A new species of the Cymbasoma longispinosum species-group (Copepoda, Monstrilloida) from the northern South China Sea

95 A new species of the Cymbasoma longispinosum species-group (Copepoda, Monstrilloida) from the northern South China Sea Zhiqian Zhou1,2 , Yanjiao Lai3, Xiping Lian1,2 , Yehui Tan1,2 , Wei Shi1 1 South China Sea Marine Biodiversity Collections / Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, China 2 University of Chinese Academy of Sciences, Beijing, 100049, China 3 South China Sea Development Research Institute, Ministry of Natural Resources (Remote Sensing Technology Application Center of South China Sea, NMR), Guangzhou 510300, China Corresponding author: Xiping Lian ([email protected]) Copyright: © Zhiqian Zhou 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 A new monstrilloid copepod species, Cymbasoma stricturum sp. nov., is described and illustrated on the basis of adult females from the Pearl River estuary, northern South China Sea. The new species is closest to C. morii Sekiguchi, 1982, C. sinopense Üstün, Terbiyik & Suárez-Morales, 2014, and C. jinigudira Suárez-Morales & McKinnon, 2016, but it can be distinguished by a combination of characters including strongly protuberant, straight oral papilla; a cephalothorax that is distinctly constricted at the anterior two-fifths in both dorsal and lateral views; and two pairs of well-developed nipple-like processes on anterior dorsal surface, among other diagnostic traits. This is the ninth nominal species known in the Cymbasoma longispinosum species-group. We provide an updated dichotomous key for females and a revised worldwide distribution map of the species-group. Key words: Copepods, Cymbasoma, South China Sea, taxonomy, zooplankton Introduction The order Monstrilloida Sars, 1901 is renowned for its peculiar life history strategies. These semiparasitic copepods undergo ontogenetic niche shift—from endoparasitic juveniles to planktonic free-living adults (Huys and Boxshall 1991). Adult specimens are characterized by non-feeding, free-swimming behaviour and the absence of mouthparts (Grygier and Ohtsuka 1995; Suárez-Morales 2011, 2018). Due to their unique life cycle, monstrilloids are rarely captured in routine zooplankton surveys, which results in significant gaps in our understanding of their biodiversity. In recent decades, records of monstrilloids have increased substantially. A new genus, Sarsimonstrillus Suárez-Morales & McKinnon, 2025, was established by unique combination of characters, including paired uniramous horn-like processes between the antennule bases (Suárez-Morales and McKinnon 2025). Eight valid genera and approximately 197 accepted species are currently recognized within the single family Monstrillidae Dana, 1849 (Walter Academic editor: Kai Horst George Received: 13 June 2025 Accepted: 17 November 2025 Published: 15 December 2025 ZooBank: https://zoobank. org/28751C64-E422-4920-B80466C0B25B7748 Citation: Zhou Z, Lai Y, Lian X, Tan Y, Shi W (2025) A new species of the Cymbasoma longispinosum speciesgroup (Copepoda, Monstrilloida) from the northern South China Sea. ZooKeys 1264: 95–108. https://doi. org/10.3897/zookeys.1264.161966 ZooKeys 1264: 95–108 (2025) DOI: 10.3897/zookeys.1264.161966 96 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea and Boxshall 2025). These species exhibit a wide distribution worldwide, but they are particularly numerous records in Australian waters (Suárez-Morales and McKinnon 2014, 2016, 2025). Currently, Cymbasoma Thompson, 1888 represents the most speciose genus, comprising about 44% of described species. New Cymbasoma species have been discovered recently in deep-sea habitats, where no previous records of Monstrilloida existed (Suárez-Morales and Mercado-Salas 2023). Despite China’s possession of a 32,000-km coastline and vast maritime territories (Gao et al. 2013), taxonomic studies on monstrilloid copepods remain disproportionately scarce, with merely three novel species documented to date (Lian and Tan 2019; Lian and Zhou 2025; Zhou et al. 2025). The Pearl River estuary, located in southern China where the Pearl River discharges into the South China Sea, sustains rich marine biological communities and contributes significantly to regional fisheries. Our recent zooplankton survey in this estuary yielded a new species presenting long ovigerous spines. The taxonomic examination of this material revealed that it belongs to the C. longispinosum species-group. This study aims to describe the new species through comparative analysis with its closest congeneric species, while providing a dichotomous key to the species-group based on foundational work by Suárez-Morales et al. (2020). Materials and methods Zooplankton sample was collected from the coast near Shenzhen (22°29'3.5"N, 113°56'4.2"E), Guangdong Province, China on 7 February 2025 by