The Co To Islands are located in the Gulf of Tongking in the northwestern part of the South China Sea. The outline of the Gulf of Tongking is crescent-shaped, and its depth is generally less than 60 m. The Red River (Song Hong River) and some other rivers flow into the Gulf of Tongking and form deltas and estuaries. The Co To Islands are an archipelago located about 30 km offshore from the northeastern coast of Vietnam (Fig. 1). The Co To Islands consist of 40 islands. The main islands are the Co To Island, Thanh Lan Island, and Tran Island (Fig. 1).

The coastal areas of the Co To Islands are characterized by rocky shores and tidal flats. In this study, we collected surface sediments at 10 sites (Fig. 1, black stars; Table 1) by dredging with a skimming net sampler either from shore on tidal flats at low tide (at Sts. 1–4) or towing the sampler from a small boat (at Sts. 5–10). The sediments were immediately preserved in 70 % ethyl alcohol and washed through 1 mm and 63 µm sieves. After being completely dried, ostracod specimens were picked out under a stereoscopic microscope (Olympus SZX7). Specimens with a carapace, disarticulated valves, juvenile valves, and fragmented valves were also counted as one specimen. Ostracod specimens were identified under the stereoscopic microscope and photographed using a scanning electron microscope (Hitachi TM-1000, Tokyo, Japan; JEOL JSM-6360LV). We conducted a cluster analysis to clarify the similarities among ostracod assemblages from different sampled sites. We used six samples (Sts. 3, 5, 6, 7, 8, 9, and 10) at which more than 50 ostracod specimens occurred. Thirty-one species (significantly occurred below 5 % rejection level) were used for the cluster analysis. Q-mode cluster analysis using the weighted pair-group arithmetic average method was performed using the Horn's overlap index (Horn, 1966). Q-mode cluster analysis was calculated with the free software package Paleontological Statistics (PAST; Hammer, 2013). To clarify the biogeographical relationships among ostracod fauna, we compared the Holocene ostracod assemblages from the Co To Islands with assemblages from adjacent areas (Table 3). The areas and studies from which we extracted information were as follows: Australia (Hartmann, 1981; Howe and McKenzie, 1989; Warne et al., 2006), Strait of Malacca (Whatley and Zhao, 1987a, 1988; Yamada et al., 2014), Sunda Shelf (Zhao and Whatley, 1989; Mostafawi, 1992), southern China (Cai, 1982, 1988; Zheng, 1994; Hu and Tao, 2008), East China Sea (Zhao, 1984; Wang et al., 1988; Zhao and Wang, 1988; Tabuki and Nohara, 1990; Hu and Tao, 2008), and Japan (Tanaka et al., 1998; Irizuki et al., 2008; Tanaka, 2008). To reveal the similarity of ostracod assemblages in northeastern Vietnam, we calculated the value of the Jaccard similarity coefficient in terms of species (Js) and genera (Jg) between the Co To Islands and the northeastern coast of Vietnam (Fig. 5).

A total of 77 species were identified from nine samples from the Co To Islands (Table 2). Q-mode cluster analysis revealed three biofacies (I, II, and III), all bounded by a within-group similarity value of 0.5 (Fig. 2) as follows. Biofacies I: this biofacies was composed of four samples (Sts. 3, 6, 7, and 9), which were from rocky shore, bay-mouth, and offshore sites with depths of less than 6.5 m. Aurila hataii and Loxoconcha japonica commonly occurred in this biofacies. Neonesidea elegans sensu lato (s.l.), Neonesidea sp. 1 and Xestoleberis hanaii also predominated in this cluster (Figs. 3, 4). The ratio of carapaces or living individuals present in these samples was relatively high, except for N. elegans s.l. and Neonesidea sp. 1. Biofacies II: this biofacies was represented by two samples (Sts. 8, 10), which were collected offshore at a depth of 5.0 m and also near a sandbar at a depth of 3.5 m. It was characterized by the dominance of Xestoleberis hanaii. Also, other dominant species differ between the two samples: Propontocypris crocata in St. 8 Loxoconcha uranouchiensis in St. 10 (Figs. 3, 4). Biofacies III: this biofacies consisted of one sample (St. 5) from the midbay at a depth of 2.5 m. This biofacies was dominated by Caudites huyeni and Loxoconcha cotoensis sp. nov. Species Tanella glacilis s.l. and Loxoconcha vietnamensis (Figs. 3, 4) are also characterized by this biofacies. The total ratio of living individuals to all specimens was relatively high (about 50 %) in biofacies III. Whole synonym list of dominant species is shown in the Supplement.

