Monstrillopsis plumosa, Lian & & Tan & Yehui, 2025

Lian, Xiping, , Zhiqian Zhou, Tan, & Yehui, 2025, First record of Monstrillopsis G. O. Sars, 1921 (Copepoda: Monstrilloida) from the South China Sea, with description of a new species, Raffles Bulletin of Zoology 73, pp. 198-204 : 199-203

Monstrillopsis plumosa , new species

( Figs. 2–5)

Materialexamined. Holotype, adultmale (SCSMBC-031012), 1.46 mm total length (TL), Sanya Bay , Hainan Province, China ( 18.21°N, 109.47°E), vertical haul, 5–0 m, 0.505 mm-mesh plankton net, coll. X. Lian, 10 December 2016 . Paratype, adult male (SCSMBC-031013), 1.43 mm total length (TL), same data as holotype.

Diagnosis. Medium-sized male Monstrillopsis ( 1.46 mm total body length), with body divided in relatively short prosome representing about half of total body length, pedigerous somites 2–4 tapering posteriorly, and slender, cylindrical urosome. Cephalothorax with low, rounded medial frontal projection, Antennule 5-segmented, geniculate. Geniculation between segments 4, 5. Last segment relatively short, lacking inner expansion, with apical claw less than half length of segment. Fifth pedigerous somite separate from preceding somite, 5 th legs absent. Legs 1–4 with outer seta on basis; exopods, endopods 3-segmented, basipodal setae absent. Genital somite with no obvious transverse striations in dorsal field. Urosome with two postgenital somites; anal somite as long as genital double-somite. Ventral genital complex represented by pair of slender distally diverging lappets; lappets medial surface smooth. Caudal rami subquadrate, with four subequally long caudal setae.

Description. Adult male. Total body length of holotype 1.46 mm, as measured from anterior end of cephalic somite to posterior margin of anal somite ( Fig. 3A, B). Cephalothorax length about 0.4 times whole body length ( Fig. 3a). Forehead medially flat in dorsal view, lacking integumental ornamentation. Eyes represented by one ventral, two lateral cups; pigment cups moderately developed, weakly pigmented; ventral cup slightly smaller than lateral cups ( Fig. 3b). Oral papilla small, located on about anterior one-third of ventral surface of cephalothorax ( Fig. 3c). One pair of rounded protuberances on dorsal surface below ocelli ( Fig. 3d).

Antennule relatively long, measuring 0.49 mm, about 0.34 times total body length. Antennule 5-segmented with geniculation between 4 th, 5 th segments ( Fig. 4A, B). Length ratio of segments 10.85%: 23.25%: 8.52%:28.68%: 29.45%. First segment with element 1 arising dorsally on inner corner. Second segment armed with spinous elements 2d 1, 2d 2, 2v 1, long strap-like, outward seta IId; Dorsal spinous elements (2v 1) slightly longer than ventral ones (2d 1, 2d 2). Third segment with elements 3d 1, 3d 2, 3v 1, IIId, IIIv. Long IIIv, IIId setae located on inner side, extended downward to second segment. Fourth segment with 6 elements (4d 1, 4d 2, 4v 1-3, IVd), all arising at inner side. Terminal antennular segment modified: with slight inner proximal expansion, rest of distal part relatively thin, elongate, short and sabre-like. Terminal segment armed with unbranched elements 1–5. Among these, elements 1–3 located on outer distal margin, and element 5 located on inner side and longer than elements 1–4 ( Fig. 4A).

First thoracic pedigerous somite incorporated into cephalothorax; this and 3 free succeeding pedigerous somites each bearing pair of well-developed legs, with endopodites, exopodites 3-segmented ( Fig. 5 A). Pedigerous somites 2–4 accounting for 34.93% of total body length in dorsal view segment on leg 1 also spiniform ( Fig. 5A). Setal patterns of legs 1–4 shown in Figure 5 and Table 1 (Roman numerals indicate numbers of spines, and Arabic numerals indicate numbers of setae).

Urosome consisting of 4 somites: 5 th pedigerous somite (5 th legs absent), genital somite with genital apparatus, 1 free postgenital somite, long anal somite ( Fig. 4C). Genital somite rounded in lateral view, no obvious transverse striations, with enlarged base of cylindrical shaft. Cylindrical shaft smooth, with short bud at insertion of lappets, visible in lateral, ventral views ( Fig. 4D, E). Distal part armed with two genital lappets. Lappets reaching beyond midlength of anal somite. Lappets smooth, flake-like, without protrusion or ornamentation, transparent, slightly tapering distally in ventral view ( Fig. 4).

Anal somite trapezoidal; lateral margin smooth. Caudal rami subquadrate, smooth both on dorsal, ventral surfaces. Each ramus armed with 4 subequally long, non-plumose setae: 3 dorsal apical, 1 inner ventral.

