Euphilomedes biacutidens (Ostracoda, Myodocopida, Philomedidae), a new species from China Sea

Ostracods are one of the major groups of marine benthos, inhabiting virtually all oceanic environments worldwide, and a total of 31 species have been recorded in genus Euphilomedes Kornicker, 1967. In the present study, we describe a new species Euphilomedes biacutidens collected from the Taiwan Strait and South China Sea. E. biacutidens sp. nov. differs from the related species of the genus Euphilomedes in having a unique combination of the characteristics of spines on carapace, the filaments on sensory seta, the arrangement of setae on tip of the first antenna, the numbers of setae on appendages, the claws on fifth limb, the teeth on the comb of the seventh limb and furcal claws. It is particularly obvious that it has a bifurcated and pointed ventral corner of the rostrum, two spines on the posterior margin of right valve, a row of teeth along the inner margin of article 3 of the endopod of the second antenna, and some long claws instead of setae on the fifth limb.


INTRODUCTION
Ostracoda is a class of the phylum Arthropoda (Martin & Davis, 2001). The ostracods are small bivalved aquatic crustaceans and can be benthos or plankton. Ostracods are one of the major groups of marine meiobenthos and also macrobenthos which inhabit virtually all oceanic environments worldwide with various feeding habits and high taxonomic diversity (Karanovic, 2010). Studies on ostracods from China began in the 1950's on fossil species (Chang, 1955). About thirty years later, we initiated investigations of the taxonomy and ecology of living marine ostracods in China (Chen, 1982;Chen, 1984). So far, 237 species of recent marine ostracods have been recorded from the China Sea (Chen, 2012;Chen et al., 2015a;Chen et al., 2015b;Xiang et al., 2017).

MATERIALS AND METHODS
Samples were obtained from two cruises of the South China Sea and Taiwan Strait in 1984-1985and 1994-1995 (Fig. 1). There are no specific permissions required for the sampling activities in the research areas.
All samples were collected using a sampling net with mouth diameter of 80 cm and a mesh aperture of 0.505 mm by vertical dragging from 200 m (or bottom) to surface water. Samples were fixed with 5% buffered formaldehyde for preservation.
Specimens were dissected under a zoom-stereomicroscope (Zeiss Discovery V2.0) and mounted in permanent slides with CMC-9AF mounting medium (Masters Company Inc., Wood Dale, IL, USA). Observations and photomicrographs were obtained with a transmitted-light binocular microscope combined with a differential interference contrast system and AxioVision Image-Pro software (Axio Imager Z2; Carl Zeiss Inc., Oberkochen, Germany). Line drawings were made from photomicrographs and observations of preserved specimens and dissected appendages in slides by Adobe Photoshop CS6 software (Adobe Inc., San Jose, CA, USA).
The type specimens were deposited in the Marine Biological Sample Museum of the Chinese Offshore Investigation and Assessment, the Third Institute of Oceanography, State Oceanic Administration, China (Xiamen, China), under the collection numbers TIO-OMPEu 326-TIO-OMPEu 329 for the new species.

Nomenclatural acts
The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:557FB253-93C6-473E-9E07-227D9D9C1A60. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.
Diagnosis. Height about 60% of length. Carapace oval, external surface with tiny circular pits and small setae ( Figs Frontal organ: Frontal organ extremely long and thin with two articles, article 2 longer with a sharp tip (Fig. 2D).
First antenna: First antenna uniramous with eight articles (Fig. 2E). Articles 1 and 2 long. Article 2 with one disto-dorsal and one disto-ventral plumose setae. Article 3 short with two spinose setae on disto-dorsal margin. Article 4 with one long and one short plumose setae on disto-dorsal margin, three long setae on mid-ventral margin, and one grand sensory seta with about thirty-two very long soft ventral filaments on disto-ventral margin. Article 5 bare. Article 6 very short with one short and bent plumose seta on disto-dorsal margin. Article 7 and 8 fused, very small with seven setae on tip: a-seta very short, spinose b-and g-setae very grand and long, c-seta with one mid filament, d-seta long with bifurcated tip, e-and f-setae long and bare.
Second antenna: Second antenna biramous. Endopod with three articles (Figs. 2F-2G). Article 1 short with three short ventral dorsal setae and one disto-ventral seta; article 2 long and slightly bent with corpulent ventral part, and two grand setae on ventro-distal margin; article 3 thin and bent, approximately equilong to article 2, with one bent proximo-dorsal seta, two small distal setae, twelve small ventral acute teeth, rugged dorsal margin, uneven inner side, and ten small tines on tip. Exopod with nine articles (Fig. 2H). Articles 1-8 with a line of fine spines on medio-distal margin; articles 2-8 with one disto-vental long plumose swimming seta, respectively; articles 3-8 with one spine on disto-dorsal edge; article 1 very long, article 2-9 more and more shorter; article 9 very short with four long plumose setae on tip.
Mandible: Limb biramous (Fig. 2I). Coxale grand, endite with bifurcated tip and cluster of spines (Figs. 2J, 3H). Basale grand, dorsal margin slightly humped with one mid-dorsal seta and two distal plumose setae; ventral margin with a group of proximal short setae, seven plumose setae and one short medio-ventral seta. Exopod tiny with two equilong plumose setae on tip (Figs. 2L, 3G). Endopod with three articles. Article 1 with a group of five setae on disto-ventral margin (two long plumose and three short). Article 2 longer than one; dorsal margin with a group of two long proximo-dorsal setae (one bare and one plumose), a group of four mid-dorsal long setae; ventral margin with a group of two mid-ventral setae (one short and one long plumose), a group of three bare disto-ventral setae (inner one short, outer two equilong). Terminal article very small with two claws and four setae on tip (Figs. 2K, 3I): disto-dorsal claw biggest with numerous spines on distal half ventral margin, short seta, big claw with numerous spines on distal half ventral margin, longest seta, long seta and shortest disto-ventral seta.
Maxilla (Figs. 2M, 3J): Coxale with one plumose seta on disto-dorsal edge. Basale with two disto-ventral long plumose setae. Exopod small with three long distal plumose setae. Endopod with two articles. Article 1 long with one long and two short dorsal setae, and three disto-ventral setae. Article 2 very short, with one very small seta, two plumose setae, three claws and four plumose setae on tip. Maxilla with three endites. Endite I with seven plumose and one serrated setae. Endite II with two plumose and one serrated setae. Endite III with nine plumose and one serrated setae.
Fifth limb (Figs. 4A, 5A-5B): Coxale with three endites. Endite I with four plumose setae. Endite II with two plumose setae and one claw. Endite III with three plumose setae and five claws, inner claw very strong. Exopod with five articles. Article 1 with one plumose and one bare setae on mid-distal margin, main tooth comprising two slices of constituent teeth, medial teeth smooth, lateral teeth jagged. Article 2 with one long bare and one small plumose setae, and two long claws on posterior side. Article 3 with two plumose and one long bare setae on inner lobe and two short slender plumose setae on outer lobe. Articles 4 and five fused, with nine distal plumose setae. All claws of this limb with numerous disto-half ventral spines.
Seventh limb: Limb with about fifty-two articles (Figs. 4C, 5D). All articles very short. Article 40 with one disto-ventral seta with two bells. Article 41 with one disto-dorsal seta with two bells and one disto-ventral seta with three bells. Article 42 with one dorsal seta with three bells. Article 44 with one ventral seta with three bells. Article 45 with one disto-dorsal seta with three bells. Article 52 with two long setae with five and three bells respectively. Terminal article with one long and two short dorsal setae with five, four and three bells, respectively. Comb with six teeth, side opposite comb with two bare bent small pegs. Comb teeth from outside to inside from long to short sequence (Figs. 4D, 5E).