Table 3 shows the distributions of 43 species that have been reported in Holocene materials from the Co To Islands and its adjacent areas. Three species (Caudites huyeni, Cornucoquimba gibboidea, and Loxoconcha vietnamensis) were excluded from this comparison because they occurred at the northeastern coast of Vietnam only in Holocene and late Holocene materials (e.g., Tanaka et al., 2009, 2011b). The remaining species were used for further analysis of the faunal assemblage in the Gulf of Tongking. Thirty-seven species from the Co To Islands have also been reported from southern China (Cai, 1982, 1988; Zhao and Wang, 1988; Gou, 1990; Zheng, 1994; Hu and Tao, 2008). Twenty-seven species from the Co To Island are found in the East China Sea (Zhao, 1984; Zhao and Wang, 1988; Tabuki and Nohara, 1990; Zhou, 1995; Hu and Tao, 2008). Three species (Keijia demissa s.l., Kotoracythere inconspicua s.l., and Robustaurila splendideornata) widely occur in the west Pacific region (Australia, Strait of Malacca, Sunda Shelf, Vietnam, southern China, and East China Sea). Among them, K. demissa s.l. and K. inconspicua s.l. are known to be stenothermal species, which have an extremely wide habitat across the Indo-Pacific Ocean and in Australia (Teeter, 1973; Titterton and Whatley, 1988; Zhao and Whatley, 1989). Eleven species (Aurila cymba, Aurila hataii, Bicornucythere bisanensis s.l., Bythoceratina cassidoidea, Callistocythere asiatica, Cythere omotenipponica, Loxoconcha japonica, Loxoconcha kattoi, Loxoconcha ocellata, Loxoconcha uranouchiensis, and Pontocythere miurensis) have been distributed in East Asia (southern China, East China Sea, and Japan). In summary, the ostracod assemblage from the Co To Islands is most closely similar to that of the southern China one.

To investigate the relationships between characteristic ostracod species of the Co To Islands and its adjacent areas, we focused on the dominant species of three ostracod biofacies (I–III). The dominant species (Aurila hataii and Loxoconcha japonica) of Biofacies I live on seaweed and algae in the littoral zone (Kamiya, 1988; Nakao et al., 2001). These two species are widely distributed along the coasts of Japan and China (e.g., Ishizaki, 1968, 1971; Wang et al., 1988; Ruan, 1990; Masuma and Yamada, 2014). In terms of fossil records, Biofacies I is similar to cluster II of the Shimajiri Group reported by Tanaka and Nomura (2009). Xestoleberis hanaii is the most abundant species in Biofacies II. X. hanaii lives on calcareous algae on intertidal rocky shores, around Zostera beds, and Sargassum beds (Okubo, 1984; Kamiya, 1988; Ikeya and Kato, 2000). In the Co To Islands, X. hanaii lives around Sargassum beds. X. hanaii has been reported in Japan and China (Kamiya et al., 2001; Nakao et al., 2001; Irizuki et al., 2006; Wang et al., 1988). The species composing Biofacies III (St. 5) is similar to the northeastern coast of Vietnam (Sts. 14 and 15 of Tanaka et al., 2009). We compared the differences in faunal composition among the Co To Islands and the northeastern coast of Vietnam by using the Jaccard similarity coefficient (J). The results revealed that the assemblages of the Co To Islands showed low similarity to that of the northeastern coast of Vietnam (Js = 0.16, Jg = 0.39). The fauna of the Co To Islands and the northeastern coast of Vietnam were less similar than might be expected, although these two areas are relatively close (shortest distance is about 35 km between St. 4 of this study and St. 15 of Tanaka et al., 2009). In the Co To Islands, the ratio of phytal species to the total number of specimens is quite high. On the other hand, the number of species which live in a wide range of salinity such as Sinocytheridea impressa is low or the species is absent in the Co To Islands.

The result of biogeographic comparison showed that the ostracod assemblage in the Co To Islands has a strong connection with the East Asia seas (Table 3). Our report supports the opinion of Tanaka et al. (2009) that the northern Vietnamese shallow-marine ostracod fauna belongs to the Kymerian Provence of Titterton and Whatley (1988), even for islands situated in open-sea areas. Although, the ostracod assemblage in Weizhou Island is reported by Ruan (1990) (Fig. 5). The ostracod assemblage in Weizhou Island is characterized by the dominance of Loxoconcha japonica, Robustaurila splendideornata, and Loxoconcha alata. In addition to these three species, some phytal species, such as Aurila hataii, Neonesidea elegans s.l., and Xestoleberis hanaii, are also common there, and these species also occurred commonly in the Co To Islands and some other Kymerian Province areas. The results show that the ostracod assemblages in the Gulf of Tongking, including northern Vietnam, have little relationship with tropical fauna in the East Indian Province.