Female. Unknown.

Etymology. The species epithet is derived from the Latin adjective ‘plumosa’ which means feathery or feathered, in allusion to the plumose (feather-like) setae of the swimming legs on the endopodite and exopodite of the new species; used as an adjective (feminine).

( Fig. 3 A, B). Protopods consisting of large coxal portion, relatively small basis ( Fig. 5A, B). Articulation between coxa, basis clearly expressed. Basis of legs 1–4 lacking setae. Ramus setae all long except for spiniform outer setae on exopodal segments 1, 3, inner seta of first exopodal

Distribution. Known only from the type locality at Sanya Bay, Hainan, China.

Remarks. The male type specimens of the present new species are easily assignable to the genus Monstrillopsis by virtue of several characters including the sabre-like spine on the tip of antennule, the expansion in the last segment of the attennule, the oral papilla located far anteriorly on the cephalothorax, and the fully developed eyes ( Huys & Boxshall, 1991; Suárez-Morales et al., 2006). The most obvious distinguishing feature of Monstrillopsis plumosa , new species, is the modified fifth antennular segment, with a slight inner expansion and an attenuated, short sabre-like spine on the tip. Suárez-Morales & McKinnon (2014) recognised two main types of male genital complexes in Monstrillopsis . Type I has a long, well-developed cylindrical shaft and relatively short, rounded lappets, and type II has a short shaft and relatively long, basally separated lappets, as in M. plumosa , new species. The type II genital complex was previously reported in eight other congeneric species, including M. chilensis Suárez-Morales, Bello-Smith & Palma, 2006 , M. chathamensis Suárez-Morales & Morales-Ramírez, 2009 , M. coreensis Lee, Kim & Chang, 2016 , M. hastata Surarez-Morales & McKinnon, 2014 , M. boonwurrungorum Surarez-Morales & McKinnon, 2014 , M. longilobata Lee, Kim & Chang, 2016 , M. pontoeuxinensis Suárez-Morales & Ustun, 2018 , and M. paradoxus Jeon, Lee, Soh & Eyun, 2019 (Suárez-Morales et al., 2014). However, six species of the type II group, i.e., M. chilensis , M. chathamensis , M. hastata , M. boonwurrungorum , M. pontoeuxinensis , and M. paradoxa , can be immediately excluded from further morphological consideration because of significant differences in body size compared to the new species. The body length of these species is less than 1.0 mm (i.e., 0.93 mm in M. chathamensis , 0.81 mm in M. hastata , 0.91 mm in M. boonwurrungorum , 0.561 in M. pontoeuxinensis , 0.78 mm in M. paradoxa , and 0.78 mm in M. chilensis ) ( Suárez-Morales & Morales-Ramírez, 2008, 2009; Suárez-Morales et al., 2014; Suárez-Morales & Üstün, 2018; Jeon et al., 2020). In addition, among the known type-II Monstrillopsis species, M. paradoxus is unique in having the minimum number of postgenital somites, specifically just one segment, while M. plumosa and M. hastata have two postgenital somites, and the rest have three postgenital somites ( Suárez-Morales et al., 2006; Lee et al., 2016) ( Table 2).

Monstrillopsis plumosa , new species, also differs from its abovementioned congeners in other characters, such as the slight inner expansion and the short, sabre-like apical element in the distal antennulary segment ( Fig. 3d). The quantity of plumose setae on swimming legs 1–4 of the new species is another remarkable feature. In the genus Monstrillopsis , other species either have only one plumose seta on the outer apical exopodites or have entirely glabrous legs. Both M. plumosa and M. coreensis , on the other hand, are exceptional for having several plumose setae on their swimming legs 1–4 ( Lee et al., 2016). Nonetheless, the plumose setae of M. coreensis and M. plumosa also differ from one another. Every ramus seta on the swimming legs of M. coreensis is plumose and heterogeneously ornamented with setules, whereas only specific ramus setae of M. plumosa exhibit plumose characters ( Fig. 5) ( Lee et al., 2016). In M. plumosa , Leg 1 has 11 plumose setae on the endopodites and exopodite, Leg 2 has 5, and Leg 3 and 4 have 4 plumose setae each on their endopodites ( Fig. 5).

The new species can be distinguished from all the other species of Monstrillopsis on the basis of this suite of characters: 1) a type II genital complex; 2) total body length greater than 1.0 mm ( 1.46 mm & 1.43 mm for holotype & paratype, respectively); 3) two postgenital somites (containing anal somite); 4) a slight inner expansion and a short, sabre-like apical element in the distal antennulary segment; 5) the fifth genital somite with a short bud at the insertion of the lappets; and 6) the distinct pattern of plumose setae on the swimming legs.