A new species of Nitocra Boeck, 1865 (Copepoda, Harpacticoida) from Vietnam

277 A new species of Nitocra Boeck, 1865 (Copepoda, Harpacticoida) from Vietnam Ngoc-Son Tran1, Thi-Phuong Pham1, Khanh Chuyen Phung1, Samuel Gómez2 1 The University of Danang – University of Science and Education, 459 Ton Duc Thang St., Danang City 550000, Vietnam 2 Universidad Nacional Autónoma de México, Instituto de Ciencias del Mar y Limnología, Unidad Académica Mazatlán, Joel Montes Camarena s/n, Mazatlán, 82040, Sinaloa, Mexico Corresponding author: Samuel Gómez ([email protected]) Copyright: © Ngoc-Son Tran et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract An undescribed taxon of the species-rich genus Nitocra was found during a survey on the harpacticoid copepods from the hyporheic zone in Vu Gia–Thu Bon Rriver, Central Vietnam. The new species, N. duyxuyenensis sp. nov., is morphologically similar to the American N. evergladensis Bruno & Reid, 2002. These two species can be separated by the number of marginal spinules on the anal operculum, the relative length of the baseoendopod of the female fifth leg, the structure of the antenna, the number of setae on the female fifth leg, the number of setae on the inner margin of the first endopodal segment of the second to fourth swimming legs of the males, and the number of setae on the third endopodal segment of the third swimming leg. Additionally, some comments on the groups and subgroups of the genus defined so far are given, with the proposal of the lacustris species subgroup for group III of Gómez et al. (2012). Key words: Groundwater fauna, hyporheic zone, new species, Nitokra, Southeast Asia, species groups, taxonomy Introduction With 74 species and 26 subspecies, the genus Nitocra Boeck, 1865 is one of the genera with the largest number of species in the family Ameiridae. Of these, nine species (N. arctolongus Shen & Tai, 1973; N. karanovici Chullasorn, Kangtia & Klangsin, 2014; N. koreanus Chang 2007; N. malaica Kiefer, 1929; N. platypus platypus Daday, 1906; N. platypus bakeri Chappuis, 1930; N. sewelli Gurney, 1927; N. vietnamensis Tran & Chang, 2012; N. quangnamensis Tran, Trinh-Dang, Nguyen & Brancelj, 2025) have been found in Asia. Species of Nitocra are common in several kinds of soft bottoms in fresh-water, brackish, and marine habitats (Boxshall and Halsey 2004). Records of the genus Nitocra from Vietnam are still scarce, and only two species, N. vietnamensis found in anchialine caves at Ninh Binh province (Tran and Chang 2012), and N. quangnamensis found in the hyporheic zone of the Vu Gia-Thu Bon River in Quang Nam Province (Tran et al. 2025b), had been reported from this region. Here we describe a new species, N. duyxuyenensis sp. nov., collected from the hyporheic zone in Vu Gia–Thu Bon river (Quang Nam province), Central Vietnam. Academic editor: Maria Cristina Bruno Received: 5 August 2025 Accepted: 29 September 2025 Published: 6 November 2025 ZooBank: https://zoobank.org/ E675055E-CB6D-4D4A-A0F1B8B197FAB766 Citation: Tran N-S, Pham T-P, Phung KC, Gómez S (2025) A new species of Nitocra Boeck, 1865 (Copepoda, Harpacticoida) from Vietnam. ZooKeys 1258: 277–304. https://doi. org/10.3897/zookeys.1258.167766 ZooKeys 1258: 277–304 (2025) DOI: 10.3897/zookeys.1258.167766 278 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Materials and methods Harpacticoid copepods were collected from the hyporheic zone in Vu Gia–Thu Bon river, Quang Nam province, Central Vietnam, by the Karaman-Chappuis method (Karaman 1934; Chappuis 1942). Samples were filtered with a standard zooplankton net with a 50-μm mesh, and subsequently fixed and preserved in 4% formalin. Dissected specimens were mounted in glycerin, and the slides were sealed with transparent nail varnish. Dissected appendages and body ornamentation were examined at 1000 × magnification under an Axio Lab A1 Carl Zeiss compound microscope. All drawings were digitally inked using CORELDRAW v. 19.0®. Morphological nomenclature follows Huys and Boxshall (1991). Abbreviations used throughout the text are A1, antennule; A2, antenna; ae, aesthetasc; Exp, exopod; Enp, endopod; Benp, baseoendopod; P1–P6, first to sixth thoracopod; Exp(Enp)-1(2, 3) denotes the proximal (middle, distal) segment of a ramus. The type material was deposited at the Zoological Collection of Duy Tan University (ZC-DTU), Da Nang city, Vietnam. Two different spellings, Nitocra and Nitokra, have been widely used [see Gómez in litt., Huys in litt. (Walter and Boxshall 2025)]. The spelling Nitokra is regarded as the alternative representation of Nitocra, and both are in use (Walter and Boxshall 2025). Here we adopt the spelling Nitocra Boeck, 1865 over Nitokra Boeck, 1865. Taxonomy Family Ameiridae Boeck, 1865 Genus Nitocra Boeck, 1865 Type