A bathypelagic ostracod Conchoecissa nigromaculatus sp. nov. (Myodocopa, Halocyprididae) from the South China Sea

Pelagic ostracods are one of the main groups of zooplankton and are abundant in marine ecosystems worldwide. The record of marine planktonic ostracod species in the central and southern part of the South China Sea accounts over for one-third of the total recorded marine planktonic ostracods in seas around China. In this study, we examined and compared the specimens from a recent cruise in this region that appeared to be different from previously described species of genus Conchoecissa, and then confirmed them as a new bathypelagic species Conchoecissa nigromaculatus. These specimens clearly differed from the other species of genus Conchoecissa with differences observed in the size, carapace, locations of glands, mandible, maxilla, sixth limb, and furca. In this species, mandibular coxal endite has no ventral finger process, maxilla has prominently large endites and has only two claws on the tip, the sixth limb has very simple endites, and this species has distinctive features not previously observed in the tribe Conchoeciini before. It is therefore necessary to emend the diagnosis of this group.


INTRODUCTION
Marine zooplankton form a main link between primary producers and higher trophic levels in the food web of marine ecosystems, and are a key element in the fluxes of carbon through the ocean surface to bottom. Pelagic ostracods occur throughout oceanic water columns and are often the second-or third-most abundant taxon in mesoplankton samples particularly at sub-thermocline depth. Despite their abundance, the importance of their role in influencing carbon fluxes are often overlooked because they are generally considered to be difficult to identify. The classification of pelagic ostracods is based on morphological features that are highly diverse. The group may not be monophyletic, and phylogenetics of the species remains ambiguous (Fortey & Richard, 1997;Yamaguchi & Endo, 2003). In last two decades, the taxonomy and ecology of marine ostracods has been extensively

MATERIALS AND METHODS
The collections that included the novel species were made during a cruise in the southern South China Sea from December 2013 to January 2014, between latitudes 7 • N to 12 • N and longitudes 110 • E to 115 • E.
All zooplankton samples were collected using a Hydro-Bios Multinet system (Type Midi, mouth 0.25 m 2 , mesh-size aperture 200 µm; HydroBios Inc., Kiel, Germany) by vertical and stratified hauls from bottom to surface. Specimens were photographed live using a Canon 7D camera fitted with an EF100 mm USM Micro lens (Canon Inc., Tokyo, Japan), before being preserved in 5% buffered formaldehyde for preservation.
The specimens were dissected under a zoom-stereomicroscope system (Discovery V20; Zeiss, Oberkochen, Germany) and mounted on permanent slides with CMC-9AF mounting medium (Masters Company Inc., Wood Dale, IL, USA). Observations, measurements, micrographs and drawings were made following the methodology of Chavtur & Angel (2011) using a transmitted-light binocular microscope combined with a differential interference contrast system (Axio Imager Z2) and an AxioVision Image-Pro software (Carl Zeiss Inc., Oberkochen, Germany). All drawings were made of preserved specimens using a camera Lucida and drawing apparatus. Figures were finally prepared with Adobe Photoshop CS6 software (Adobe Inc., San Jose, CA, USA). Specimens/appendages have been archived in the Marine Biological Sample Museum, the Third Institute of Oceanography, SOA (Xiamen, China), under the collection Nos. TIO-OHHZe-01 and TIO-OHHZe-02.

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:D5DFB0CC-37C5-4A75-BB4B-5A4FF9A81747. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.  Segment 2 twice as long as segment 1, and with one long dorsal plumose seta. Segments 3-6 quite short. Segments 3 and 4 bare. Segment 5 with two ventral sensory setae: a-and b-setae. Segment 6 with two equilong ventral sensory setae and one distal sensory seta: cto e-setae. A-to d-setae subequal, bare and a third length of e-seta. E-seta with short spines along its trailing edge. Second antenna (Figs. 2D, 2F, 2I, 5A-5D): Antenna biramous. Protopodite extremely large with powerful muscles. Endopod without c-, d-, e-setae. Endopod with three segments. Segment 1 with inflated base, small processus mamillaris, a-seta short curved and half length of b-seta also curved with two clusters of long cilia on its mid-ventral margin. Segments 2 and 3 fused with five setae: h-and i-setae thin and spinous; f-and g-setae three times length of h-seta, with flat distal half; j-seta two times length of h-seta; all five setae with bare distal half. Exopod with nine segments. Segment 1 long and bare, segments 2 to 8 with one long plumose swimming seta with tiny spines, respectively; seta 1 with flat distal half part. Segment 9 with one plumose seta, one long acerose seta and one very small acerose seta, arranged from long to short sequence. Mandible (Figs. 2E, 2G-2H, 3A, 5E-5H): Basale large. Exopod tiny thumb shaped process with a single long plumose seta on tip. Endopod with three segments. Segment 1 with one disto-dorsal and three ventral long spinose setae. Segment 2 with two short setae and one long claw with disto-half ventral short spines, on disto-dorsal margin; one quite long and one short setae with disto-half ventral short spines, on disto-ventral margin. Segment 3 with one row of dorsal cilia on distal part and seven setae on tip: four short disto-ventral setae, two long knife-shaped claws with several disto-half short ventral spines, and one small bare seta between claws. Toothed edge of basale with four long slim spinose setae on proximal margin, three clusters of short setae on medio-dorsal side, short medial cilia on proximo-ventral margin, and two lists of teeth, eight on distal list and seven on proximal list. Coxal endite constituting by two parts: medio-ventral part with one big triangular tooth, four comb teeth and three little teeth; medio-dorsal part with four long teeth, four plate teeth, one comb tooth, one little tooth, one cluster of short soft cilia, one cluster of long soft cilia, one row of long sclerotic outer spines and one row of short sclerotic inner spines. Coxal endite without ventral finger process. Maxilla (Figs. 3B-3C, 6A-6C): Maxilla with three large endites. Each endite with a row of medial cilia. Endite I with about seven spinose setae and five papillae. Endite II with two large medio-anterior plumose setae, two plumose distal setae and about seven long papillae.

