Monstrilla ciqroi (Suárez-Morales, 1993 a)

Figs 4, 5

Type material.

Holotype • adult female, undissected, deposited in the collection of Crustacea, U. S, National Museum of Natural History, Smithsonian Institution. USNM 251656 . Female paratype USNM 251700.

Type locality.

Bahía de la Ascensión, Caribbean coast of Mexico (19°47.00'N, 87°33.20'W). Date of collection 5 September 1991.

Description of adult female holotype.

Body length of holotype 3.1 mm. Cephalothorax long, cylindrical, with weakly expanded lateral margins (Fig. 4 A), cephalothorax representing almost 64 % of total body length. Oral cone moderately developed, prominent, papilla-like (Figs 4 B, C, 5 F, oc), located at 12 % of body along ventral surface of cephalothorax (Fig. 4 B). Cephalic region with flat ‘ forehead’ and weak integumental corrugation, field of transverse integumental wrinkles between antennule bases (Fig. 4 C); ventral preoral surface with integumental ornamentation including two pairs of nipple-like processes in both the holotype and paratype specimens (Figs 4 C, 5 F, nlp) and medial low protuberance (in Fig. 5 F, nlp). Eyes comprising two lateral cups and medial cup (Fig. 4 C, lec, mec); lateral eye cups strongly pigmented, with a diameter ~ 0.6 × as that of larger, weakly pigmented medial eye cup (Figs 4 C, 5 F); small hyaline bodies (sensu Suárez-Morales 2018) visible anteriorly to lateral eye cups (Figs 4 C, 5 F, dotted lines).

Urosome consisting of four somites: fifth pedigerous somite (with fifth legs), genital double-somite ventrally carrying paired ovigerous spines barely reaching beyond distal end of caudal rami and attached egg cluster (Fig. 5 D), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) 28.3: 48.3: 13.3: 10.1 (Fig. 5 D). Genital double-somite with weakly expanded lateral margins on proximal 1 / 2 (Fig. 4 D), with incomplete transverse suture visible in dorsal and lateral view (Fig. 5 C, D arrowheads); pair of slender ovigerous spines on ventral surface, carrying eggs mass (Figs 4 B, 5 D, os). Caudal rami subrectangular, ~ 1.5 × as long as broad, each armed with six caudal setae (I – VI), seta VI being shortest (Fig. 5 B, E).

Antennules 0.64 mm in length, representing ~ 21 % of total body length and almost 32 % of cephalothorax length (Fig. 4 A, B); as usual in female monstrilloids, antennules distinctly 4 - segmented, anteriorly directed, weakly divergent (Fig. 4 A); segments 1–3 divided, segments 3 and 4 partly fused (Fig. 5 A); length ratio of antennular segments (proximal to distal) 16.5: 14.3: 12.1: 57.1 (Fig. 5 A). Following Grygier and Ohtsuka’s (1995) setal nomenclature for female antennules, first segment with reduced, slender setal element 1, second segment bearing setiform element IId, and long, curved spiniform elements 2 v 1-3, 2 d 1, 2,; third segment with spiniform smooth element 3 and adjacent setiform elements IIId and IIIv, fourth segment longest of antennule, separated from third by deep intersegmental suture, proximal 1 / 2 armed with short spiniform elements 4 v 1-3, 4 d 1, 2, long, biserially setulated setiform elements IVd and IVv; distal 1 / 2 armed with setiform elements Vv (biserially setulated), Vd, short spiniform element 5, and several setae of the “ b-group ” (b 1, b 2, b - 3, b 6) on outer distal margin; apical elements 6 1, 6 2, and 6 aes reduced (Fig. 5 A).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 4 A, B), all with exopodite longer than endopodite. Setal armature pattern as in M. barbata (this document). Armature 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 (Fig. 5 B) represented by a single oblong exopodal lobe armed with four setae, three setae inserted terminally, subequal in length and breadth; fourth seta representing the endopod inserted laterally on midlength of inner margin adjacent to small inner beak-like protuberance (Fig. 5 B, arrowheads).

Remarks.

This species was originally assigned to Monstrillopsis Sars, 1921 based on the well-developed eyes and the forward location of the oral cone (Suárez-Morales 1993 a). Affinities with the invalid genus Strilloma Isaac, 1975 were also suggested in its original description (Suárez-Morales and Gasca 2004). It was compared therein with other species of Monstrillopsis recognized by Isaac (1975) like M. angustipes Isaac, 1974, M. dubia (T. Scott, 1904), M. gracilis (Gurney, 1927), and M. reticulata Davis, 1949 . The main character used to separate M. ciqroi from species of Monstrillopsis was the fifth leg structure and armature; M. reticulata fifth leg was deemed morphologically closest, but differences in the antennule armature were also mentioned to distinguish these two species. Further research (Suárez-Morales and Gasca 2004) confirmed Strilloma as invalid, and the first taxonomic revision of Monstrillopsis by Suárez-Morales et al. (2006) resulted in the inclusion of both Monstrillopsis reticulata and M. ciqroi as members of Monstrilla .

Monstrilla ciqroi has relevant affinities with other Caribbean species sharing a fifth leg with the same setal armature of the fifth legs (3 exopodal, 1 endopodal) like M. rebis Suárez-Morales, 1993, M. barbata, and M. xcalakensis Suárez-Morales, 2024, but M. ciqroi diverges in the size and structure of the endopodal lobe. In both M. rebis and M. barbata (see Fig. 3 B) the inner lobe is a relatively well defined, armed with a distal seta and in M. xcalakensis a strongly developed endopodal lobe is present, approximately the same size of the exopodal lobe (Suárez-Morales, 2024: fig. 3 A). In M. ciqroi the fifth leg inner seta likely represents the endopod, lacking a structured endopodal lobe. The number of caudal setae has been a relatively strong character related in the definition of some monstrilloid genera: three setae in Cymbasoma (Suárez-Morales and McKinnon, 2016), four setae in Monstrillopsis (Suárez-Morales et al. 2006), and five or six setae in Monstrilla and Caromiobenella (Jeon et al. 2018; Suárez-Morales and McKinnon 2025). In the original description of M. ciqroi, only five caudal setae were observed; this re-examination of the holotype specimen allowed me to determine that the actual number of caudal setae is six (Fig. 5 B, C, E); this finding supports the decision of including M. ciqroi as a member of Monstrilla . Overall, this species can be distinguished from other Mexican Caribbean species of Monstrilla by its strongly pigmented eyes, fifth leg armed with four setae, and the lack of a structured endopodal lobe.