species. Nitocra typica typica Boeck, 1865. Other species. Nitocra affinis Gurney, 1927; N. alperi Yıldız & Karaytuğ, 2024; N. arctolongus Shen & Tai, 1973; N. australis Soyer, 1975; N. balli Rouch, 1972; N. balnearia Por, 1964; N. bdellurae (Liddell, 1912); N. bisetosa Mielke, 1993; N. blochi Soyer, 1975; N. californica Lang, 1965b; N. cari Petkovski, 1954; N. chelifer Wilson, C. B., 1932; N. colombiensis Fuentes-Reinés & Suárez-Morales, 2014; N. delaruei Soyer, 1975; N. divaricata caspica Behning, 1936; N. divaricata divaricata Chappuis, 1924; N. dubia Sars, G. O., 1927; N. elegans (Scott, T., 1905); N. elongata Marcus, 1968; N. esbe Karanovic, Eberhard, Cooper & Guzik, 2015; N. evergladensis Bruno & Reid, 2002; N. fallaciosa baltica Lang, 1965a; N. fallaciosa fallaciosa Klie, 1937; N. fluviatilis Galhano, 1968; N. fragilis fragilis Sars, G. O., 1905, N. fragilis paulistana Jakobi, 1956; N. galapagoensis Mielke, 1997; N. hamata Bodin, 1970; N. hibernica bulgarica (Apostolov, 1976); N. hibernica hibernica (Brady, 1880); N. hibernica hyalina Jakubisiak, 1929; N. humphreysi Karanovic & Pesce, 2002; N. husmanni Kunz, 1976; N. hyperidis Jakobi, 1956; N. karanovici Chullasorn, Kangtia & Klangsin, 2014; N. kastjanensis Kornev & Chertoprud, 2008; N. koreanus Chang, 2007; N. lacustris azorica Kunz, 1983; N. lacustris colombianus Reid, 1988; N. lacustris lacustris (Schmankevitsch 1875); N. lacustris pacifica Yeatman, 1983; N. lacustris sinoi Marcus & Por, 1961; N. laingensis Fiers, 1986; N. langi Karanovic, Eberhard, Cooper & Guzik, 2015; N. loweae Yıldız & Karaytuğ, 2024; N. malaica Kiefer, 1929; N. marinae Jaschnov, 279 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam 1935; N. mediterranea jakubisiak Karanovic, Eberhard, Cooper & Guzik, 2015; N. mediterranea mediterranea Brian, 1928; N. medusaea Humes, 1953; N. minor Willey, 1930; N. mozambica Huys, 2021; N. oligochaeta Giesbrecht, 1882; N. phlegraea Brehm, 1909; N. phreatica Bozic, 1964; N. pietschmanni Chappuis, 1933; N. platypus platypus Daday, 1906; N. platypus bakeri Chappuis, 1930; N. pontica Boeck, 1865; N. pori Karanovic, Eberhard, Cooper & Guzik, 2015; N. psammophila Noodt, 1952; N. pseudospinipes Yeatman, 1983; N. puebloviejensis Fuentes-Reinés, Suárez-Morales & Silva-Briano, 2022; N. pusilla mediterranea Brian, 1928; N. pusilla pusilla Sars, G. O., 1911; N. quadriseta Wells & Rao, 1987; N. quangnamensis Tran, Trinh-Dang, Nguyen & Brancelj, 2025; N. reducta Schäfer, 1936; N. reunionensis Bozic, 1969; N. rijekana Petkovski, 1954; N. salina (Schmankevitsch, 1875); N. serdarsaki Yıldız & Karaytuğ, 2024; N. sewelli Gurney, 1927; N. simplex Schmeil, 1894; N. sonmezi Yıldız & Karaytuğ, 2024; N. sphaeromata Bowman, 1988; N. spinipes orientalis Sewell, 1924; N. spinipes spinipes Boeck, 1865; N. stygia Por, 1968; N. taylori Gómez, Carrasco & Morales-Serna, 2012; N. tenuicornis Brady & Robertson, 1873; N. typica adriatica Petkovski, 1954; N. typica pontica Jakubisiak, 1938; N. uenoi Miura, 1962; N. vietnamensis Tran & Chang, 2012; N. wolterecki Brehm, 1909; N. yahiai (Blanchard & Richard, 1891); N. yeelirrie Karanovic, Eberhard, Cooper & Guzik, 2015. Nitocra duyxuyenensis sp. nov. https://zoobank.org/1D504676-1266-40E7-B0CA-983FE7F6F921 Figs 1–9 Type material. • Adult female holotype dissected and mounted on one slide (ZC-DTU-COPEPODA-0014) • adult male allotype dissected and mounted on one slide (ZC-DTUCOPEPODA-0015) • two female and two male paratypes preserved in 70% ethanol (ZC-DTU-COPEPODA-0013). Type locality. Hyporheic zone in Vu Gia – Thu Bon river (15°50'15"N, 108°09'05"E), Quang Nam province, Central Vietnam. Differential diagnosis. Caudal rami 1.2× as long as wide. Anal operculum with six marginal spinules. Female antennule eight-segmented; male antennule haplocer, eight-segmented. Antenna with allobasis. P1 Exp-2 with inner seta; Exp-3 with five elements in all; P1 Enp-1 reaching approximately the middle of Exp-2, with inner seta; Enp-2 with three elements. P2–P4 Exp-2 with inner seta, Exp-3 with seven setae/spines; P2–P4 Enp-1 without, Enp-2 with inner seta, Enp-3 with four, five, and five setae/spines respectively. Female P5 Exp with six, Benp with five setae and reaching the middle of Exp; male P5 Exp with six, baseoendopods fused medially, each with three setae. Description of the adult female. Habitus (Fig. 1A) semicylindrical; total body length measured from tip of rostrum to posterior margin of caudal rami ranging from 492 to 546 μm (average = 519 μm, n = 5; holotype = 546 μm). Nauplius eye not visible. Rostrum (Fig. 1A) linguiform, small, not reaching end of first antennule segment, with pair of tiny sensilla subapically. Prosome four-segmented, comprising cephalothorax with completely fused first pedigerous somite, and three free pedigerous