DISCUSSION
According to Chen's key of family Philomedidae Müller, 1906(Chen & Lin, 1995, the current specimens separated from the other philomedids with the following characteristics defining the genus Euphilomedes: (1) the carapace is elongate oval in lateral view with pits and setae, the posterior margin is evenly rounded; (2) the rostrum is broad anteriorly, and the incisure is shallow (compared with other philomedids); (3) article 4 of the first antenna has one to four ventral setae; (4) the endopodal article 2 of the second antenna has two ventral setae; (5) the anterior triangular protuberance of the main tooth of the fifth limb has denticulate margin, the inner lobe of article 3 has three setae, and the outer lobe has two setae; (6) the seventh limb has six to nineteen cleaning setae, the comb has less than fifteen teeth; (7) the furcal lamella is not fused with the main claws, the secondary claws are alternating with the main claws, the edge between furcal lamella and claws has long cilia. With this new species, the genus Euphilomedes contains 32 recent species thus far (Brandão et al., 2017).
Like E. africanus (Klie, 1940), E. bradyi Poulsen, 1962 andE. walfordi Poulsen, 1962, the new species has a row of teeth along the inner margin of article 3 of the endopod of the second antenna. However, E. biacutidens sp. nov. differs from these three closely related species (Table 1)    of setae on the tip of the first antenna between these species (detailed differences are given in Table 1); (7) the endopodal article 3 of the second antenna has about twelve small ventral acute teeth and an uneven inner margin (Figs. 2F-2G); (8) the numbers of setae on the endopod of the mandible, endopod and endites of the maxilla, endopod and endites of the sixth limb have significant differences (detailed numbers are given in Table 1); (9) some setae on the fifth limb have developed into long claws (Figs. 4A, 5B); (10) the comb of the seventh limb has six teeth and the side opposite comb has two bare bend pegs (Figs. 4C-4D, 5D-5E); (11) the furcal lamella has twelve claws, the first claw has dorsal and ventral sawteeth (Figs. 4E-4F, 5F-5G). The obvious characteristics of E. biacutidens sp. nov. are the postero-dorsal and posteroventral spines on the right valve; E. sinister Kornicker, 1974 (including two subspecies: E. sinister sinister Kornicker, 1974 andE. sinister pentathrix Kornicker &Caraion, 1977) also shows posterior spines, which is known only in the adult female. However, both species can be easily distinguished from each other by the following remarkable differences ( Table 2): (1) they have different carapace ornamentation; (2) E. biacutidens sp. nov. has the postero-dorsal and postero-ventral spines on the right valve (Fig. 2C, Fig. 3F), but in E. sinister the spines are on the left valve; (3) there are about 32 very long filaments on the sensory seta of E. biacutidens sp. nov., and only five short filaments and three long bifurcated filaments on the sensory seta of E. sinister, and there are significant differences of the setae on the tip of the first antenna between these species (detailed differences are given in Table 2); (4) E. biacutidens sp. nov. has two mandibular claws, E. sinister has three; (5) they have significant differences in the numbers of setae on the endites of the maxillae and the fifth limbs (except endite III of maxilla, with detailed numbers given in Table 2); (6) E. biacutidens sp. nov.has more cleaning setae on the seventh limb, but fewer teeth on the comb (Figs. 4C-4D, 5D-5E).
Additionally, E. biacutidens sp. nov. shows some long claws instead of setae on the fifth limb (Figs. 4A, 5B); this is a diagnostic characteristic of the species and is an unusual characteristic in the genus. The rostrum has a pointed bifurcated ventral corner (Figs. 2B, 3E), which is also a distinctive characteristic not previously observed in the genus.
Finally, the distance between the sampling localities of the holotype and paratypes indicates that the new species may be widely distributed southeast off China (Fig. 1).