Holotype • adult female, undissected, deposited in the collection of Crustacea, U.S, National Museum of Natural History, Smithsonian Institution. USNM 251756.

Bahia de la Ascension, central part of eastern Yucatan Peninsula coast (19°47.00'N, 87°33.20'W). Depth 1.5 m. Date of collection 6 September, 1991.

Total body length measured from forehead to posterior margin of anal somite: 1.82 mm. Cephalothorax 0.947 mm long, representing ~52.1% of total body length. Antennules moderately divergent, representing 16.6% of total body length and 31.5% of cephalothorax length. Oral papilla small, located anteriorly, ~26% posteriorly on ventral surface of cephalothorax (Fig. 1A, C). Eyes represented by relatively small medial cup and two larger, weakly pigmented lateral cups (Fig. 2A, mec, lec). Forehead flat, corrugate, with two large sensilla and field of small cuticular papillae extending to most of cephalic area, including ventral and dorsal surfaces (Fig. 2A, B). Ventral surface of cephalic area bearing: 1) single pair of nipple-like cuticular processes between antennule bases and oral papilla, 2) ventral surface with coarsely corrugate medial keel-like protuberance (Fig. 2B) visible in lateral view of the whole specimen (Fig. 1C, arrowhead); 3) pale pattern of cuticular wrinkles and minute papillae present adjacent to these structures (Fig. 2A).

Antennules (Fig. 2D) 0.34 mm long, almost 17% of total body length; antennules 4-segmented; segments 1–4 clearly divided. Following antennule armature nomenclature by Grygier and Ohtsuka (1995), first segment with short, spiniform element 1; second segment carrying long spiniform elements 2v _1-3 and 2d_1,2, dorsal seta IId long; third segment with short, spiniform element 3 and IIIv; elements IIId and IIIv setiform, slender; fourth segment with reduced armature including proximal spiniform element 4d₁, flexible setae IVd, IV v, and slender ventral aesthetasc 4aes. Fourth segment carrying short spiniform elements 4v_1-3, setiform elements IVd, IVv, Vm, Vv, Vd, spiniform element 5, three setae of the “b” group inserted on the outer distal margin (b₁, b₂, b₃, b₆), and reduced apical elements 6₁ and 6₂.

First pedigerous somite incorporated into cephalothorax; this and succeeding three free pedigerous somites each bearing pair of biramous swimming legs. Pedigerous somites 2–4 together accounting for 26% of total body length in dorsal view. Intercoxal sclerites of legs 1–4 subrectangular, without ornamentation on surface or along distal margin. Basis of legs articulating with rectangular coxa along diagonal line. Basis with thin, simple lateral basipodal seta on legs 1, 2, and 4; on leg 3, this seta thicker, lightly setulate, and 4× longer than on other legs (Fig. 3C–F, bs). Endopods and exopods of swimming legs 1–4 tri-articulate (Fig. 3C–F). Ramus setae all lightly and biserially plumose except for spiniform outer setae on exopodal segments 1 and 3, and inner seta of first exopodal segment, all these being short and slender. Outer apical exopodal seta of swimming legs 1–4 with outer margin smooth, inner margin lightly setose. Armature formula as (not provided in original description):

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, 1, 2, 3

Fifth legs biramous. Outer lobe represented by subrectangular exopodal lobe armed with three setae, outermost seta lightly pinnate, being longer, slightly wider than adjacent two exopodal setae; innermost exopodal seta lightly shorter, narrower. Inner endopodal lobe reduced, drop-like, arising proximally from inner margin of exopodal lobe, armed with short, slender apical seta (Fig. 3B).

Urosome consisting of four somites: fifth pedigerous somite, genital double-somite, one free postgenital somite, and short anal somite (Fig. 3B). Genital somite with pair of lateral protuberances on distal 1/2 visible in frontal and ventral views (Fig. 1B, plp; Fig. 3B, arrowheads). Ventral surface of genital somite forming enlarged base of cylindrical shaft with distal genital lappets. Relative length of urosomites, from proximal to distal as: 40: 35: 16.25: 8.76. Caudal rami subquadrate, symmetrical, weakly divergent, ~1.1× longer than wide, each ramus bearing six setae (I–IV), seta VI being shortest (Fig. 3B).

Male. Unknown.