Systematic account
Endite III with about three short plumose setae, two short and two long papillae. Exopod with two spinose setae on tip. Endopod 1 with two long anterior setae, four antero-distal setae, one long spinose lateral seta, three posterior spinose setae, one disto-posterior plumose seta and one spinose medial seta. Endopod 2 with six setae arranged from long to short sequence: seta 1with dorsal spines, seta 2 plumose, setae 4 and 6 with ventral spines, setae 3 and 5 claw-shaped with several ventral spines; setae 2 and 4 with one cluster of long cilia on base. Fifth limb (Figs. 3D, 6D): Endopod with three segments. Segment 1 large with four groups of setae: proximo-ventral group with two long plumose and three short setae; mid-ventral group with two bare setae; disto-ventral group with one long spinose and two shorter setae; disto-dorsal group with two longer setae. Segment 2 long and thin with two ventral and one dorsal setae. Segment 3 short and small with three bare and long acerose setae on tip.
Epipod with one small bare inner seta and about fourteen long flexible setae. Sixth limb (Figs. 3E, 6E): Coxale broad and short with one pair of short disto-ventral setae, one long disto-ventral seta, one long and one short disto-medial setae. Basale large and long, with one pair of long mid-dorsal setae, one disto-dorsal seta (vestige of exopod), one proximal long seta, one pair of long mid-ventral setae, one pair of disto-ventral setae and one long seta on disto-ventral edge. Endopod with three segments. Segment 1 long, with one mid-ventral seta. Segment 2 equilong to segment 1 with one mid-dorsal, one mid-ventral setae, and one long acerose on disto-ventral edge. Segment 3 very short and bluntly conical, with three distal strong long acerose setae. Limb with three simple endites, endites with two bare setae on tip, respectively. Epipod with one small bare inner seta and about seventeen long flexible setae. Seventh limb (Figs. 3F, 6F): Limb simple, uniramous with two segments. Segment 1 slender and bare. Segment 2 very short and conical with two long flexible setae, the dorsal one about twice the length of another. Furca (Figs. 3G, 6G): Each furcal lamella with one large ringed dorsal claw seta and seven claws with small ventral spines, arranged from long to short sequence. Furca without unpaired seta.