The present results are also useful for the reconstruction of the paleoenvironment in the coast of the Indo-Chinese peninsula. For example, Tanaka et al. (2011b) investigated the transitions of the ostracod assemblage about 10 000 years based on three sediment cores in the Red River delta, northeastern coast of Vietnam, and reconstructed past sea-level changes. They reported that the open-marine ostracods were detected during the period of maximum sea levels, and the assemblages at this time were characterized by Loxoconcha malayensis, Pistocythereis bradyformis, Pistocythereis bradyi, Pistocythereis euplectella, Pistocythereis cribriformis, and Cytherelloidea cingulata. These species are common in the southern coast of the Indo-Chinese peninsula, but rare in the Co To Islands (especially C. cingulata and P. euplectella, which do not occur at the islands). The ostracod assemblages from the Co To Islands could thus serve as environmental indicators representing the conditions in littoral areas with argal/seaweeds in the eastern or southeastern margin of the Eurasian continent, especially during the Quaternary.

From type locality, Co To Island.

Adult female carapace (VNMN.0099), (Fig. 6, 1–6)

One adult male carapace (VNMN.0100), (Fig. 6: 7–12), one adult male left valve (VNMN.0088), (Fig. 4: 4), one A-1 instar female right valve (VNMN.0101), one A-1 instar male carapace (VMNH.0102), one adult female carapace (divided into left and right valve for dissection and internal observation) (VNMN.0103), (Fig. 7: 1); female antenna (VNMN.0104), antennule, mandible, maxilla, first leg, second leg, and third leg (VNMN.0105), male copulatory organ (VNMN.0106).

Holocene fine sand from the Co To Islands, northeastern Vietnam (St. 5; 20∘58′57′′ N, 107∘46′46′′ E)

Female (holotype) (VNMN.0099), L = 374 µm, H = 226 µm; female (aratype) (VNMN.0103), L = 379 µm, H = 227 µm; female, A-1 instar (paratype) (VNMN.0101), L = 329 µm, H = 199 µm; male (paratype) (VNMN.0100), L = 450 µm, H = 224 µm; (VNMN.0088), L = 441 µm, H = 218 µm; male, A-1 instar (paratype) (VMNH.102), L = 356 µm, H = 199 µm.

Small subrhomboidal-shaped Loxoconcha with widely arched anterior margin, straight dorsal margin. Surface covered by punctuation and characterized by trifurcated muri at anteroventral area. Male copulatory appendage with funeral-bell-like clasping apparatus, ox-shaped distal lobe and copulatory duct being short with a tapered distal end.

Outer morphology of carapace (Fig. 6: 1–14). Carapace subrhomboidal in lateral view; anterior margin broadly arched toward anteroventral direction, dorsal margin straight, posterior margin broadly rounded toward posteroventral direction, ventral margin sinuated and concaved at one-third from anterior end. Surface ornamented with punctation. Three muri run parallel from anterior to midventral margin, most dorsal one trifurcated at anteroventral area. Eye tubercle prominent. Caudal process absent. Ellipsoidal in dorsal view, Ovate in anterior view. Strong sexual dimorphism: female shorter and rounder in lateral view. A-1 instar: ventral muri are unclear. Prominent sexual dimorphism; male is longer than female, weak punctation of posteroventral area of male.

Inner morphology of valve (Fig. 7: 1, 2). Marginal pore canal straight: nine in anterior, five in ventral, and five posterior pore canals. Duplicature developed. Anterior vestibulum developed. Hinge gongylodont: left valve anterior element a rounded tooth, median element an elongate bar, posterior element an elongated socket with a stepped tooth. One heart shaped frontal scar. Four adductor muscle scars in a vertical low: upper most one is rounded, second and lower most scars are oblong, third one is kidney shaped. Two oblong mandible scars at anteroventral area of adductor muscle scars.

