Cymbasoma stricturum, Zhou & Lai & Lian & Tan & Shi, 2025

Zhou, Zhiqian, Lai, Yanjiao, Lian, Xiping, Tan, Yehui & Shi, Wei, 2025, A new species of the Cymbasoma longispinosum species-group (Copepoda, Monstrilloida) from the northern South China Sea, ZooKeys 1264, pp. 95-108 : 95-108

Cymbasoma stricturum sp. nov.

Figs 2 , 3 , 4 , Tables 1, 2

Type material.

Holotype: adult female ( SCSMBC 240260 ); Yanjiao Lai leg.; 7 February 2025; partially dissected, formaldehyde preserved .

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-somite 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.

Description of adult female holotype.

Body elongate, with numerous oil droplets inside (Fig. 2 A – 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. 2 B ); 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 2 A, B , 3 A ); 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. 2 A, 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. 3 A, 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. 3 B – D ).

Antennule short, extending downwards (Fig. 4 A, 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 2 d 1, 2 v 1–3, 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 4 d 1, 4 d 2, 4 v 1–3; elements 4 v 1 well developed, thick, and remarkably long; setae IVd, IVv, Vv, Vm, and aesthetasc 4 aes also present on same segment except Vd; element 5 spiniform, appressed in a subdistal position. Subterminal element 6 1, 6 aes present, element 6 2 absent, element b 1–3 branched and b 4, 5 unbranched (Fig. 4 A, B ).

Legs 1–4 (Fig. 3 H – 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. 3 H – 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. 3 F, G ).

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. 3 E ), ridges converging together in lateral surface (Fig. 3 G ); 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. 2 A – C ); spines basally conjoined, individual spines arise beyond posterior margin of caudal rami (see the arrow in Fig. 3 E – G ); spines slender, straight at their base and along shaft, both with distally swollen sections and then tapering apically, one spine slightly shorter (Fig. 2 D, 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 McKinnon 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:

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.

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 .

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) .

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) .

Rounded ventral process between antennules, which is absent in C. morii and C. jinigudira , but shell-like in C. sinopense .

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.

Figures

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.

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.

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.

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).