There are several characters of this species that were not properly described in its original description in 1992 . The holotype habitus was originally provided in dorsal view; I was able to add both ventral and lateral views. The armature of the antennules was largely incomplete and at least one setal element (element 1 sensu Grygier and Ohtsuka 1995) was erroneously illustrated in the original drawings (Suárez-Morales and Gasca-Serrano 1992: fig. 1b). The setal elements present in the holotype specimen are identified, illustrated, and labelled in this redescription, thus allowing a more accurate comparisons among the Caribbean species of Monstrilla. Also, the coarse forehead surface noticed in the re-examination was not mentioned or illustrated by Suárez-Morales and Gasca-Serrano (1992); the same is true for the eye cups, which were originally described as present and moderately developed; it was possible now to provide more details about their relative size and pigmentation, recognizing the lateral and medial cups. The cephalic integumental ornamentations were not typically deemed relevant in the taxonomy of monstrilloid copepods, but they can aid to separate species (Cruz Lopes da Rosa et al. 2021). In this redescription of M. barbata, I was able to provide new data on the cephalic ventral ornamentation, which includes fields of papillae and wrinkles, a pair of slender sensilla, a pair of nipple-like processes, and details of the medial keel-like corrugate antero-ventral process that is distinctive of the species (Suárez-Morales and Gasca-Serrano 1992). Also, the swimming legs 1–4 were not illustrated in the original description, but drawings of these legs are newly provided in this redescription.

This species was originally compared with M. longicornis Thompson, 1890 and M. lata Desai & Bal, 1963 based on sharing a bilobed fifth leg carrying four setae. Its fifth leg was also compared with that of M. reticulata Davis, 1949. The most distinctive character of M. barbata is the beard-like protuberant medial process and was considered unique among species of Monstrilla (Suárez-Morales and Gasca-Serrano 1992), but this structure was compared recently with those of other Monstrilla species described after M. barbata, like M. pustulata Suarez-Morales & Dias, 2001 from Brazil, and M. brevicornis Isaac, 1974 from Java (see Suárez-Morales and McKinnon 2025). A further comparison of M. barbata’s fifth leg with Australian congeneric species was presented by Suárez-Morales and McKinnon (2025). The presence of paired lateral protuberances on the genital double-somite of M. barbata was also mentioned by Suárez-Morales and Gasca-Serrano (1992) as another distinctive character of this species and is documented in this paper.

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.

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

Body length of holotype 3.1 mm. Cephalothorax long, cylindrical, with weakly expanded lateral margins (Fig. 4A), cephalothorax representing almost 64% of total body length. Oral cone moderately developed, prominent, papilla-like (Figs 4B, C, 5F, oc), located at 12% of body along ventral surface of cephalothorax (Fig. 4B). Cephalic region with flat ‘forehead’ and weak integumental corrugation, field of transverse integumental wrinkles between antennule bases (Fig. 4C); ventral preoral surface with integumental ornamentation including two pairs of nipple-like processes in both the holotype and paratype specimens (Figs 4C, 5F, nlp) and medial low protuberance (in Fig. 5F, nlp). Eyes comprising two lateral cups and medial cup (Fig. 4C, lec, mec); lateral eye cups strongly pigmented, with a diameter ~0.6× as that of larger, weakly pigmented medial eye cup (Figs 4C, 5F); small hyaline bodies (sensu Suárez-Morales 2018) visible anteriorly to lateral eye cups (Figs 4C, 5F, 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. 5D), 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. 5D). Genital double-somite with weakly expanded lateral margins on proximal 1/2 (Fig. 4D), with incomplete transverse suture visible in dorsal and lateral view (Fig. 5C, D arrowheads); pair of slender ovigerous spines on ventral surface, carrying eggs mass (Figs 4B, 5D, os). Caudal rami subrectangular, ~1.5× as long as broad, each armed with six caudal setae (I–VI), seta VI being shortest (Fig. 5B, E).

Antennules 0.64 mm in length, representing ~21% of total body length and almost 32% of cephalothorax length (Fig. 4A, B); as usual in female monstrilloids, antennules distinctly 4-segmented, anteriorly directed, weakly divergent (Fig. 4A); segments 1–3 divided, segments 3 and 4 partly fused (Fig. 5A); length ratio of antennular segments (proximal to distal) 16.5: 14.3: 12.1: 57.1 (Fig. 5A). 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 2v_1-3, 2d_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 4v_1-3, 4d_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” (b₁, b₂, b_-3, b₆) on outer distal margin; apical elements 6₁, 6₂, and 6aes reduced (Fig. 5A).

