Monstrilla pseudograndis, Zhou & Lian & Tan, 2025

Monstrilla pseudograndis sp. nov.

Figs 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , Tables 1 View Table 1 , 2 View Table 2

Type material.

Holotype: • adult female ( SCSMBC 240185 ); Zhiqian Zhou leg.; 7 June 2023; partially dissected, formaldehyde preserved GoogleMaps .

Type locality.

China • Guangxi Province; coast near Fangchenggang   GoogleMaps . 21°22'53"N, 108°19'38"E; salinity 29.42, temperature 31°C; depth 17 m.

Etymology.

The species name is derived from the Greek word pseudo, meaning “false”, and the name of the closely similar M. grandis Giesbrecht, 1891 .

Diagnosis.

Female Monstrilla with smooth cuticle on cephalothorax; forehead medially concave, bearing a pair of short sensilla and small setae bilaterally near antennule bases. Cephalothorax ventrally marked by four pairs of small, nipple-like scars, arranged symmetrically anterior to oral papilla. Oral papilla located at approximately midlength of cephalothorax, ventrally posteriorly-bent. Antennule two-segmented, segments fused distally, reaching 37.8 % of total body length. Legs 1–4 with relatively short outer exopodal spines. Fifth legs bilobed, outer lobe elongate, with three plumose setae; inner lobe shorter, bearing two plumose setae and a basal protuberance on inner margin. Caudal rami 2.1 times as long as wide, divergent posteriorly, each armed with six well-developed caudal setae.

Description of adult female holotype.

Body moderately elongate (Fig. 2 A – C View Figure 2 ), about 1.68 mm, measured from anterior end of cephalothorax to posterior margin of caudal rami, excluding antennules and caudal setae. Cephalothorax rather large and relatively long, accounting for about 57.7 % of total body length, transparent, dorsal surface smooth; anterior 2 / 5 slightly swollen laterally and ventrally. Nauplius eye present, weakly developed, elliptical, ocelli unpigmented with separate oval hyaline bodies, separated by 1 ½ eye diameter (Fig. 2 A View Figure 2 ). Anteriormost part of cephalothorax with ventral, rounded convex protuberance with irregular margin in lateral view (Fig. 2 B View Figure 2 ); cuticular ornamentation observed on the surface of cephalothorax in lateral and ventral view. Forehead slightly concave medially between antennulary bases in dorsal view, without rostral protrusion, bearing a pair of short, slender sensilla in the middle and a pair of setae near first antennule (Fig. 3 B View Figure 3 at arrow); weak, fine, longitudinal and transverse wrinkles running behind antennular bases on each side of lateroventral surfaces, flanked by four pairs of small nipple-like scars ahead of oral papilla (Fig. 3 A View Figure 3 at arrow), without sensory pore. Oral papilla situated slightly posterior to midlength of cephalothorax, accounting for about 52.9 %, protruding ventrally, with distal half posteriorly-bent (Fig. 3 A View Figure 3 ).

Antennule long (Fig. 4 A, B View Figure 4 ), about 37.8 % of total body length, about 65.5 % of the cephalothorax; antennule two-segmented, only first segment distinctly separate, remaining segments fused, with constrictions along antennular body representing places of intersegmental divisions (purported 2–5), length ratio of antennule segments, from basal to distal one: 14.0: 86.0 (= 100). In terms of the pattern described by Grygier and Ohtsuka (1995) for female monstrilloid antennulary armature, setae (Roman numerals) and spines (Arabic numerals), short, slender element 1 present on first segment; purported segment two with elements 2 d 1, 2 d 2, 2 v 1, 2 v 2, 2 v 3, IId; purported segment three with elements 3, IIId and IIIv; purported segment four with elements 4 v 1–2, 4 d 1, IVd and IVv as well as 4 aes (aesthetasc); purported segment five with elements 5, Vd, Vv and Vm; setae b 1–3, b 5 all dichotomously branched from proximal half or third, setae b 4 simple, without b 6. Apical elements 6 1, 6 2, and 6 aes present, but 6 aes absent on right antennule in dorsal view.

Legs 1–4 (Fig. 3 D – G View Figure 3 ) all with both endopod and exopod three-segmented. Coxa without setae and lacking marginal rows of setae or spines. Basis not fully divided medially from coxa; all outer basal setae on legs 1–4 slender, naked; seta on leg 3 much longer. First and second exopodal segments of legs 1–4 slightly swollen, third exopodal segement undulate along outer distal margin; outer margins of all endopodal segments swollen and smooth. Outer distal spines on first and third exopodal segments of legs 1–4 feeble, much shorter than segments bearing them. Seta / spine armature of swimming legs 1–4 as in Table 1 View Table 1 .

Leg 5 bilobed, both lobes confluent basally and divided distally. Outer lobe elongate, armed with three long, plumose setae apically or subapically, of subequal lengths. Inner lobe relatively short, with a basal protuberance on inner margin (arrowed in Fig. 3 C View Figure 3 ), its tip exceeding the half of the outer lobe, armed with two plumose setae apically or subapically.

