Bathyconchoecia incisa sp. nov. (Myodocopa, Halocyprididae), a new species of ostracod from the neritic zone of the South China Sea

Specimens previously identified as Bathyconchoecia paulula sensu Yin & Chen, 1991 and Bathyconchoecia galerita sensu Yin & Chen, 1991, together with two similar specimens, are reclassified as a new species, Bathyconchoecia incisa sp. nov., which we describe and illustrate in detail here. The present species is similar to Bathyconchoecia sagittarius and Bathyconchoecia kornickeri in having a shallow notch on the posterodorsal corner of the carapace, but can be easily distinguished from these species by the structure of the posterior margin of the carapace and the striated sculpturing on the surface of the valves. The specimens on which this new classification was based were collected at shallow depths (<125 m) off the east coast of Hainan Island, in the northern South China Sea and in the Zengmu (James) Shoal and adjacent areas in the southern South China Sea. B. incisa is likely to be a shallow-water species or has a wide bathymetric range.


MATERIAL AND METHODS
During the 1980s and 2000s researchers from the South China Sea Institute of Oceanology, Chinese Academy of Sciences, using 0.505 or 0.169-mm mesh plankton nets, collected large numbers of samples of zooplankton from the southern South China Sea and the northwest continental shelf of the South China Sea, respectively (Yin & Chen, 1991;Xiong et al., 2012). Among the samples, four specimens of Bathyconchoecia incisa sp. nov. were recorded, and the sites where they were found are shown in fig. 1.
The carapace length (CL), height and width of each specimen were measured using a stereomicroscope at 36× magnification with a calibrated eyepiece micrometer. The specimen was dissected, and the limbs were placed on a series of slides under a stereomicroscope. The limbs and setae were measured using a microscope and an eyepiece micrometer. These measurements were standardized by expressing them as percentages of the carapace length (% CL). The complete animal and its limbs were drawn using camera lucidas. The illustrations for publication were inked from these penciled drawings. One of the paratype specimens (SCSMBC008023) was photographed using a Leica MC120 HD microscope camera. DESCRIPTION  Bathyconchoecia pauluta Deevey, 1968-Yin & Chen, 1991. Bathyconchoecia galerita Deevey, 1968-Yin & Chen, 1991. Not Bathyconchoecia pauluta Deevey, 1968: 547-551, figs. 1-3. -Poulsen, 1972Kornicker, 1991: 28-30, figs. 14-19. Not Bathyconchoecia galerita Deevey, 1968: 557-561, figs. 8, 9. Holotype.   . 3C).-The limb is typical of the genus. It is approximately 24.8% CL and appears to comprise six segments (Angel & Graves, 2013). The fifth segment carries distodorsally a plumose seta (26.4% CL) and ventrally a cluster of 200-210 sensory filaments (approximately 40.5% CL), which are arranged in ten rows, each row comprising 20-21 filaments. The plumose seta is approximately two-thirds of the length of the sensory filaments. The terminal segment carries four setae distally. The principal seta is obviously longer than the sensory filaments, and the remaining three setae are almost subequal and shorter than the sensory filaments. The main seta is 53.7% CL and the other three setae are 33.1, 33.9 and 31.4% CL, respectively.
Second antenna ( fig. 3D, E).-The limb is typical of the genus. The protopodite is ham-shaped (40.5% CL), and its length is obviously longer than its width. The exopodite comprises nine segments. The first exopodite segment (23.1% CL) is slightly bent and carries a small terminal seta that reaches the third segment. Segments 2-9 each carry a long swimming seta and have a combined length of 9.9% CL. The endopodite comprises two segments. The first endopodite segment (13.2% CL) is elongate and almost rectangular, and one third of the length of the protopodite. It carries two distodorsal setae. The a-seta (4.8% CL) is bare and less than half of the length of the b-seta (11.7% CL). The b-seta has distally marginal spinules and is shorter than the first endopodite segment. The second endopodite segment carries five setae. The f-, g-, h-, i-and j-setae are 71.9, 49.6, 13.2, 24.8 and 13.2% CL, respectively. They are all longer than the b-seta of the first endopodite segment, the longest of which is longer than the swimming setae. The swimming seta of the second exopodite segment is 47.1% CL. Mandible ( fig. 3F-H).-The toothed edge of the coxal endite has five cutting teeth, which diminish in size along the distal edge; the anterior edge is serrated. The proximal and distal tooth lists have a quantity of denticles, which are irregular in size and arrangement. The masticatory pad has numerous robust, long columnar spines with spinous tips, and a large finger-like spinose process. The toothed edge of the basal endite has six triangular cutting teeth with secondary serrations, and two pointed spine teeth. The basal endite has four plumose setae near the insertion of the endopodite and four bare setae on the anterior edge and lateral surface of the endite. The first endopodite segment has one distodorsal seta and three ventral setae. The second endopodite segment has three distodorsal setae and one ventral seta. The end segment has seven terminal setae, one of which is extremely long (35.5% CL) and exceeds the length of the endopodite.
Maxilla ( fig. 4A, B).-The precoxal endite has six setae, two of which are tubelike. The coxal endite is distally divided into two lobes. It has a total of 10 setae, three of which are tube-like. The basis has one plumose and one bare setae. The endopodite comprises two segments. The anterior margin of the first endopodite segment has a group of three bare setae proximally and one setae distally; the posterior margin has a stout claw-like seta and three plumose setae medially. The terminal segment has two stout terminal claws with spines on the posterior edges, and a pair of slender setae on either side.