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

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 1993a). 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. 3B) 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. 3A). 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. 5B, 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.

Holotype • adult female, undissected, deposited in the collection of Crustacea, U.S, National Museum of Natural History, Smithsonian Institution. USNM 251655. Adult female paratype ECO-CHZ-00067.

Bahía de la Ascensión, Caribbean coast of Mexico) (19°45.09'N, 87°30.00'W). Date of collection 4 September 1991.

Body length of holotype 2.98 mm, paratype body length 2.82 mm. Cephalothorax cylindrical with weakly expanded lateral margins, relatively short, representing almost 55% of total body length. Oral cone well developed, prominent, papilla-like (Fig. 6B, oc), located 22% of way back along ventral surface of cephalothorax. Cephalic region anteriorly expanded in dorsal view, ‘forehead’ rounded, smooth, weakly produced (Fig. 6A); ventral preoral surface with integumental ornamentation including two pairs of nipple-like processes with adjacent integumental wrinkles in both the holotype and paratype (Fig. 6B, nlp). Eyes comprising two relatively large lateral cups (Fig. 6A, lec) and ventral medial cup (Fig. 6A, mec), latter smaller than lateral cups; lateral eye cups weakly pigmented, with a diameter ~1.2× as that of smaller unpigmented medial eye cup (Fig. 6A).

Urosome consisting of four somites : fifth pedigerous somite (carrying fifth legs), genital double-somite with pair of ovigerous spines reaching well beyond distal end of caudal rami (Suárez-Morales 1993a: fig. 1a), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) 17.5: 23.5: 33.25: 12.5: 13.25 (Fig. 6C, D). Genital double-somite longest of urosome, with weakly expanded lateral margins (Fig. 6D, E); pair of slender ovigerous spines on ventral surface; spines equally long, both ending in thin, straight parallel points (Fig. 6F). Caudal rami subrectangular (Fig. 6D), ~1.4× as long as broad, each armed with five caudal setae (I–V), seta V being shortest (Fig. 6D).

Antennules 0.64 mm in length, representing ~23% of total body length and almost 44% of cephalothorax length (Suárez-Morales 1993a: fig. 1a); as usual in female monstrilloids, antennules distinctly 4-segmented, relatively slender, anteriorly directed, weakly divergent; intersegmental divisions segments 1–3 complete (Fig. 7E); length ratio of antennular segments (proximal to distal) 10.71: 25.71: 14.28: 49.3 (Fig. 7E). 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 stout, slender spiniform elements 2v_1-3 and 2d_1,2; third segment with slender, stout spiniform element 3 and adjacent setiform elements IIId and IIIv, fourth segment longest of antennule, proximal 1/2 armed with short spiniform elements 4v₁, setiform elements IVv and IVd, and short aesthetasc 4 aes. Distal 1/2 of fourth segment with long, biserially setulated setiform elements IVd and IVv, Vm, and Vv plus short spiniform element 5; outer distal margin carrying several unbranched, slender setae of the “b-group” (b_1-6); apical elements 6_1,2 spiniform, acute, with reduced adjacent aesthetasc 6aes (Fig. 7E).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Suárez-Morales 1993a: fig. 2b, c, d), all with exopodite longer than endopodite. Swimming legs 1–4 slender, with setal armature pattern as shown in Fig. 7A–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 (Fig. 6E, P5) biramous, represented by long, subrectangular exopodal segment armed with two equally long terminal setae; inner (endopodal) lobe reduced, comprising small inner protuberance armed with two apical setae. Fifth leg setae long, reaching beyond distal margin of anal somite (Fig. 6E).

Male. Unknown.

In the original description, the position of the oral cone (at the anterior ¼ of the cephalothorax) was proposed as the main distinctive character of this species (Suárez-Morales 1993a); this remark was based on Isaac’s (1975) definition of Monstrilla, which, in reference to this particular character, is misleading; however, at that time it was one of the most used sources on monstrilloid morphology. In M. rebis, the oral cone was originally described as located at 15% of the body on the cephalothorax ventral surface, but in fact it is positioned at 22–25% in both the holotype and the paratype. The antennule length < ½ cephalothorax length (44%) was considered as the second most relevant character to separate M. rebis from its known congeneric species; it was compared with monstrilloid species having clearly longer antennules, like M. conjunctiva Giesbrecht, 1902 and C. helgolandica Jeon et al. 2018. The third distinctive character mentioned was the structure and armature of the fifth legs, showing a unique pattern of two exopodal, two endopodal setae. It was compared with other species of Monstrilla with a total of four setae on the fifth leg arranged in a different combination (i.e., 3 exopodal, 1 endopodal), such as M. wandelii Stephensen, 1913, C. helgolandica (Claus, 1863), and M. conjunctiva Giesbrecht, 1902. Other species of Monstrilla with four setae (3, 1) on the female fifth leg include M. longiremis Giesbrecht, 1893, M. longicornis Thompson, 1890, and related species.