Urosome consisting of four urosomites: fifth pedigerous somite, genital double-somite, free postgenital somite and anal somite, accounting for 17.9 % of total body length, excluding caudal setae; ratio of lengths 30.3: 44.0: 15.4: 10.3 (= 100). Genital double-somite representing almost half-length of urosome (44.0 %), somewhat swollen laterally, partial suture visible; about 1.7 times longer than the combined length of the next two segments. Genital double-somite bearing pair of long ovigerous spines, these being inserted on middle of ventral surface, basally separated, with pointed tips extending far beyond tips of caudal setae, in total equal to about 38.2 % of total body length. Anal somite trapezoidal; lateral margin nearly smooth in dorsal but with apparent notch in ventral; lacking wrinkles or striae both on dorsal and ventral surfaces.

Caudal rami long (Fig. 3 C View Figure 3 ), about 2.1 times as long as wide; divergent outward; with small cuticular protuberance at basal part of outer face and slightly swollen at distal part of inner face; each ramus armed with six well-developed caudal setae, consisting of two distal, two lateral, one inner distal, and one dorsomedial setae.

Remarks.

The new species is assigned to the genus Monstrilla based on the presence of one free postgenital somite and anal somite in the female, six caudal setae, and the oral papilla located ventrally at nearly midlength of the cephalothorax ( Isaac 1975; Huys and Boxshall 1991; Suárez-Morales 2011). Among the females of Monstrilla , there are two main types of fifth leg, one of which is formed by a single lobe, such as M. mariaeugeniae Suarez-Morales & Islas-Landeros, 1993 ( Suárez-Morales and Islas-Landeros 1993). The type II fifth leg is bilobed. Six species, M. annulata Suárez-Morales, 2024 , M. cymbula A. Scott, 1909 , M. gibbosa Suárez-Morales & Palomares-García, 1995 , M. grandis , M. grygieri Suárez-Morales, 2000 , and M. investigatoris Sewell, 1949 , are similar to the new species in having the type II fifth leg and the antennule that exceeds ½ the length of the cephalothorax (Table 2 View Table 2 ) ( Giesbrecht 1891; Scott 1909; Sewell 1949; Suárez-Morales and Palomares-García 1995; Suárez-Morales 2000 b, 2000 a, 2024; Chang 2014). Notably, only two species, M. cymbula and M. grandis , share similar features with M. pseudograndis sp. nov., including six well-developed caudal setae on the caudal rami and five setae on the bilobed fifth leg. These three species exhibit the same setation pattern in the fifth leg, a plesiomorphic character state with three exopodal and two endopodal setae, largest number of setae found on the fifth leg in monstrilloids ( Huys and Boxshall 1991; Suárez-Morales 2024).

The forehead between the antennular bases exhibits different cuticular dorsal ornamentations in various species. This area may appear concave with a pair of small setae ( Lee and Chang 2016), they may form a protrusion ( Suárez-Morales et al. 2013, 2017; Suárez-Morales and Castellanos-Osorio 2019; Suárez-Morales 2024), or they present a flat surface lacking both sensilla and setae ( Suárez-Morales and Gasca 2003; Suárez-Morales et al. 2020; Suárez-Morales 2021). Notably, setae and sensilla are either present singly or absent in other species. The new species possesses two distinctive features that render it readily distinguishable among its congeners and support its status as a new member of Monstrilla . First, M. pseudograndis sp. nov. bears a pair of bilateral setae on the forehead and a pair of medial sensilla (Fig. 3 B View Figure 3 ), marking a unique combination that has not been previously reported in Monstrilla . Second, the new species has four pairs of small, nipple-like scars on the cephalothorax, flanked in ventral view, which are commonly recorded as 1–3 pairs in Monstrilla ( Suárez-Morales 2011, 2018; Jeon et al. 2024).

Among all known species of Monstrilla , the new species is most closely related to M. grandis , which was reported by Giesbrecht (1891) in the southeastern Atlantic Ocean from southern Patagonia. As a widespread species, M. grandis has been extensively redescribed by researchers worldwide ( Huys and Boxshall 1991; Suárez-Morales 2000 b; Chang 2014). Monstrilla grandis and the new species share similar body proportions, including total body length, the position of the oral papilla, and the relative lengths of the cephalothorax and ovigerous spines. In addition to the two distinctive features above mentioned, several significant differences also exist: 1) the relative length of the antennule and the number of segments, which is 37.8 % and two in M. pseudograndis sp. nov., compared to 48 % of the total body length and three in M. grandis ; 2) the ventral sensory pores of cephalothorax are absent in M. pseudograndis sp. nov., whereas they are present in M. grandis ; 3) M. pseudograndis sp. nov. bears a protruding ventral oral papilla, with the distal half posteriorly-bent, in contrast to the slightly protruding, unbent midventral papilla in M. grandis .; 4) the setae on the first segment of endopod of leg 2 present in M. pseudograndis sp. nov., whereas absent in M. grandis ; 5) a thumb-like process is absent on the distal part of the inner margin of the outer lobe in M. pseudograndis sp. nov., and it is present in M. grandis ; 6) the inner margin of the inner lobe bears a basal protuberance in M. pseudograndis sp. nov., which is absent in M. grandis .