Monstrilla ciqroi (Suárez-Morales, 1993 a)

Monstrilla ciqroi (Suárez-Morales, 1993 a) View in CoL

Figs 4 View Figure 4 , 5 View Figure 5

Type material.

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

Type locality.

Bahía de la Ascensión   GoogleMaps , 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 View Figure 4 ), cephalothorax representing almost 64 % of total body length. Oral cone moderately developed, prominent, papilla-like (Figs 4 B, C View Figure 4 , 5 F View Figure 5 , oc), located at 12 % of body along ventral surface of cephalothorax (Fig. 4 B View Figure 4 ). Cephalic region with flat ‘ forehead’ and weak integumental corrugation, field of transverse integumental wrinkles between antennule bases (Fig. 4 C View Figure 4 ); ventral preoral surface with integumental ornamentation including two pairs of nipple-like processes in both the holotype and paratype specimens (Figs 4 C View Figure 4 , 5 F View Figure 5 , nlp) and medial low protuberance (in Fig. 5 F View Figure 5 , nlp). Eyes comprising two lateral cups and medial cup (Fig. 4 C View Figure 4 , lec, mec); lateral eye cups strongly pigmented, with a diameter ~ 0.6 × as that of larger, weakly pigmented medial eye cup (Figs 4 C View Figure 4 , 5 F View Figure 5 ); small hyaline bodies (sensu Suárez-Morales 2018) visible anteriorly to lateral eye cups (Figs 4 C View Figure 4 , 5 F View Figure 5 , 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 View Figure 5 ), 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 View Figure 5 ). Genital double-somite with weakly expanded lateral margins on proximal 1 / 2 (Fig. 4 D View Figure 4 ), with incomplete transverse suture visible in dorsal and lateral view (Fig. 5 C, D View Figure 5 arrowheads); pair of slender ovigerous spines on ventral surface, carrying eggs mass (Figs 4 B View Figure 4 , 5 D View Figure 5 , 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 View Figure 5 ).

Antennules 0.64 mm in length, representing ~ 21 % of total body length and almost 32 % of cephalothorax length (Fig. 4 A, B View Figure 4 ); as usual in female monstrilloids, antennules distinctly 4 - segmented, anteriorly directed, weakly divergent (Fig. 4 A View Figure 4 ); segments 1–3 divided, segments 3 and 4 partly fused (Fig. 5 A View Figure 5 ); length ratio of antennular segments (proximal to distal) 16.5: 14.3: 12.1: 57.1 (Fig. 5 A View Figure 5 ). 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 View Figure 5 ).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 4 A, B View Figure 4 ), 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 View Figure 5 ) 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 View Figure 5 , 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 View Figure 3 ) 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 View Figure 5 ); 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.