Most importantly, there is only another species of Monstrilla sharing the same fifth leg setation pattern with M. rebis (two endopodal, two exopodal setae), the Philippine M. grygieri Suárez-Morales, 2000, from which it can be distinguished by several characters, including the length and segmentation of the antennules: in M. grygieri the antennules are clearly longer (73% of cephalothorax length) than in M. rebis (40%) and its segments 2–4 are partly fused (Suárez-Morales 2000: fig. 2A, B) vs completely divided antennular segments 1–4 in M. rebis; the position of the oral cone clearly diverges in these two species, this structure is located approximately halfway along the cephalothorax ventral surface in M. grygieri (Suárez-Morales 2000: fig. 1A) vs the anterior ¼ of the cephalothorax in M. rebis; and the number of caudal setae, six in M. grygieri (Suárez-Morales 2000: fig. 2D), vs five in M. rebis.

Holotype • male, undissected, deposited in the U.S. National Museum of Natural History (USNM-251698). Paratype adult male, undissected. Specimens preserved in 70% ethanol, vial USNM-251699.

Bahia de la Ascension, central part of the eastern coast of the Yucatan Peninsula (19°45.09'N, 87°30.00'W), Caribbean coast of Mexico.

Total body length of holotype 2.3 mm (Fig. 8), length of paratype 2.4 mm (Fig. 9). Cephalothorax robust, representing 48.3% of body (Fig. 8A, B). Oral cone moderately developed, located at 28% from anterior margin of cephalothorax ventral surface in both the holotype and paratype specimens (Figs 8A, 9A, 10E, oc). Eyes weakly pigmented, represented by two lateral eye cups and medial eye cup at anterior part of cephalothorax, medial and lateral cups ~the same diameter in both the holotype (Fig. 10E; mec, lec) and paratype (Fig. 9B). Cephalothorax integumental ornamentation including pair of medial preoral pores (Fig. 11A, pop), two pairs of nipple-like processes with adjacent field of wrinkles (Figs 10E, 11A, nlp).

Urosome relatively slender (Fig. 10B), almost 21% of total body length; urosome comprising fifth pedigerous somite, genital somite, one free somite, preanal and anal somites, the latter holding pair of caudal rami; relative length of urosomites, from proximal to distal: 26.66: 22.22: 24.44: 15.55: 11.11. Fifth pedigerous somite with straight lateral margins, ventral surface moderately produced, carrying pair of reduced fifth leg buds (Figs 8A, 9A, 10D, arrowheads). Succeeding genital somite carrying genital apparatus. Free postgenital somite subquadrate, with smooth ventral and lateral surfaces. Preanal and anal somites subequally long, with smooth lateral and dorsal surfaces; anal somite carrying caudal rami. Genital complex arising ventrally on genital somite (Figs 8A, 9A, 10B, GC), apparatus comprising long cylindrical shaft with smooth lateral margins; shaft distally branching into pair of short, divergent rounded genital lappets (Figs 10B, 11F, GL); lappets medially conjoined by rounded concave process with medial genital opening with everted spermatophore (Figs 9A, 10B, 11F, SPO). Caudal rami subrectangular, 1.6× as long as wide (Fig. 10C); rami armed with six caudal setae subequal in length and width; seta VI relatively shorter (Figs 8B, 10C, 11A).