Fifth limb ( fig. 4C).-The epipodial setae are in three groups of five, four, and four, respectively (not shown). The precoxa has three bare ventral setae. The coxa has eight setae (two plumose and six bare) and two claws on the ventral face. The basis has four bare ventral setae, a pair of plumose lateral setae, and one long distodorsal seta (a remnant of the exopodite, see Angel, 2012) that extends past the end of the limb. The endopodite comprises three segments. The first segment has four bare ventral setae. The second segment has a bare dorsal seta and four bare ventral seta. The third segment has three terminal setae, two of which are clawlike and stout. The dorsal seta (12.4% CL) is slightly longer than the central one (10.7% CL). Sixth limb ( fig. 4D).-The epipodial setae are arranged in three groups of seven, five and five, respectively. The coxa has a short, bare seta distoventrally. The basis has four bare ventral setae, and an extremely long distodorsal seta (a residual exopodite, see Angel, 2012) that extends beyond the end of the limb.
The endopodite comprises three segments. The first segment has three pairs of bare ventral setae. The second segment has a ventral and a dorsal setae. The third segment has three terminal setae, of which the dorsal seta (16.5% CL) is slightly longer than the central setae (15.7% CL).
Seventh limb (fig. 4E).-The limb is small and short, and the suture between the two segments is indistinct. The terminal segment bears two unequal and bare setae terminally.
Caudal furca ( fig. 4F).-Each lamella has eight long, slim, weak claws, all with two rows of spines on the posterior edge. The first claw is five jointed (20.7% CL). There is a long unpaired seta behind the claws.
Immature male (A-1 developmental stage, see Kornicker & Angel, 1975). Carapace ( fig. 5A-C).-The carapace is little shorter than that of the female, but is slightly relatively longer. The length is 1.20 mm, the height is 0.77 mm (64.2% CL), and the width is 0.74 mm (61.7% CL). The incisure is 13.3% CL. The ventral margin is almost straight. Otherwise, most of the carapace features are similar to those of the female.
Frontal organ.-There is no frontal organ. First antenna ( fig. 5D).-This limb is similar to that of the female, apart from the fifth segment, which only carries 170-180 ventral sensory filaments (approximately 36.7% CL), which are arranged in ten rows, each comprising 17-18 filaments. The plumose seta on the fifth segment is 20.8% CL and the four terminal setae are 47.5, 25.0, 27.5 and 25.0% CL, respectively.
Second antenna ( fig. 5E, F).-The left and right limbs are similar. The protopodite is pear-shaped (38.3% CL), and is almost as broad as it is long. The basal half of the first exopodite segment (19.2% CL) is bent, but is straight distally. The overall length of the remaining eight exopodite segments is 10.0% CL. The endopodite comprises three segments. The first endopodite segment is almost rectangular and relatively short (12.5% CL). The second endopodite segment has two terminal setae that are relatively short (37.5 and 55.8% CL, respectively). The third endopodite segment is small and knob-like, and bears three terminal setae of various lengths.
-This limb appears to be similar to that of the female, except that one of the two spine teeth on the toothed edge of the basal endite is tubular. The longest terminal seta is 26.6% CL.
Maxilla ( fig. 6A, B).-This limb is similar to that of the female, apart from the fact that all the claws and setae on the endopodite are bare.
Fifth limb ( fig. 6C).-The epipodial setae are in three groups of four (not shown). The precoxa has three bare ventral setae. The coxa has seven setae (two plumose, five bare) and two claws on the ventral face. The basis has five bare  ventral setae, one bare lateral seta, and one long distodorsal seta that extends past the end of the limb. The endopodite comprises three segments, but its segmentation is indistinct (see Angel & Graves, 2013). The first segment has three bare ventral setae. The second segment has a bare dorsal seta and three bare ventral seta. Unlike in the female, the dorsal and central setae of the third segment are almost equal in length (approximately 10.0% CL). Sixth limb ( fig. 6D).-This limb is similar to that of the female, except that the first endopodite segment has only two pairs of bare ventral setae. The dorsal and central setae of the third endopodite segment are equal in length (15.0% CL).
Seventh limb ( fig. 6E).-This limb is similar to that of the female. Caudal furca ( fig. 6F).-This has only seven pairs of claws and a long unpaired seta behind the claws. The first claw is four-jointed and it is shorter than that of the female (15.0% CL).
Remarks.-The present species differs from other species of the genus Bathyconchoecia, except from B. sagittarius and B. kornickeri, in that the posterodorsal corner of carapace has a shallow notch (Deevey, 1968;Angel, 1973Angel, , 2012. However, it is easily distinguished from these two species by lacking a row of serrations or a flange of sculpturing on the posterior margin of the carapace and by the absence of vertical or irregular striations on the surface of the valves.

DISCUSSION
Most Bathyconchoecia are deep-water species, with only B. georgei and B. angeli (excluding Scottoecia species) having been collected in the upper 200 m (George, 1971(George, , 1977Kornicker & Rudjakov, 2004). Angel & Graves (2013) highlighted that all records of Bathyconchoecia species from depths <500 m are from upwelling regions. Unusually, all specimens of B. incisa sp. nov. were collected at shallow depths (<125 m) off the east coast of Hainan Island and in the Zengmu (James) Shoal and adjacent areas. The former is a coastal upwelling region (Yin et al., 2011), but the latter is not. Therefore, it is likely that B. incisa is a shallow-water species or that it has a wide bathymetric range.