somites; cephalothorax and P2–P4-without surface ornamentation other than sensilla as shown (Fig. 1A). Urosome five-segmented, comprising fifth pedigerous somite, genital double-somite, two free uroso- 280 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam mites, and anal somite with caudal rami (Fig. 1A, B); P5-bearing somite without spinular ornamentation, with few posterior sensilla; genital double-somite (Fig. 1A–C) formed by the fusion of genital and third urosomite, with dorsolateral suture indicating original division, completely fused ventrally, proximal half with dorsolateral sensilla close to suture and with short row of lateral spinules as depicted (Fig. 1B), ventrally with medially interrupted row of minute spinules and Figure 1. Nitocra duyxuyenensis sp. nov. Female, holotype. A. Habitus, dorsal; B. Urosome, lateral; C. Urosome, ventral (P5-bearing somite omitted). Scale bars: 100 µm. 281 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Figure 2. Nitocra duyxuyenensis sp. nov. Female, holotype. A. Fifth urosomite, anal somite and caudal rami, dorsal; B. Fifth urosomite, anal somite and caudal rami, ventral. Scale bar: 50 µm. with sensilla along partial ventral suture, genital field close to anterior margin of first half of somite (Figs 1C, 6B), with large median copulatory pore; fourth urosomite with posterior spinular row ventrally (Fig. 1C); fifth urosomite with medial ventrolateral row of small spinules and with continuous row of small ornaments close to posterior margin, without sensilla (Fig. 2A, B); anal somite (Figs 1A, 2A, B) with median ventrolateral row of spinules, with strong spinules close to caudal rami dorsally (Figs 1A, 2A), and with comparatively smaller ornaments ventrally (Fig. 2B), anal operculum semicircular with six strong marginal spinules and flanked by pair of sensilla (Figs 1A, B, 2A). Caudal rami (Figs 1B, C, 2A, B) short, subquadrate, 1.2× as long as wide, with seven elements each; anterolateral seta I slender, short, ~0.3× as long as caudal ramus; anterolateral seta II and posterolateral seta III smooth, the former slightly shorter than the latter, with short strong spinules at base of seta II; outer apical seta IV and middle apical seta V well-developed, with fracture planes, seta V longest; distal inner accessory seta VI slender, smooth, 0.9 as long as caudal ramus, with several small spinules at its base; dorsal seta VII tri-articulated, ~2.4× as long as caudal ramus, issuing close to posterior margin of ramus. Antennule (Fig. 3A) 8-segmented, long and slender; all segments smooth except for first segment with a row of spinules; all setae smooth except for pinnate seta on first segment. Aesthetasc on fourth segment, fused to long seta, reaching well beyond distal end of last segment. Armature formula as follows: 1(1), 2(7), 3(2), 4(2+(1+ae)), 5(2), 6(1), 7(4), 8(5+acrothek). 282 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Figure 3. Nitocra duyxuyenensis sp. nov. Female holotype. A. Antennule; B. Antenna; C. Mandible; D. Maxillule; E. Maxilla; F. Maxilliped. Scale bars: 50 µm. Antenna (Fig. 3B) relatively short, composed of coxa, allobasis, one-segmented endopod, and one-segmented exopod. Coxa short, unornamented. Allobasis 2.7× as long as wide, with a short row of inner spinules proximally, and several minute proximal spinules close to coxa. Exopod with three distal unequal setae (one bare, two unipinnate). Endopod with inner longitudinal spinular row; lateral armature consisting of one inner spine and one strong inner seta; distally with one smooth seta, four geniculate setae and one distal outer geniculate seta fused basally to slender element. Mandible (Fig. 3C) with well-developed coxa; gnathobase with ~11 teeth as shown, and single lateral seta. Mandibular palp 2-segmented, first segment (basis) with one strong plumose seta; endopod with one lateral and four smooth apical setae. Maxillule (Fig. 3D) with well-developed praecoxa; arthrite with two surface setae, and four large robust distal spines. Coxal endite with two smooth setae. 283 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Basis with five normal unequal setae distally. Endopod 1-segmented, with four slender smooth setae. Maxilla (Fig. 3E) without spinular ornamentation on syncoxa, the latter with two endites; proximal endite somewhat bulbous, with one strong plumose seta; distal endite cylindrical, with three subequal setae. Allobasis drawn out into long claw accompanied by a shorter bare seta. Endopod short, one-segmented; with two subequal, smooth setae. Maxilliped (Fig. 3F) with short and stout syncoxa, the latter 1.3× as long as wide, with short row of posterior spinules, with one small bare seta subapically. Basis ~2.2× as long as wide, with short row of inner spinules and with