Monstrilla gibbosa Suarez-Morales & Palomares-Garcia, 1995

Monstrilla gibbosa Suarez-Morales & Palomares-Garcia, 1995 View in CoL

Figs 16 View Figure 16 , 17 View Figure 17 , 18 View Figure 18 , 19 View Figure 19 , 20 View Figure 20

Type material.

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

Type locality.

Puerto Escondido coastal system   GoogleMaps , 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 View Figure 16 ; Suárez-Morales and Palomares-García 1995: fig. 1 a). Oral cone well developed, prominent, papilla-like (Figs 17 B View Figure 17 , 18 A View Figure 18 , oc), located at 32 % on ventral surface of cephalothorax. Cephalic region with weakly produced, lightly rugose ‘ forehead’ (Figs 16 B View Figure 16 , 17 B View Figure 17 ) with pair of small spherical processes near insertion of antennules (Figs 16 A – C View Figure 16 , 17 B View Figure 17 , 19 A View Figure 19 , arrowheads); forehead with pair of minute sensilla (Fig. 20 A View Figure 20 , sl). Ventral preoral surface with medially divided hump-like medial protuberance (Figs 17 B View Figure 17 , 18 A View Figure 18 , 19 A View Figure 19 , poh). Integumental ornamentation consisting of single pair of well-developed nipple-like processes with adjacent integumental wrinkles (Figs 18 A, B View Figure 18 , 19 B View Figure 19 , nlp), and field of minute longitudinal wrinkles stretching ventrally between nlps and oral cone (Figs 17 B View Figure 17 , 18 A View Figure 18 ). Eyes comprising two lateral cups (Figs 17 B View Figure 17 , 20 A View Figure 20 , lec) and ventral medial cup (Figs 17 B View Figure 17 , 20 A View Figure 20 , 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 View Figure 18 ; 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 View Figure 1 , 20 C View Figure 20 ). 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 View Figure 18 , 20 C View Figure 20 ); pair of long, slender ovigerous spines inserted on ventral surface (Figs 16 A – C View Figure 16 , 17 B View Figure 17 , 18 C View Figure 18 , os); spines basally conjoined, equally long, both ending in acute, parallel points. Caudal rami subquadrate (Fig. 18 C View Figure 18 ), ~ 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 View Figure 18 , 20 C, D View Figure 20 ).

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 View Figure 16 ); antennules 4 - segmented, segments 3 and 4 partly fused; Intersegmental division between segments 1 and 2 and 2 and 3 complete (Fig. 17 A View Figure 17 ); length ratio of antennular segments (proximal to distal) 19.20: 16.55: 17.88: 46.36 (Fig. 20 B View Figure 20 ). 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 View Figure 17 , 20 B View Figure 20 ); third segment with slender, stout spiniform element 3 and setiform elements IIId and IIIv (Fig. 17 A View Figure 17 ), 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 View Figure 17 , 20 B View Figure 20 ). 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 View Figure 20 ).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 19 C – E View Figure 19 ), 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 View Figure 19 ). Exopodal spines of legs 1–4 apically rounded (Fig. 19 C – E View Figure 19 , arrowheads). Basipodal seta of third swimming leg longest (Fig. 19 D View Figure 19 ) 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 View Figure 18 , 20 C, D View Figure 20 ) biramous, represented by large, weakly corrugate subrectangular exopodal segment armed with three equally long terminal setae (Fig. 18 C, D View Figure 18 ). Endopod represented by small thumb-like lobe armed with short distal seta (Fig. 18 D View Figure 18 ; Suárez-Morales and Palomares-García 1995: fig. 2 a). Fifth leg setae long, reaching proximal margin of caudal rami (Fig. 18 C View Figure 18 ; 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 View Figure 19 ), but not discernible as such in lateral view (Fig. 18 A View Figure 18 ). 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 View Figure 17 , 20 B View Figure 20 ). 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 View Figure 17 , 18 C View Figure 18 ). 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 View Figure 18 , 20 D View Figure 20 ). 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.