Monstrilla gibbosa Suarez-Morales & Palomares-Garcia, 1995

Figs 16, 17, 18, 19, 20

Type material.

Holotype • adult female, undissected, deposited in the collection of Crustacea, U. S, National Museum of Natural History, Smithsonian Institution, USNM 259488 . Adult female paratype USNM -259665.

Type locality.

Puerto Escondido coastal system, southern part of the eastern coast of the Baja California Peninsula, Mexico (25°49'N, 111°18'W). Date of collection 18 November 1993.

Description of adult female holotype.

Body length of holotype 4.2 mm. Cephalothorax moderately robust, representing almost 60 % of total body length, with lateral expansions at distal end of cephalothorax (Fig. 16 A – C; Suárez-Morales and Palomares-García 1995: fig. 1 a). Oral cone well developed, prominent, papilla-like (Figs 17 B, 18 A, oc), located at 32 % on ventral surface of cephalothorax. Cephalic region with weakly produced, lightly rugose ‘ forehead’ (Figs 16 B, 17 B) with pair of small spherical processes near insertion of antennules (Figs 16 A – C, 17 B, 19 A, arrowheads); forehead with pair of minute sensilla (Fig. 20 A, sl). Ventral preoral surface with medially divided hump-like medial protuberance (Figs 17 B, 18 A, 19 A, poh). Integumental ornamentation consisting of single pair of well-developed nipple-like processes with adjacent integumental wrinkles (Figs 18 A, B, 19 B, nlp), and field of minute longitudinal wrinkles stretching ventrally between nlps and oral cone (Figs 17 B, 18 A). Eyes comprising two lateral cups (Figs 17 B, 20 A, lec) and ventral medial cup (Figs 17 B, 20 A, mec), lateral and medial cups with approximately the same diameter, eyes weakly pigmented.

Urosome consisting of four somites: fifth pedigerous somite (carrying fifth legs), genital double-somite with pair of ventral ovigerous spines reaching well beyond distal margin of caudal rami (Fig. 18 C; Suárez-Morales and Palomares-García 1995: fig. 1 B), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) being: 36.66: 39.96: 13.98: 9.49 (Figs 1 C, 20 C). Genital double-somite longest of urosome, with weakly expanded proximal 1 / 2 ornamented with transverse ridges on dorsal surface; distal 1 / 2 ornamented with scattered integumental wrinkles in lateral and dorsal surfaces (Figs 18 C, 20 C); pair of long, slender ovigerous spines inserted on ventral surface (Figs 16 A – C, 17 B, 18 C, os); spines basally conjoined, equally long, both ending in acute, parallel points. Caudal rami subquadrate (Fig. 18 C), ~ 1.3 × as long as broad, each ramus armed with five caudal setae (I – V), proximal outer seta I longest, outer terminal seta III proximally expanded, seta V shortest, slender, inserted dorsally (Figs 18 C, 20 C, D).

Antennules relatively robust, slender, divergent, 0.42 mm in length, ~ 20 % of total body length and almost 35 % of cephalothorax length (Fig. 16 B, C); antennules 4 - segmented, segments 3 and 4 partly fused; Intersegmental division between segments 1 and 2 and 2 and 3 complete (Fig. 17 A); length ratio of antennular segments (proximal to distal) 19.20: 16.55: 17.88: 46.36 (Fig. 20 B). Following Grygier and Ohtsuka’s (1995) setal nomenclature for female antennules, first segment armed with slender, spiniform element 1; second segment carrying complete armature comprising slender spiniform elements 2 v 1-3 and 2 d 1, 2, and setiform element IId (Figs 17 A, 20 B); third segment with slender, stout spiniform element 3 and setiform elements IIId and IIIv (Fig. 17 A), fourth segment longest of antennule, proximal 1 / 2 armed with short, slender spiniform elements 4 v 1, 2, setiform elements IVv and IVd, and aesthetasc 4 aes (Figs 17 A, 20 B). Distal 1 / 2 of fourth segment armature comprising setiform elements Vd, Vm, Vd, and spiniform element 5; apical armature including apical aesthetasc 6 aes, apical spine 6 1, 2, the former being short and blunt. Outer distal margin with setae of the “ b-group ”; b 1-3, 5, 6, only b 1, b 3 branched (Fig. 20 B).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 19 C – E), all with exopod longer than endopod. Swimming legs 1–4 robust, with setal armature pattern as described by Suárez-Morales and Palomares-García (1995) (Fig. 19 C – E). Exopodal spines of legs 1–4 apically rounded (Fig. 19 C – E, arrowheads). Basipodal seta of third swimming leg longest (Fig. 19 D) Armature formula of swimming legs 1–4 as:

Legs Basis Endopod Exopod:

Leg 1 1-0 0-1; 0-1; 1, 2-2 I- 1; 0-1; I, 2, 2

Legs 2–4 1-0 0-1; 0-1; 1, 2, 2 I- 1; 0-1; I, 2, 3

Fifth legs (Figs 18 C, D, 20 C, D) biramous, represented by large, weakly corrugate subrectangular exopodal segment armed with three equally long terminal setae (Fig. 18 C, D). Endopod represented by small thumb-like lobe armed with short distal seta (Fig. 18 D; Suárez-Morales and Palomares-García 1995: fig. 2 a). Fifth leg setae long, reaching proximal margin of caudal rami (Fig. 18 C; Suárez-Morales and Palomares-García 1995: figs 2 a, 3 a, b).

Male. Unknown.

Remarks.

In the original description of this species, Suárez-Morales and Palomares-García (1995) emphasized the structure and armature of the fifth leg as the main distinctive character of M. gibbosa . They compared it with several other congeneric species sharing a 3 exopodal- 1 endopodal fifth leg setal armature, like M. turgida Scott, 1909, M. reticulata Davis, 1949, M. longicornis Thompson, 1890, M. lata Desai & Bal, 1963, and M. barbata . Details of the fifth leg lobes and setae were compared among species of this group to reliably recognize M. gibbosa . In addition, Suárez-Morales and McKinnon (2025) included M. gibbosa among the species of Monstrilla with a relatively long fifth leg endopodal lobe, together with the Baja Californian M. hendrickxi Suárez-Morales & Velázquez-Ornelas, 2024 and the Australian M. janetgrieveae Suárez-Morales & McKinnon, 2025; the inner lobe of M. gibbosa is clearly the shortest among these species, barely reaching the midlength of the exopodal lobe inner margin; in addition, the endopodal lobe is unarmed in M. hendrickxi . The antennules length and segmental fusion patterns were also evaluated to distinguish M. gibbosa . It was also noticed that the rhomboid shape of the apical spiniform antennular element 6 1 (sensu Grygier and Ohtsuka 1995) is unique among the compared species of Monstrilla . The presence of an antero-ventral hump-like cephalic process was described as the main distinctive character of M. gibbosa; in fact, this process, previously described as two processes, is a single bipartite one (Fig. 19 A), but not discernible as such in lateral view (Fig. 18 A). Another unique character of this species is the presence of a pair of globular processes in the frontal area adjacent to the antennules insertion; this kind of process has not been described in other known species of Monstrilla . It was depicted by Suárez-Morales and Palomares-García (1995: fig. 3 d) but was not included in the description. Also, Suárez-Morales and Palomares-García (1995: fig. 2 b, c) depicted the antennular setal armature as comprising three branched setae, but there are only two branched setae (b 1, b 3) in the holotype (Figs 17 A, 20 B). A set of branched “ b-group ” setae has been reported also in other species of the genus like M. grandis Giesbrecht, 1891, M. bahiana Suárez-Morales & Dias, 2001, M. bernardensis Davis & Green, 1974 (Suárez-Morales and McKinnon 2025) and recently also in the Chinese M. pseudograndis Zhou, Lian & Tan, 2025 .

In Suárez-Morales and Palomares-García (1995: fig. 2 a), the fifth leg exopodal setae are depicted as having a relatively shorter middle exopodal seta, whereas the innermost seta is in fact the shortest in the holotype (Figs 17 B, 18 C). Another relevant character that was depicted (Suárez-Morales and Palomares-García (1995: figs 1 a, 3 b) but not described is the presence of a proximally swollen caudal seta (seta III) in M. gibbosa (Figs 18 C, 20 D). According to Suárez-Morales and McKinnon (2025), several Australian species of Monstrilla exhibit proximally swollen caudal setae (setae III and IV), thus diverging from M. gibbosa, exhibiting a single swollen seta.