one outer subdistal spinule. Endopod drawn out into long curved smooth claw accompanied by single short slender seta. P1–P4 (Figs 4A, B, 5A, B) with short unornamented intercoxal sclerites, the latter with dorsal tines as shown. Coxa of P1 and P2 with, of P3 and P4 without outer subdistal spinules. Basis of P1 with spinules at the base of inner and outer spine, with additional ornaments between rami; basis of P2–P4 with spinules at the base of outer spiniform (P2) and setiform elements (P3–P4), of P2 and P3 with, of P4 seemingly without spinules close to the base of the endopod. Rami three-segmented. Exopodal segments with outer and subdistal spinules as shown; Exp-1 with outer spine, without inner armature; Exp-2 with outer spine and inner seta; Exp-3 with three outer spines, two distal geniculate setae and without inner armature (P1) or with one distal outer spiniform and one distal inner setiform element and with two inner setae (P2–P4). Endopodal segments with outer ornaments as figured; P1 Enp visibly longer than Exp, Enp-1 not reaching beyond Exp-2; Enp-1–2 with inner seta, Enp-3 with three elements; P2–P4 Enp shorter than Exp; P2–P3 Enp reaching distal tip of Exp-2, P4 Enp reaching slightly above the middle of Exp-2; Enp-1 short, wider than long, unarmed; Enp-2 with one inner seta and a projection at the inner distal corner; Enp-3 with one (P2) or two (P3 and P4) inner setae, two distal elements, and one outer spine; length ratio of armature elements on P3 Enp-3 (starting from outer most apical spine) 1: 1.9:4.7:5.2:2.9. Armature formula of P1–P4 as in Table 1. Table 1. Armature formula of swimming legs (P1–P4) of Nitocra duyxuyenensis sp. nov. Leg Basis Exp Enp 123123 P1 I-I I-0 I-1 III,2,0 0-1 0-1 I,1,1 P2 0-I I-0 I-1 III,2,2 0-0 0-1 I,2,1 P3 0-1 I-0 I-1 III,2,2 0-0 0-1 I,2,2 P4 0-1 I-0 I-1 III,2,2 0-0 0-1 I,2,2 P5 (Fig. 6A) with exopod and baseoendopod separated. Baseoendopod with outer unipinnate seta; endopodal lobe well-developed, with longitudinal row of outer spinules, with three inner spinulose spines, and two robust distal setae of which distal inner longest. Exopod oval, 1.6× as long as wide, with six unequal smooth setae of which two outer and outer subdistal seta shortest. P6 (Fig. 6B) represented by narrow plate with one single plumose seta on each side. Description of the adult male. Body shape and general appearance as in female (Fig. 7A); total body length measured from tip of rostrum to posterior 284 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Figure 4. Nitocra duyxuyenensis sp. nov. Female, holotype. A. P1; B. P2. Scale bar: 50 µm. margin of caudal rami ranging from 476 to 508 μm (average = 492 μm, n = 2; holotype = 508 μm). Rostrum, prosomites, anal operculum, and caudal rami as in female (Fig. 7A, C, D). Sexual dimorphism expressed in urosomal segmentation (genital and third urosomite not fused), spinular ornamentation of urosomites, antennule, basis of P1, P5, and P6. Genital somite (Fig. 7A, B) with continuous posterior dorsolateral spinular row; third urosomite with dorsolateral spinular row interrupted ventrolaterally; 285 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Figure 5. Nitocra duyxuyenensis sp. nov. Female holotype. A. P3; B. P4. Scale bar: 50 µm. fourth and fifth urosomites (Fig. 1A–D) with continuous posterior and median spinular row, respectively, and fifth urosomite with short spinular row ventrally. Antennule (Fig. 8A) haplocer, 8-segmented, with geniculation between sixth and seventh segments; all segments smooth except for first segment with some spinules. All setae smooth except for plumose seta on first segment. 292 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Table 3. Differentiating characters of the species of the lacustris subgroup. uk = unknown. Species P1 Enp-1 P5 female Exp:Benp P5 male Exp:Benp Female P5 Benp:Exp A2 A2 Exp-2 Female P6 Inner seta Enp-1 P1 Anal operculum CR relative length # setae # setae relative length allobasis/ basis # setae # setae position, ornamentation, and relative length ornamentation l/w N. duyxuyenensis sp. nov. ~Exp-2 6:5 6:3 1/2 Exp allobasis 3 1 1/2 Enp-1; bipinnate; ~Enp-2 6 strong spinules ~1 N. karanovici 2/3 Exp-3 5:5 6:3 1/3 Exp basis 3 1 3/4 Enp-1; plumose; ~Enp-3 11 strong spinules ~1 N. evergladensis ~Exp-2 6:5 7:4 2/3 Exp basis 3 uk 2/3 Enp-1; smooth; ~1/2 Enp-3 7 strong spinules ~1 N. balli 1/2 Exp-3 5:4 5–6:2 1/3 Exp basis 3 2 2/3 Enp-1; smooth; ~Enp-2 9–11 spinules ~0.7 N. pseudospinipes 1/2 Exp-2 6:3–4 5–6:3 1/3 Exp basis 3 uk 1/2 Enp-1; smooth; ~1/2 Enp-3 8 small spinules 0.4 N. lacustris pacifica 1/2 Exp-2 6:5 6:2 1/2 Exp basis uk 1 uk 2–4 large spines 1.5 N. fallaciosa baltica ~Exp-2 5:5 6:4 uk uk uk 2 2/3 Enp-1; smooth; > Enp-3 ~10 spinules 1.3 N. minor beyond Exp-3 5:5 5:2 uk uk 3 uk 