a vertical tow net (505 μm mesh size, 0.8 m diameter at a towing speed of 0.5 m/s) from the surface to a depth of 5 m (Fig. 1). The specimens were immediately preserved in 5% formaldehyde. Observation and measurements were performed using a stereomicroscope (SMZ18, Nikon, Japan), and drawings were prepared based on the images captured with a digital camera (DS-Fi3, Nikon, Japan). Morphologic terminology follows Huys and Boxshall (1991). The nomenclature for the female monstrilloid antennulary armature proposed by Grygier and Ohtsuka (1995) is followed. The type specimen is deposited in the South China Sea marine biodiversity collections, Chinese Academy of Sciences (SCSMBC), Guangzhou. Taxonomy Subclass Copepoda Milne Edwards, 1840 Order Monstrilloida Sars, 1901 Family Monstrillidae Dana, 1849 Genus Cymbasoma Thompson, 1888 Cymbasoma stricturum sp. nov. https://zoobank.org/AD1FE586-06E3-4900-AEBC-9A59A9C28FB9 Figs 2–4, Tables 1, 2 Type material. Holotype: adult female (SCSMBC 240260); Yanjiao Lai leg.; 7 February 2025; partially dissected, formaldehyde preserved. 97 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea Figure 1. Sampling site (blue dot) of Cymbasoma stricturum sp. nov. near Shenzhen, Guangdong Province, China. Type locality. China • Guangdong Province; coast near Shenzhen; 22°29'3.5"N, 113°56'4.2"E; depth 5 m. Etymology. The new species name is derived from the Latin noun strictūra, meaning “constriction” or “narrowing,” in reference to the characteristic constricted region of the cephalothorax in the new species. The neuter ending –um is adopted (stricturum) to agree with the neuter gender of the generic name Cymbasoma. The proposed Chinese name is “缩缢舟形怪水蚤”. Diagnosis. Female Cymbasoma having the cephalothorax distinctly constricted laterally and ventrally at the anterior 2/5, bearing distinctive transverse belt-like striae at the same level; two pairs of well-developed nipple-like processes on anterior dorsal surface, both bearing shallow concentric reticulation (faintly visible but discernible upon specimen tilting). Antennule 4-segmented, short, and extending downwards, reaching 17.7% of total body length. Swimming legs 1–4 and leg 5 with well-developed plumose setae. Genital double-so- 98 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea Figure 2. Cymbasoma stricturum sp. nov., female holotype. A. Habitus, dorsal; B. Habitus, lateral; C. Habitus, ventral; D. Distal of ovigerous spine, dorsal; E. Distal of ovigerous spine, lateral. A–C. Share the same scale bar; D, E. Share the same scale bar. mite bearing transverse pattern of deep cuticular ridges on proximal dorsal surface. Caudal ramus rectangular, 1.3 times as long as wide, armed with three subequally long lightly setulated caudal setae. Ovigerous spines paired, long, 1.7 times as long as body length, spines proximally fused, with bifurcation beyond distal margin of caudal rami. 99 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea Description of adult female holotype. Body elongate, with numerous oil droplets inside (Fig. 2A–C), 2.37 mm, measured from anterior end of cephalothorax to posterior margin of caudal rami, excluding antennules and caudal setae; cephalothorax (incorporating first pedigerous somite) mostly transparent, 1.70 mm long, representing 71% of total body length; oral papilla conical, straight, strongly protruding located ventrally at anterior 1/5 of cephalothorax (Fig. 2B); cephalothorax distinctly constricted at the anterior 2/5 in both dorsal and lateral views, bearing wide transverse belt of faint and shallow integumental striae (Figs 2A, B, 3A); pair of relatively large ocelli present, pigment cups moderately developed, medially conjoined, strongly pigmented; ventral cup indistinct; forehead flat, with conspicuous, medially convergent cuticular ridges between antennulary bases in dorsal view, bearing a pair of short, slender sensilla (Fig. 2A, B); esophagus within the cephalothorax, broad; ornamentation on anterior ventral surface: rounded cuticular protuberance and paired of simple, conical nipple-like processes between antennulary bases, without adjacent striae; a pair of well-developed nipple-like processes under antennulary bases, with conspicuous striae around (Fig. 3A, C); ornamentation on anterior dorsal surface: two pairs of well-developed nipple-like processes, both with faint and shallow concentric reticulation and three additional dorsal sensilla adjacent to these dorsal nipple-like processes (Fig. 3B–D). Antennule short, extending downwards (Fig. 4A, B), representing almost 18% of total body length and 25% of cephalothorax length; antennule 4-segmented; relative length of segments, from proximal to distal as: 18.5; 21.5; 14.2; 45.8 = 100. In terms of the pattern described by Grygier and Ohtsuka (1995) for female monstrilloid antennulary armature, setae (Roman