Antennula (Fig. 7: 3). Five articulated podomeres, length ratio among them from proximal to distal 80:24:19:25:40. Walls well developed, especially first to fourth podomeres. First podomere with simple seta at posterior distal end, pseudochaetae at anterior proximal end, and very short setulae along anterior margin. Second podomere with stout simple seta at anterior distal end. Third podomere with simple seta and claw-like seta at anterior distal end. Fourth podomere with two simple setae and one stout seta at anterior distal end, one long simple seta at posterior distal end. Fifth podomere with four setae at distal end, two simple setae with different length, one claw-like seta, one spoon-like seta.

Antenna (Fig. 7: 4). Four articulated podomeres, length ratio among them from proximal to distal 48:20:50:5. First podomere with several pseudochaetae at anterior distal end. Second podomere with exopodite (meaning spinneret seta) at distal end, one plumose seta at posterior distal end, pseudochaetae at midanterior margin. Third podomere with simple seta at posterior distal end, two setae with different length at middle part of the posterior margin (short simple seta and long plumose seta), pseudochaetae along posterior margin from the distal part of the two setae to the distal end, and several pseudochaetae (two distal pseudochaetae longer than other pseudochaetae) near anterior proximal end. Fourth podomere with two stout claw-like setae.

Mandibula (Fig. 7: 5). Five articulated podomeres: length ratio among them from proximal to distal 20:7:3:3:1. First podomere (coxa) bearing six teeth to distal end. Second podomere (basis) with five simple setae on proximal margin. Third podomere with two simple setae at posterior distal end and one feather-like seta at anterior distal end. Fourth podomere with five simple setae at anterior distal end, two short plumose setae at middle of posterior margin, and one stout simple seta at posterior distal end. Fifth podomere with five unequal simple setae at distal end.

Maxillula (Fig. 7: 6). Extremely thin branchial plate (exopodite) with 16 plumose setae. Basal podomere bearing palp and three endites. Palp with two indistinct podomeres; first podomere with four long plumose setae at distal end, second podomere with three stout setae at distal end and one stout seta at proximal end. Outer masticatory process with four simple setae and one plumose seta at distal end. Middle and inner masticatory processes with six simple setae.

Fifth limb (Fig. 7: 7). Four articulated podomeres: length ratio among them from proximal to distal 100:57:28:37. First podomere with two plumose setae at dorsal distal end, and two simple setae at half way from distal end of anterior margin. Third podomere with one knob-like structure at anterior distal end. Fourth podomere with stout distal claw.

Sixth limb (Fig. 7: 8). Four articulated podomeres: length ratio among them from proximal to distal 98:77:35:42. First podomere with two plumose setae at middle of dorsal margin, one short plumose seta at anterior distal end, and one plumose seta at posterior proximal end. Second podomere with simple seta at anterior distal end. Fourth podomere with stout distal craw.

Seventh limb (Fig. 7: 9). Four articulated podomeres, length ratio among them from proximal to distal 120:94:35:50. First podomere with one plumose seta at anterior distal end, one plumose seta at half way from distal end of anterior margin, one plumose seta at outer lateral surface of half way from distal end, one plumose seta at posterior proximal end. Second podomere with simple seta at anterior distal end.

Male Copulatory appendage (Fig. 7: 10). Capsule trapezoidal and wall, well developed along margin. Ox-like distal lobe developed on distal end of capsule. One funeral-bell-like clasping apparatus. Copulatory duct short, one turn around, tapered distal end.

Loxoconcha cotoensis sp. nov. is similar to Loxoconcha taiwanensis Zhao (1988) in Wang et al. (1988) from the Holocene surface sediment of the East China Sea in terms of its broadly arched posterior margin, but differs from it by its widely arched anterior margin, trifurcated muri, and surface ornamentation being covered by punctuation. The new species resembles Loxoconcha pulchra Ishizaki (1968) from the Holocene sediments of Uranouchi Bay, Shikoku Island, Japan. However, Loxoconcha cotoensis sp. nov. differs from L. pulchra in having a prominent eye tubercle, straight dorsal margin, and trifurcated muri. The male copulatory organ of L. cotoensis sp. nov. differs from this because of its funeral-bell-like clasping apparatus, ox-shaped distal lobe, and copulatory duct being short with a tapered distal end (see Nakao and Tsukagoshi, 2002, about the copulatory organ of L. pulchra). Focused on their habitat, L. taiwanensis is reported below the depth of 20 m in the Yellow Sea and the Bohai Sea (Wang et al., 1988). L. pulchra is abundant in sandy tidal flats in Japan (Nakao and Tsukagoshi, 2002). The depth of the locality of L. cotoensis is 2.5 to 4 m. Thus, some species that resemble L. cotoensis show annidations that depend on the water depth.