2/3 Enp-1; smooth; ~1/2 Enp-2 11 spinules ~0.7 N. phreatica ~Exp-3 5:3 4:2 uk uk 3 uk 2/3 Enp-1; smooth; ~Enp-2 12 spinules ~1 N. psammophila 1/2 Exp-3 6:5 6:3 1/2 Exp uk 3 uk 1/2 Enp-1; smooth; ~Enp-3 ~6 spinules 1.2 N. uenoi ~Exp-2 5:4 uk 1/2 Exp basis 2 1 3/4 Enp-1; smooth; ~Enp-2 11 spinules 1.2 293 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Table 4. Differentiating characters of the species of the fragilis subgroup. uk = unknown. Species P1 Enp-1 Female P5 Exp:Benp Male P5 Exp:Benp Female P5 Benp A2 Female P6 Inner seta P1 Enp-1 P1 Enp-3 Anal operculum CR relative length # setae # setae relative length allobasis/ basis # setae position, ornamentation, and relative length # setae ornamentation l/w N. australis ~Exp--3 5:5 6:4 1/2 Exp basis 3 2/3 Enp-1; plumose; ~Enp-3 3 ~22 large spinules ~0.7 N. fragilis 1/2 Exp-3 5:5 6:3 1/2 Exp basis uk 2/3 Enp-1; smooth; ~Enp-3 2 ~12 spinules ~0.7 N. intermedia 2/3 Exp-3 5:5 6:3 1/2 Exp allobasis uk 2/3 Enp-1; bipinnate; ~Enp-3 3 ~7–8 spinules uk N. husmanni ~Exp-2 6:5 6:3 ~2/3 Exp uk uk 2/3 Enp-1; bipinnate; ~Enp-2 3 ~7 spinules ~1 N. pietschmani 1/2 Exp-2 6:5 6:3 2/3 Exp allobasis uk 2/3 Enp-1; plumose; ~Enp-3 3 ~4–6 spinules ~1 N. laingensis uk 5:4 uk 1/2 Exp uk 2 uk uk 6 large spinules ~1 N. koreanus ~Exp-2 6:5 6:3 1/3 Exp allobasis uk 2/3 Enp-1; bipinnate; ~Enp-3 3 4–5 spinules ~1 N. taylori 2/3 Exp-2 6:5 6:2 1/3 Exp allobasis 2 2/3 Enp-1; bipinnate; > Enp-3 3 3 strong spinules 1.2 N. vietnamensis 1/2 Exp-3 6:5 6:3 1/2 Exp allobasis 2 2/3 Enp-1; bipinnate; ~Enp-3 3 5–6 strong spinules ~1 N. puebloviejensis 2/3 Exp-3 5:5 6:4 1/2 Exp basis 1 1/2 Enp-1; bipinnate; ~Enp-3 3 5–6 spinules ~1 N. spinipes 1/2 Exp-3 5:5 6:5 1/2 Exp basis 3 2/3 Enp-1; bipinnate; > Enp-3 3 8–12 spinules ~1 N. sphaeromata beyond Exp-3 5:5 5:4 1/2 Exp basis uk 2/3 Enp-1; unipinnate; ~Enp-2 3 With row spinules ~1 N. reunionensis ~Exp-2 5:4 6:2 1/2 Exp uk 1 2/3 Enp-1; bipinnate; ~Enp-2 3 ~7 large spinules ~1 N. humphreysi ~Exp-2 4–5:4 5–6:3 1/2 Exp basis 1 2/3 Enp-1; bipinnate; ~Enp-3 3 ~5 strong spines ~1 N. quangnamensis 2/3 Exp-3 6:4 5–6:3 1/2 Exp basis 1 1/2 Enp-1; bipinnate; ~Enp-2 3 6 strong spines ~1 294 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Table 5. Differentiating characters of the species of the mediterranea subgroup. uk = unknown. Species P1 Enp-1 P2–P4 Enp-1 P2–P4 Enp-3 P2–P4 Exp-3 P4 Exp-3 Female P5 Exp:Benp Female P5 Benp Male P5 Exp:Benp A2 relative length # inner setae # setae # setae # inner setae # setae relative length # setae allobasis/ basis N. mediterranea jakubisiak uk 1-1-1 5-4-5 6-6-6 1 6:5 uk uk uk N. mediterranea mediterranea ~Exp-3 1-1-1 4-4-5 6-6-6 1 6:5 1/2 Exp 5:4 basis N. typica adriatica 2/3 Exp-3 1-1-1 4-4-4 6-6-6 1 6:5 1/3 Exp 6:4 uk N. elongata ~Exp-3 1-1-1 4-4-4 6-6-7 2 5:5 1/2 Exp 6:4 basis N. langi uk 1-1-1 4-4-4 4-6-7 2 5:5 uk uk uk N. fluviatilis ~Exp-2 1-1-1 3-3-3 5-5-7 2 6:4 1/2 Exp 6:3 basis N. kastjanensis ~Exp-2 0-0-0 5-5-5 7-7-7 2 5:5 uk 6:3 basis N. quadriseta ~Exp-3 0-0-0 4-4-4 7-7-7 2 5:5 uk 6:4 basis N. cari ~Exp-3 0-0-0 3-5-5 7-7-8 3 6:4 uk 5:2 uk N. arctolongus ~Exp-2 0-0-0 3-4-5 7-7-7 2 5:5 1/3 Exp 6:2 allobasis N. yeelirrie 1/2 Exp-2 0-0-0 3-4-4 7-7-7 2 6:4 1/3 Exp 6:2 allobasis N. bisetosa ~Exp-3 0-0-0 3-4-4 7-7-6 1 5:2 1/3 Exp 6:2 basis N. esbe ~Exp-2 0-0-0 3-4-4 6-6-7 2 7:4 1/3 Exp 6:2 allobasis 295 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam The dense river network in Central Vietnam, originating from the Annamite Range, characterized by steep terrain and widespread sandy sediments, has created particularly favorable geomorphological and hydrological conditions for the formation and development of the hyporheic zone. This is an intermediate zone between surface water and groundwater, where metabolic processes and micro-flows take place, providing a stable habitat for many species of benthic organisms and plankton. To date, only four species of copepods (Parastenocaris sontraensis Tran, Trinh-Dang & Brancelj, 2021; P. vugiaensis Tran, Trinh-Dang & Brancelj, 2021; Nitocra quangnamensis, Phyllognathopus vietnamensis Tran, Nguyen & Brancelj, 2025) have been discovered from the hyporheic region in Vietnam, suggesting that the copepod biodiversity in this ecosystem remains considerably underestimated (Tran et al. 2025b). Identification keys to the species groups and species of each subgroup of Nitocra are given below. The identification of the species of Nitocra is difficult mainly because of poor original descriptions of some species. The user is suggested to also check the original descriptions in case of doubt. Key to the groups and subgroups of Nitocra 1 P1 Exp-3 with four spines/setae ........................................................group I – P1 Exp-3 with five spines/setae ...................................................................2 2 P1 Exp-2 without inner seta ...............................................................group II – P1 Exp-2 with inner seta .............................................................3 (group III) 3 P2–P4 Enp-3 with 4:5:5 spines/setae .......................................................... 