numerals) and spines (Arabic numerals), element 1 present on first segment; elements 2d1, 2v1–3, and setae IId present on second segment. Third segment with elements 3, setae IIId, and IIIv. Fourth segment long, representing 45.8% of antennule length; segment bearing elements 4d1, 4d2, 4v1–3; elements 4v1 well developed, thick, and remarkably long; setae IVd, IVv, Vv, Vm, and aesthetasc 4aes also present on same segment except Vd; element 5 spiniform, appressed in a subdistal position. Subterminal element 61, 6aes present, element 62 absent, element b1–3 branched and b4,5 unbranched (Fig. 4A, B). Legs 1–4 (Fig. 3H–K) all with both endopod and exopod three-segmented; pedigerous somites 2–4 together accounting for 19.8% of total length in lateral view. Coxa without setae and lacking marginal rows of setae or spines. Basis not fully divided medially from coxa; basal seta of legs 3–4 with biserially plumose, that of leg 4 being thicker; seta on leg 3 being longest. Endopod segments 1–2 of legs 1–4 with swollen outer margins; exopod segment 3 of leg 1 and 3 bearing a small convex protuberance on outer margin. Ramus setae all biserially plumose except uniserially plumose outer seta on exopod segments 3; exopod segments 2–3 and endopod segments 1–2 with setules on outer margins (marked with an asterisk in Fig. 3H–K); outer distal spines on exopod segments 1 and 3 shorter than segments bearing them. Seta/spine armature of swimming legs 1–4 as in Table 1. Leg 5 bilobed, medially conjoined; inner (endopod) lobe rounded, not reaching the half-length of outer (exopod) lobe; outer lobe armed with two long setae apically and one subapical short seta, all heavily plumose; innermost seta relatively slender, shortest, not reaching the half-length of outer two (Fig. 3F, G). 100 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea Figure 3. Cymbasoma stricturum sp. nov., female holotype. A. Cephalosome view, ventral; B. Anterior cephalothorax, emphasizing the striae of the surface, dorsal; C. Same, ventral; D. Same, lateral; E. Genital double and anal somites, lateral; F. Same, ventral; G. Same, lateral; H–K. Legs 1–4, nlp, nipple-like process. B–G. Share the same scale bar; H–K. Share the same scale bar. 101 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea Urosome consisting of three urosomites: fifth pedigerous somite, genital double-somite and anal somite, accounting for 10.5% of total body length, excluding caudal setae; length ratio of urosomites as: 43.3:34.4:22.3 (= 100); genital double-somite subquadrate, with transverse pattern of deep, transverse integumental ridges on proximal half of dorsal surface (Fig. 3E), ridges converging together in lateral surface (Fig. 3G); anal somite trapezoidal, smooth; Caudal rami subrectangular, 1.3 times as long as wide, armed with three subequally long lightly setulated caudal setae; ovigerous spines paired, 4.22 mm long, 1.7 times as long as body length (Fig. 2A–C); spines basally conjoined, individual spines arise beyond posterior margin of caudal rami (see the arrow in Fig. 3E–G); spines slender, straight at their base and along shaft, both with distally swollen sections and then tapering apically, one spine slightly shorter (Fig. 2D, E). Remarks. The monstrilloid copepod described herein from the Pearl River estuary is assigned to the genus Cymbasoma based on the presence of a single free somite between the caudal rami and the genital double-somite, and the caudal rami bearing only three setae (Huys and Boxshall 1991; Benz 2005; Suárez-Morales 2011). It is placed in the Cymbasoma longispinosum species-group by its body proportions—specifically the elongate cephalothorax (65–71% of total body length), long, proximally fused ovigerous spines (1.7 times the body length), and the presence of conspicuous cuticular dorsal ornamentation on the genital double-somite (Grygier 1994; Suárez-Morales 2011; Üstün et al. 2014; Suárez-Morales et al. 2020). Since the original description of C. longispinosum s. str. from the English Channel (Bourne 1890), the nominal species has been reported worldwide (Giesbrecht 1893; Sars, 1921; Rose 1933; Dakin and Colefax 1940; Wilson 1950; Marques 1961; Martin-Thompson 1973; Dias 1996). However, subsequent studies revealed that some specimens initially identified as C. longispinosum represent undescribed cryptic species, including C. cf. longispinosum recorded from Brazil (Dias and Bonecker 2007; Leite et al. 2010; Suárez-Morales et al. 2020). These specimens exhibit subtle yet consistent differences (such as the proportions of the cephalothorax and genital double-somite), limited geographical distributions, and have consequently been classified under the C. longispinosum species-group (Suárez-Morales et al. 2020). Eight species belonging to this group have been recognized worldwide (Fig. 5), including C. longispinosum (Bourne, 1890), C. morii Sekiguchi, 1982, C. chelemense Suárez-Morales & Escamilla, 1997, C. californiense Suárez-Morales & Palomares-García, 1999, C. janetae Mageed, 2010, C. sinopense Üstün, Terbiyik & Suárez-Morales, 2014, C. jinigudira Suárez-Morales & McKinnon, 2016, and C. belizense Suárez-Morales, Vásquez-Yeomans & Santoya, 2020 (Bourne 1890; Sekiguchi 1982; Suárez-Morales and Escamilla 1997; Suárez-Morales and Palomares-García 1998; Mageed 2010; Üstün et al. 2014; Suárez-Morales and McKTable 1. Armature of legs 1–4 including coxa, basis, exopods, and endopods in Cymbasoma stricturum sp. nov. Roman numerals indicate numbers of spines, and Arabic numerals indicate numbers of setae. Coxa Basis Endopod Exopod Leg 1 0-0 1-0 0-1; 0-1; 2,2,1 I-1; 0-1;I,2,2 Leg 2–4 0-0 1-0 0-1; 0-1; 1,2,2 I-1; 0-1;I,2,3 102 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea Figure 4. Cymbasoma stricturum sp. nov., female holotype. A. Left antennule, in dorsal view but tilted little from the medial side; B. Right antennule, lateral. A, B. Share the same scale bar. 1 Ⅱd Ⅲd 3 Ⅴv Ⅳd 4aes Ⅴm b4 b3 b5 61 b1 b2 6aes Ⅳv 1 2v1 2v2 2v3 Ⅱd 2d1 ⅢvⅢd 3 4v1 4v2 Ⅴm 4d1 Ⅴv Ⅳv Ⅳd b4b3 b5 61 b2 5 4v3 4d2 b1 6aes 100 μm 2v2 2d1 4d1 4d2 5 AB Figure 5. Worldwide distribution of valid species of the Cymbasoma longispinosum species-group. C. longispinosum s.str. conforms with Bourne’s (1890) description; the Brazilian C. cf. longispinosum is a tentative identification requiring further taxonomic verification (Dias and Bonecker 2007). 103 ZooKeys 1264: 95–108 (2025), DOI: 10.3897/zookeys.1264.161966 Zhiqian Zhou et al.: Cymbasoma stricturum sp. nov. from the South China Sea innon 2016; Suárez-Morales et al. 2020). The present species, C. stricturum sp. nov., constitutes the ninth one. The new species most closely resembles C. morii, C. sinopense, and C. jinigudira, mainly by its relatively long cephalothorax (over 65% of body length) and ovigerous spines exceeding 150% of body length. Cymbasoma stricturum sp. nov. is readily distinguishable from its congeners by the following combination of characters: 1) Oral papilla strongly protuberant and straight, contrasting with the regularly developed condition in other members of the species group, except in C. morii, in which it is posteriorly curved. 2) Conspicuous, medially convergent forehead integumental ridges, which are absent in C. morii, longitudinally regular pattern in C. sinopense, shallow and simple transverse pattern in C. jinigudira. 3) Two pairs of well-developed nipple-like processes on anterior dorsal surface, both with shallow concentric reticulation that becomes discernible upon tilting the specimen; within the species group this character appears unique, although a similar condition has been described in the Mexican C. quintanarroense Suárez-Morales, 1994, which was originally described as belonging to the genus Thaumaleus (Suárez-Morales 1994). 4) Wide, transverse, belt-like striae, faint and shallow on anterior 2/5 of cephalothorax, shared only with C. belizense within the species group; distinguishingly, unlike C. belizense, the new species is uniquely characterized by a distinct transverse and ventral constriction of the cephalothorax at this same level; in addition, a similar but narrower belt-like striation is present in the Mexican C. boxshalli Suárez-Morales, 1993, originally described as Thaumaleus boxshalli (Suárez-Morales 1993). 5) Rounded ventral process between antennules, which is absent in C. morii and C. jinigudira, but shell-like in C. sinopense. 6) Armature of the outer lobe of leg 5, with all three setae being heavily plumose and the innermost seta not reaching the half-length of outer two. The strongly plumose condition is shared only with C. longispinosum; this contrasts with the sparsely plumose setae in C. chelemense, C. californiense, and C. sinopense, and with the naked setae in C. morii, C. jinigudira, and C. belizense. Additionally, C. stricturum sp. nov. is among the largest members within the species-group based on key morphometric parameters: total body length (2.38 mm), cephalothorax–body length ratio (71%), and ovigerous spine length/ body length ratio (1.7×). Notably, the plumose setae of legs 1–5 exhibit the most strongly developed observed among the known members of this species-group. Taken together, these characters confirm the recognition of Cymbasoma stricturum sp. nov. as a valid new species of Cymbasoma. To facilitate interspecific comparisons, we critically re-evaluated the published diagnostic characters (Table 2) and synthesized a revised diagnostic key (Üstün et al. 2014; Suárez-Morales et al. 2020). Characters unverifiable from literature—notably the exopod/endopod length ratio of leg 5—were excluded. Illustrations of dissected fifth legs are rare, and the legs are usually depicted in situ from different angles.