As usual in male Monstrilla, antennules 5-segmented, geniculate. Ratio of length of the antennule segments, from proximal to distal: 7.79: 15.58: 9.74: 24.67: 18.83. Fourth segment is the longest (Fig. 10A). Antennules ~35% of total body length and 75% of cephalothorax length. Following nomenclature by Grygier and Ohtsuka (1995) for antennular segments 1–4, first segment with short, spiniform element 1; second segment carrying slender spiniform elements 2d_1,2 and 2v_1,3 and relatively short, lightly setulated dorsal seta IId; third segment with long, stout, spiniform element 3 and lightly setulated setiform elements IIIv and IIId; fourth segment comprising proximal elements 4d_1,2 and 4v_1-3, and lightly setulated setiform elements IVv and IVd; distal 1/2 of segment with straight lateral margins, armature absent. Following Huys et al. (2007) nomenclature for the setation of the male antennular fifth segment, outer margin with slender elements 3–7, the last two (6 and 7) dichotomously branched. Inner margin armed with slender, unbranched elements A, B, and C. Short spiniform elements 1 and 2 and aesthetasc E present in apical position (Fig. 10A).

Incorporated first pedigerous somite and succeeding three pedigerous somites each bearing a pair of well-developed swimming legs, subequal in length. Basis of four swimming legs armed with single basipodal seta inserted on outer margin (Fig. 11B–E, bs); seta on leg 3 lightly setulated, longer and thicker than in other legs (Fig. 11D). Endopods and exopods of swimming legs1–4 each with three segments. Armament formula of swimming legs 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 reduced, represented by pair of buds arising ventrally from fifth pedigerous somite. Urosome consisting of fifth pedigerous, genital double and two free abdominal somites, length ratio of anterior to posterior segments being 30.9:24.6: 16.5: 18.2:9.8.

Female. Unknown.

In the original description (Suárez-Morales 1993b: 718) of this species the armature formula of the swimming legs 1–4 was incorrect, failing to show that the exopodal rami setation pattern differs between the first leg (with one seta less on the third segment, 5 in total) and that of the other swimming legs (with 6 setae on the third segment). This redescription has allowed me to correct this mistake. In addition, the reduced fifth legs buds arising from the ventral surface of the fifth pedigerous somite was unnoticed by Suárez-Morales (1993b) and not included in the original description. The presence of a reduced fifth leg is frequent in male Monstrilla. Its development is variable, and the Caribbean M. mahahualensis and related species likely have the strongest fifth leg development within the genus Monstrilla. Contrastingly, fifth legs are absent in the closely related genus Caromiobenella (Jeon et al. 2018), with the exception of C. brasiliensis (Cruz Lopes da Rosa et al. 2021).

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

Puerto Escondido coastal system, eastern coast of the Baja California Peninsula Puerto Morelos, northern part of the Mexican Caribbean coast (20°51.40'N, 86°54.15'W). Date of collection 18 November 1993.

Body length of holotype 4.2 mm. Cephalothorax robust, cylindrical, representing 58% of total body length, with lateral expansions at distal end of cephalothorax (Fig. 12C; Suárez-Morales 1994: fig. 1A). Oral cone well developed, prominent, papilla-like (Figs 12B, 14D, oc), located 32–35% along ventral surface of cephalothorax from head. Cephalic region of holotype with flat, smooth ‘forehead’ (Fig. 12C); ventral preoral surface with integumental ornamentation consisting of two pairs of nipple-like processes with adjacent integumental wrinkles in both the holotype and paratype (Figs 12A, B, 14A, nlp), field of minute wart-like integumental processes on outer surface of cephalic area adjacent to insertion of antennules (Fig. 15A, B, small arrowheads), and medial preoral pore with adjacent field of integumental wrinkles (Fig. 15B, pop). Eyes comprising two lateral cups (Fig. 15A, lec) and ventral medial cup (Fig. 15A, lec, mec), lateral cups slightly smaller in diameter than medial cup; lateral eye cups weakly pigmented.

Urosome consisting of four somites : fifth pedigerous somite (carrying fifth legs), genital double-somite with pair of ovigerous spines barely reaching distal margin of caudal rami in holotype (Fig. 12A; Suárez-Morales 1994: fig. 1F), slightly longer in paratype (Fig. 14 B, D), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) being: 34.91: 37.20: 16.27: 11.62 (Fig. 15D, E). 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 14B, F, 15D); pair of relatively short, corrugate ovigerous spines inserted on ventral surface (Figs 13F, 14B, os); spines equally long, both ending in acute, parallel points (Fig. 15D, E). Caudal rami relatively large, subrectangular (Fig. 15C), ~2.5× as long as broad, each ramus armed with five caudal setae (I–V), lateral seta II reduced, being shortest and thinnest (Fig. 15C, E).