4 – Number of spines/setae on P2–P4 Enp-3 different ...................................... ...................................................................................mediterranea subgroup 4 P2–P4 Enp-1 without inner seta ..................................... lacustris subgroup – P2–P4 Enp-1 with one inner seta .................................................................5 5 P2–P4 Exp-3 with 7:7:8 spines/setae ................................ affinis subgroup – P2–P4 Exp-3 with 7:7:7 spines/setae ............................... fragilis subgroup Key to the species of the affinis subgroup 1 Inner middle seta of P4 Exp-3 not stronger, but longer than the other setae ...............................................................................................N. rijekana – Inner middle seta of P4 exopod-3 stronger and longer than the other setae, and spinulose ................................................................................................2 2 P5 Exp l/w ratio ≥ 4; syncoxa of maxilliped with 2 setae ............. N. hamata – P5 Exp l/w ratio < 4; syncoxa of maxilliped with 1 seta ..............................3 3 Rostrum with rostral projection ....................................................................4 – Rostrum without rostral projection ..............................................................6 4 A2 with allobasis ..............................................................................N. affinis – A2 with basis .................................................................................................5 5 Inner seta of P1 Enp-1 bipinnate, inserted at distal 1/4 of segment, reaching beyond Enp-3; all setae of Enp P2–P4 smooth ............ N. colombiensis – Inner seta of P1 Enp-1 bipinnate, inserted at 2/3 of segment, reaching approximately the tip of Enp-3; all setae of Enp P2–P4 pinnate ....N. alperi 6 P1 Enp-3 with 2 setae; P5 baseoendopod with large hyaline field ...N. stygia – P1 Enp-3 with 3 setae; P5 baseoendopod without hyaline field .................7 296 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam 7 P1 Enp-1 barely reaching tip of Exp-3; inner seta of P1 Enp-1 inserted at distal 1/4 of segment ..................................................................... N. loweae – P1 Enp-1 reaching beyond tip of Exp-3; inner seta of P1 Enp-1 inserted at distal 2/3 of segment .................................................................................... 8 8 A2 with allobasis .......................................................................N. serdarsaki – A2 with basis .................................................................................................9 9 Inner seta of P1 Enp-1 bipinnate; female P5 baseoendopod reaching proximal 1/3 of Exp .............................................................................. N. elegans – Inner seta of P1 Enp-1 strongly unipinnate; female P5 baseoendopod reaching beyond 1/2 of Exp ........................................................................10 10 L/w ratio of female P5 Exp ~1.7, baseoendopod reaching the middle of Exp, with innermost seta shortest ...............................................N. sonmezi – L/w ratio of female P5 Exp ~2–2.2, baseoendopod not reaching proximal 2/3 of Exp, with outermost seta shortest ................................N. californica Key to the species of the lacustris subgroup 1 Female P5 Exp with 5 setae ..........................................................................2 – Female P5 Exp with 6 setae ..........................................................................7 2 P1 Enp-1 reaching beyond Exp-3 ......................................................N. minor – P1 Enp-1 at most as long as Exp-3...............................................................3 3 A2 Exp one-segmented with 2 setae ................................................N. uenoi – A2 Exp one-segmented with 3 setae ............................................................4 4 Female P5, baseoendopod with 3 setae ....................................N. phreatica – Female P5, baseoendopod with 4 or 5 setae ..............................................5 5 Inner seta of P1 Enp-1 inserted at ¾ of segment; female and male P5 Exp and baseoendopod with 5 and 5, and 6 and 3 setae, respectively ............... ....................................................................................................N. karanovici – Inner seta of P1 Enp-1 inserted at 2/3 of segment; female and male P5 Exp and baseoendopod with different number of setae .............................6 6 Female and male P5 Exp and baseoendopod with 5 and 4, and 5 or 6 and 3 setae, respectively; P1 Enp-1 reaching the middle of Exp-3 .......... N. balli – Female and male P5 Exo and baseoendopod with 5 and 5, and 6 and 4 setae, respectively; P1 Enp-1 reaching the tip of Exp-2 ......N. fallaciosa baltica 7 P1 Enp-1 reaching beyond Exp-2 .........................................N. psammophila – P1 Enp-1 not reaching beyond Exp-2 ...........................................................8 8 A2 with allobasis ................................................. N. duyxuyenensis sp. nov. – A2 with basis .................................................................................................9 9 Inner seta of P1 Enp-1 inserted at 1/2 of segment; female P5 baseoendopod with 3 or 4 setae ........................................................N. pseudospinipes – Inner seta of P1 Enp-1 inserted at 2/3 of segment; female P5 baseoendopod with 5 setae ..........................................................................................10 10 Male P5 Exp and baseoendopod with 7 and 4 setae, respectively; P1 Enp-1 reaching Exp-2 .....................................................................N. evergladensis – Male P5 Exp and baseoendopod with 6 and 2 setae, respectively; P1 Enp-1 reaching ½ of Exp-2 ........................................................N. lacustris pacifica 297 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Key to the species of the fragilis subgroup 1 Female P5 Exp with 4–5 setae .....................................................................2 – Female P5 Exp with 6 setae ........................................................................10 2 Female P5 baseoendopod with 4 setae .......................................................3 – Female P5 baseoendopod with 5 setae .......................................................5 3 Female P6 with 2 setae ............................................................. N. laingensis – Female P6 with 1 seta ................................................................................... 4 4 Male P5 Exp and baseoendopod with 6 and 2 setae, respectively .............. ................................................................................................N. reunionensis – Male P5 Exp and baseoendopod with 5–6 and 3 setae, respectively .......... ..................................................................................................N. humphreysi 5 A2 with allobasis ...................................................................... N. intermedia – A2 with basis .................................................................................................6 6 P1 Enp-3 with 2 setae......................................................................N. fragilis – P1 Enp-3 with 3 setae....................................................................................7 7 P1 Enp-1 reaching beyond Exp-3 ......................................... N. sphaeromata – P1 Enp-1 not reaching beyond Exp-3 ...........................................................8 8 Female P6 with 1 short seta; inner seta of P1 Enp-1 inserted midway inner margin of segment ........................................................... N. puebloviejensis – Female P6 with 3 setae; inner seta of P1 Enp-1 inserted at distal 2/3 of segment .........................................................................................................9 9 P1 Enp-1 reaching approx. tip of Exp-3; male P5 Exp and baseoendopod with 6 and 4 setae, respectively ..................................................N. australis – P1 Enp-1 reaching the middle of Exp-3: male P5 Exp and baseoendopod with 6 and 5 setae, respectively .................................................. N. spinipes 10 Female P5 baseoendopod with 4 setae ..........................N. quangnamensis – Female P5 baseoendopod with 5 setae .....................................................11 11 Male P5 baseoendopod with 2 setae ..............................................N. taylori – Male P5 baseoendopod with 3 setae .........................................................12 12 P3 Exp-3 with middle outer seta not reaching beyond tip of segment; inner seta of P1 Enp-1 reaching approx. tip of Enp-2 ........................N. husmanni – P3 Exp-3 with middle outer seta reaching beyond tip of segment; inner seta of P1 Enp-1 reaching approx. tip of Enp-3 .........................................13 13 Female P5 Exp rounded, baseoendopod reaching ~2/3 of Exp ................... ...............................................................................................N. pietschmanni – Female P5 Exp P5 ellipsoidal, baseoendopod reaching 1/2 or 1/3 of Exp ...14 14 P1 Enp-1 reaching tip of Exp-2; female P5 baseoendopod reaching ~2/3 of Exp, all setae pinnate; male P5 Exp with fifth seta from inner to outer shortest ........................................................................................ N. koreanus – P1 Enp-1 reaching 1/2 of Exp-3; female P5 baseoendopod reaching ~1/2 of Exp, all setae smooth; male P5 Exp with second seta from inner to outer shortest .................................................................................N. vietnamensis Key to the species of the mediterranea subgroup 1 P2–P4 Enp-1 with 1:1:1 inner seta, respectively .........................................2 – P2–P4 Enp-1 without inner seta ................................................................... 7 298 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam 2 P4 Exp-3 with 1 inner seta ............................................................................3 – P4 Exp-3 with 2 inner setae ..........................................................................5 3 P2 Enp-3 with 5 setae.........................................N. mediterranea jakubisiak – P2 Enp-3 with 4 setae....................................................................................4 4 P4 Enp-3 with 5 setae; male P5 Exp and baseoendopod with 5 and 4 setae, respectively ....................................................N. mediterranea mediterranea – P4 Enp-3 with 4 setae; male P5 Exp and baseoendopod with 6 and 4 setae, respectively ........................................................................N. typica adriatica 5 P2–P4 Enp-3 with 3:3:3 setae/spines, respectively; female P5 Exp and baseoendopod with 6 and 4 setae, respectively .......................N. fluviatilis – P2–P4 Enp-3 with 4:4:4 setae/spines, respectively; female P5 Exp and baseoendopod with 5 setae each ................................................................6 6 P2–P4 Exp-3 with 6:6:7 setae/spines, respectively ...................N. elongata – P2–P4 Exp-3 with 4:6:7 setae/spines, respectively .........................N. langi 7 P4 Exp-3 with 1 or 3 inner setae ...................................................................8 – P4 Exp-3 with 2 inner setae ..........................................................................9 8 P2–P4 Enp-3 with 3:5:5 setae/spines, respectively; P4 Exp 3 with 8 setae/ spines; female and male P5 Exp and baseoendopod with 6 and 4, and 5 and 2 setae, respectively ......................................................................N. cari – P2–P4 Enp-3 with 3:4:4 setae/spines, respectively; P4 Exp-3 with 6 setae/ spines; female and male P5 Exp and baseoendopod with 5 and 2, and 6 and 2 setae, respectively ..............................................................N. bisetosa 9 A2 with allobasis .........................................................................................10 – A2 with basis ...............................................................................................12 10 P2–P4 Exp-3 with 6:6:7 setae/spines, respectively ......................... N. esbe – P2–P4 Exp-3 with 7:7:7 setae/spines, respectively ..................................11 11 P4 Enp-3 with 5 setae/spines; female P5 Exp and baseoendopod with 5 setae each............................................................................... N. arctolongus – P4 Enp-3 with 4 setae/spines; female P5 Exp and baseoendopod with 6 and 4 setae, respectively ...............................................................N. yeelirrie 12 P2–P4 Enp-3 with 5:5:5 setae/spines, respectively; P1 Enp-1 reaching tip of Exp-2; female P5 Exp and baseoendopod with 6 and 3 setae, respectively ............................................................................................. N. kastjanensis – P2–P4 Enp-3 with 4:4:4 setae/spines, respectively; P1 Enp-1 reaching tip of Exp-3; female P5 Exp and baseoendopod with 6 and 4 setae, respectively .........................................................................................N. quadriseta Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Use of AI No use of AI was reported. 299 ZooKeys 1258: 277–304 (2025), DOI: 10.3897/zookeys.1258.167766 Ngoc-Son Tran et al.: Nitocra from Vietnam Funding This research was funded by Funds from the Ministry of Education and Training, Vietnam; grant number B2023.DNA.25. Author contributions Conceptualization: SG, NST. Data curation: NST, TPP. Funding acquisition: KCP, NST. Investigation: TPP, NST, KCP. Methodology: TPP, NST, KCP. Project administration: NST, KCP. Resources: KCP, NST. Software: NST, TPP, KCP. Supervision: SG. Validation: NST, SG. 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