DISCUSSION
Initial examination identified these specimens in tribe Conchoeciini by features according to Chavtur and Angel's key (Chavtur & Angel, 2011): (1) full adult size is in the range of 0.6 to 6.5 mm; (2) one asymmetric gland opens near postero-dorsal margin of left valve; (3) postero-dorsal corner is sharply angled; (4) segment 2 of first antenna carries a dorsal seta; (5) tip of 1st antenna bears five sensory setae; (6) endopod of second antenna has a processus mamillaris on anterior margin; (7) seventh limb is degraded; (8) each furca has one large seta and seven claws. Then they were tentatively identified as a species of genus Conchoecissa (Claus, 1890) by combined display of the characteristics (Chen & Lin, 1995;Angel, Blachowiak-Samolyk & Chavtur, 2008;: (1) shape of carapace is similar, posterior part of carapace is highest; (2) carapace surface is highly ornamented with polygonal pits; (3) rostrums are long and bent downward with pointed tip; (4) postero-dorsal corners have distinct but unequal spines; (5) postero-ventral corners are developed into tubercles; (6) setae of first antenna are homothetic; (7) endopod 1 of second antenna has a-and b-setae; (8) length of mandibular coxale is more than 1/4 of total length of mandible; (9) maxillary endopod 1 has six anterior setae; (10) caudal furcas are same, claws are long sickle shaped with numerous ventral small spines. They carapace outline is very similar to that of C. plinthina. Detailed examination of the specimens showed they are not conspecific with any of the previously described Conchoecissa species (Müller, 1906;Chen & Lin, 1995;Chavtur, 2003;Chavtur & Angel, 2011;Drapun & Smith, 2012). For example, in comparisons between the specimens and the known Conchoecissa species (Table 1), it is obvious that the specimens are prominently bigger than known Conchoecissa species, and about 10% longer than C. plinthina, which is the biggest species of genus Conchoecissa, but the percentage of the 1st antenna to carapace is very much smaller in the novel species than in C. plinthina (about 49.0%: 71.4%, respectively). This novel species differs from the other Conchoecissa species in other several important characteristics (Table 2): (1) Each valve has one edge gland opening on antero-ventral margin, respectively.
(2) Postero-dorsal corners of the carapace are not-armed with spines. C. squamosa also lacks spines, but is obviously dissimilar. (3) E-seta of first antenna is armed both dorsally and ventrally with fine spines, whereas the other Conchoecissa species have only ventral spines.  Angel, 1935Angel, , 1972Angel, -1976. The data are expressed as percentages of the carapace length. (4) Mandibular coxal endite has no ventral finger process. It is noteworthy that many species of this tribe that have their gland locations shifted from basic locations are deep-living, usually associated with the secretion of bioluminescence. (Chavtur & Angel, 2011). The erection of this new species is based on carapace and locations of glands, as well as setae and structures of the limbs. However, Drapun proposed that some of the setal counts may well prove to be erroneous (Chavtur & Angel, 2011). Thus, the shape of carapace and locations of glands are just as significant as taxonomic characteristics of the tribe Conchoeciini (Müller, 1894;Angel & Iliffe, 1987). For example, the diagnosis of genus Conchoecilla (Claus, 1890) is indicated by one asymmetric gland located near postero-ventral margin of the left valve. Similarly, Poulsen (1973) identified genus Gaussicia as having one compound gland located on mid-ventral margin of the right valve. Moreover, Chavtur & Angel (2011) utilized the different locations of asymmetric gland on the dorsal margin to revise genus Metaconchoecia (Granata & Caporiacco, 1949) and erected another nine genera Juryoeoia, Deeveyoecia, Vityazoecia, Muelleroecia, Nasoecia, Austrinoecia, Clausoecia, Kyrtoecia and Rotundecia. To be consistent with these previous obsevations, we Remarkably, the mandibular coxal endite has no ventral finger process, maxilla has prominently big endites and has only two claws on tip, the sixth limb has very simple endites, all distinctive features not previously observed in tribe Conchoeciini before. It is therefore necessary to emend the diagnosis of this group.

Species
Due to the rapid development of deep-sea science in the past 10 years, many uniquely deep-sea organisms have been discovered (Brandt et al., 2007;Rogers et al., 2012;Rogacheva, Gebruk & Alt, 2013;Rouse et al., 2016;Kou, Gong & Li, 2018). However, our understanding of deep-sea organisms is still greatly inadequate, and a large number of deep-sea organisms are still unknown. The bathypelagic halocyprid specimens in this study were collected from the southern South China Sea in a region close to an ocean trench and an adjoining coral reef marine biodiversity hotspot. This kind of region represents a large reservoir of biomass with a large proportion of undiscovered biodiversity, (Roberts et al., 2002;Gianni, 2004;Danovaro et al., 2008). Our examinations of these specimens confirmed a novel bathypelagic halocyprid species, which increases the number of the planktonic ostracod species in this area. We can expect that there is also a highly diverse ostracod fauna in this region.

CONCLUSIONS
The family Halocyprididae is a diverse group of ostracods. In this study, we described a new bathypelagic species C. nigromaculatus that differed from the other halocyprid ostracods in having the combination of characteristics of size, glands, e-seta of the first antenna, coxal endite of mandible, endites of maxilla, setae and structure of the sixth limb, and unpaired seta of furca. The mandibular coxal endite has no ventral finger process; maxilla has prominently big endites and the sixth limb has very simple endites; have distinctive features not previously observed in tribe Conchoeciini before. Our examinations of the specimens confirmed a novel bathypelagic halocyprid species, which increases the records of planktonic ostracods in the central and southern part of the South China Sea.
Finally, more detailed observations of these species are needed in order to update the diagnosis of this group, and more intensive studies are needed in order to reveal cryptic diversity of ostracods in the relatively underexplored abyssal South China Sea.