Antennules remarkably long, slender, 0.64 mm in length, representing ~23% of total body length and almost 44% of cephalothorax length (Fig. 12A–C); antennules 4-segmented, but segments 2–4 partly fused; antennules weakly divergent in both holotype (Fig. 12A–C) and paratype (Fig. 14D, E) specimens. Intersegmental division between segments 1 and 2 complete (Fig. 13E); length ratio of antennular segments (proximal to distal) 10.71: 25.71: 14.28: 49.3 (Fig. 13E). Following Grygier and Ohtsuka’s (1995) setal nomenclature for female antennules, first segment unarmed, bearing thumb-like process on outer margin (Fig. 13A, E, arrows); putative second segment carrying relatively short spiniform elements 2v_1-3 and 2d_1,2, setiform element IId absent, broken off, only sockets observed (Fig. 13B); third segment with slender, stout spiniform element 3 and reduced setiform elements IIId and IIIv (Fig. 13C), fourth segment longest of antennule, proximal 1/2 armed with short spiniform elements 4v₁, setiform elements IVv and IVd, and aesthetasc 4 aes (Fig. 13D). Distal 1/2 of fourth segment lacking apical armature except for reduced spiniform element 6₁ in subdistal position (Fig. 13A). Antennules with set of two or three disc-like integumental windows of unknown function in segments 3 and 4 (Fig. 13A–C, iw).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 12A–C), all with exopodite longer than endopodite. Swimming legs 1–4 slender, with setal armature pattern as described by Suárez-Morales 1994: fig. 2J–L). 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 13F, 14B) uniramous, represented by single oblong exopodal segment armed with two equally long terminal setae; legs with subtriangular process adjacent to insertion of setae (Fig. 13F, asterisk). Fifth leg setae long, reaching distal margin of caudal rami (Fig. 12A–C; Suárez-Morales 1994: figs 1F, 2H).

Male. Unknown.

In the original description of this species, Suárez-Morales (1994) did not provide information about the eye cups morphology, which are indeed not easily discernible in the type specimens. Following Isaac’s (1975) criteria, the author incorrectly mentioned the absence of eyes as a character of the genus Monstrilla. The eye structure is described herein based on the re-examination of the type specimens.

According to Suárez-Morales (1994), the main character to distinguish this species from its congeners is the fifth leg structure and armature, consisting of a single segment armed with two setae, a character shared with several congeners like M. conjunctiva Giesbrecht, 1902, Caromiobenella helgolandica Claus, 1863 (= M. helgolandica), M. longipes Scott, 1909 (assigned to Maemonstrilla by Grygier and Ohtsuka 2008), and M. ghardaqensis Al-Kholy, 1963, likely a synonym of C. helgolandica (pers. obs.). Additional species of Monstrilla with the same fifth leg armature pattern include the recently described M. annulata Suárez-Morales, 2024, from a Mexican Caribbean reef system (see Suárez-Morales 2024), M. leucopis Sars, 1921 from Norway and the Gulf of California (Suárez-Morales 2010; Suárez-Morales and Velázquez-Ornelas 2023), and M. wandelii from Greenland (Park 1967). The antennule length is a helpful character to separate species of this group. In M. leucopis, M. annulata, and M. elongata antennules are long and slender, representing almost 50% (M. elongata) or even >60% (i.e., M. leucopis, M. annulata) of the cephalothorax length (Suárez-Morales and Velázquez-Ornelas 2023; Suárez-Morales 2024). Among these three species, only M. elongata exhibits a distinctive set of disc-like integumental processes (Fig. 13A–C). Furthermore, the recently described M. mahahualensis Suárez-Morales, 2022, based on male specimens, shares with M. elongata two interesting characters of the antennules: the presence of disc-like integumental processes and the absence of antennular element 1 (sensu Grygier and Ohtsuka 1995). In males of M. mahahualensis the disc-like integumental processes are present in antennular segments 3 and 4 (Suárez-Morales 2022: fig. 1C, E, F), whereas in M. elongata, these processes are also found on the second segment (Fig. 13B). The absence of antennular element 1 has been also reported in other species of Monstrilla like M. wandelii, M. leucopis, M. annulata, M. conjunctiva, and M. parki (see Suárez-Morales 2024; Suárez-Morales and McKinnon 2025), but only in M. elongata the first segment has a thumb-like outer process (Fig. 13A, E, arrows). Also, M. annulata has a distinctively annulated antennule and an hirsute fifth leg (Suárez-Morales 2024), thus diverging from M. elongata.

