shape of the amphids. It is similar to Paramphimonhystrella sinica sp. nov. in the shape of the amphids, but the principal differences between the two species are the buccal cavity and the head structure.
In species of Paramphimonhystrella sinica sp. nov. the head is extended and buccal cavity is wider.

Diagnosis and discussion
Paramphimonhystrella sinica sp. nov. is characterized by the circular amphids, extended head, broad buccal cavity, and cylindrical part of tail longer than conical part. It differs from Paramphimonhystrella elegans sp. nov. and Paramphimonhystrella minor sp. nov. by the structure of the head, the buccal cavity, the shape of amphids, and the proportion of cylindrical part of tail and conical part. In P. elegans sp. nov., the amphids are ovoid and the spicule has a hook at distal end, the cylindrical part of the tail is shorter than the conical part while in P. minor sp. nov. the body size is small, generally shorter than 800 mm, amphids are circular, but the head is not extended and the spicule has a sharp tip, not a hook at distal end, and cylindrical part of tail is about the same length as the conical part.