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.

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.

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

Antennules relatively robust, slender, divergent, 0.42 mm in length, ~20% of total body length and almost 35% of cephalothorax length (Fig. 16B, C); antennules 4-segmented, segments 3 and 4 partly fused; Intersegmental division between segments 1 and 2 and 2 and 3 complete (Fig. 17A); length ratio of antennular segments (proximal to distal) 19.20: 16.55: 17.88: 46.36 (Fig. 20B). 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 2v_1-3 and 2d_1,2, and setiform element IId (Figs 17A, 20B); third segment with slender, stout spiniform element 3 and setiform elements IIId and IIIv (Fig. 17A), fourth segment longest of antennule, proximal 1/2 armed with short, slender spiniform elements 4v_1,2, setiform elements IVv and IVd, and aesthetasc 4 aes (Figs 17A, 20B). Distal 1/2 of fourth segment armature comprising setiform elements Vd, Vm, Vd, and spiniform element 5; apical armature including apical aesthetasc 6aes, 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 b₁, b₃ branched (Fig. 20B).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 19C–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. 19C–E). Exopodal spines of legs 1–4 apically rounded (Fig. 19C–E, arrowheads). Basipodal seta of third swimming leg longest (Fig. 19D) 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 18C, D, 20C, D) biramous, represented by large, weakly corrugate subrectangular exopodal segment armed with three equally long terminal setae (Fig. 18C, D). Endopod represented by small thumb-like lobe armed with short distal seta (Fig. 18D; Suárez-Morales and Palomares-García 1995: fig. 2a). Fifth leg setae long, reaching proximal margin of caudal rami (Fig. 18C; Suárez-Morales and Palomares-García 1995: figs 2a, 3a, b).

Male. Unknown.

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 6₁ (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. 19A), but not discernible as such in lateral view (Fig. 18A). 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. 3d) but was not included in the description. Also, Suárez-Morales and Palomares-García (1995: fig. 2b, c) depicted the antennular setal armature as comprising three branched setae, but there are only two branched setae (b₁, b₃) in the holotype (Figs 17A, 20B). 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. 2a), 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 17B, 18C). Another relevant character that was depicted (Suárez-Morales and Palomares-García (1995: figs 1a, 3b) but not described is the presence of a proximally swollen caudal seta (seta III) in M. gibbosa (Figs 18C, 20D). 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.

Holotype

female, vial deposited at the USNM-Smithsonian Institution under number USNM-251840. Paratypes: adult female, vial deposited at the USNM (USNM-251841), Washington D.C. Additional paratypes, seven adult females, deposited in the Collection of Zooplankton at El Colegio de la Frontera Sur (ECOSUR) in Chetumal, Mexico. Types preserved in ethanol.

Reef lagoon off Puerto Morelos, state of Quintana Roo, northern part of the Yucatan Peninsula eastern coast (20°15.5'N, 86°54.5'W). Water column. Depth 1.2 m.

Body length of holotype 4.5 mm, paratypes body length ranging between 4.2 and 4.7 mm. Cephalothorax long, robust, representing almost 66% of total body length, with lateral expansions at distal end of cephalothorax (Fig. 21A–C; Suárez-Morales and Islas-Landeros 1993: fig. 1a). Oral cone weakly developed, papilla-like (Fig. 21A, B, oc), located at 43% along ventral surface of cephalothorax. Cephalic region with weakly produced, lightly rugose ‘forehead’ (Fig. 22C) with pilose integumental field between antennules insertion (Fig. 22C, pvf); ventral preoral surface with single pair of protuberant nipple-like processes with adjacent integumental wrinkles (Fig. 22C, nlp; Suárez-Morales and Islas-Landeros, 1993: fig. 1k), and adjacent field of minute integumental wrinkles. Eyes weakly pigmented, comprising two lateral cups (Fig. 21C, lec) and ventral medial cup (Fig. 21C, mec), medial cup slightly larger than lateral cups.

Urosome consisting of four somites : fifth pedigerous somite (carrying fifth legs), genital double-somite with pair of ovigerous spines (Fig. 22A, B; Suárez-Morales and Islas-Landeros, 1993: fig. 1e, f), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) being: 40.24: 24.39: 20.73: 14.64 (Fig. 22A, B). Genital double-somite with weakly expanded lateral margins, dorsal surface ornamented with transverse integumental ridges (Figs 22B, 23B, arrowhead). Ovigerous spines short, sausage-like, inserted on ventral surface (Fig. 22B, os), barely reaching distal margin of anal somite (Fig. 22A, B); spines distally tapering into acute, parallel points. Caudal rami subquadrate (Fig. 23B), ~1.2× as long as broad, each ramus armed with five caudal setae (I–V), proximal outer seta I longest, outer seta II reduced, shortest (Fig. 23B).

Antennules relatively robust, slender, parallel, ~28% of total body length and almost 43% of cephalothorax length (Fig. 21A, C); antennules 4-segmented, segments 2–4 partly fused; proximalmost intersegmental division complete (Figs 22D, 23A); divisions between successive segments marked by strong constrictions (Fig. 22D). Length ratio of antennular segments (proximal to distal, identified by segmental armature sensu Grygier and Ohtsuka 1995): 11.21: 23.36: 15.88: 49.55 (Figs 22D, 23A). Following Grygier and Ohtsuka’s (1995) setal nomenclature for female antennules, first segment unarmed, element 1 absent (Fig. 22D, asterisk); second segment carrying complete armature comprising short, slender spiniform elements 2v_1-3 and 2d_1,2, and setiform element IId (Figs 22D, 23A); third segment with slender, curved spiniform element 3 and setiform element IIId; element IIIv not observed (Fig. 22D); third segment with disk-shaped integumental window (Fig. 22D, iw1). Fourth segment longest of antennule, showing two additional constrictions. Proximal 1/3 of segment armed with short, slender spiniform elements 4v_1,2, 4d_1,2 and ornamented with disk-like integumental window (Fig. 22D, iw2). Armature of fourth segment distal 1/2 reduced, comprising setiform element Vd only; third disk-like integumental window present on this segment (Fig. 22D, iw3). Apical armature not observable in type specimens, possibly broken off during collection.

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 23C–E), all with exopod longer than endopod. Legs 2–4: identical in size and setation, carrying outer basipodal seta (Fig. 23C–E, bs); basipodal seta longest in leg 3 (Fig. 23E). Swimming legs 1–4 with relatively short, robust apical spiniform setae on the third exopod. Setal armature pattern as described by Suárez-Morales and Islas-Landeros (1993) (see also Fig. 23C–E). 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 22A, B, E, 23B) biramous, represented by large, smooth subrectangular exopodal segment armed with two equally long terminal setae (Figs 22A, 23B). Endopod represented by long digitiform, unarmed inner lobe reaching distal margin of exopodal lobe (Fig. 22A). Fifth leg setae long, barely reaching distal margin of caudal rami (Fig. 23B; Suárez-Morales and Islas-Landeros 1993: fig. 1a, e, h).

Male. Unknown (see further comments in Suárez-Morales 2022).

Monstrilla mariaeugeniae is the largest (body length = 4.2–4.7 mm) monstrilloid copepod reported from the Mexican Caribbean, followed by M. elongata (4.2 mm) and M. xcalakensis (3.51 mm). In the original description this species was considered to be closely related with M. wandelii. The first character used to separate both species was the lack of basipodal setae on swimming legs 1–4 in M. mariaeugeniae (Suárez-Morales and Islas-Landeros 1993); this is erroneous because this species has basipodal setae in all swimming legs, as revealed in this redescription (Fig. 23C–E). The antennular constrictions exhibited by M. mariaeugeniae were also mentioned as a relevant character to separate M. wandelii and M. mariaeugeniae (Suárez-Morales and Islas-Landeros 1993). Also, the size of M. mariaeugeniae females (4.4–4.7 mm) is nearly twice of that known for M. wandelii.

In their revision of the Australian species of Monstrilla, Suárez-Morales and McKinnon (2025) realized that there is a group of species related to M. conjunctiva Giesbrecht, 1902 comprising several species: (1) M. wandelii, (2) male specimens designated by Park (1967) as M. wandelii (but see Suárez-Morales 2022 for further comments), (3) M. conjunctiva, the Australian M. parki Suárez-Morales & McKinnon, 2025, and (4) the Caribbean M. mahahualensis Suárez-Morales, 2022. Overall, Monstrilla mariaeugeniae can be easily incorporated to this species group because it shares with them key distinctive characters including: the first antennular segment unarmed, antennular segments 3 and 4 fused with constrictions marking the intersegmental divisions, and the presence of disc-like integumental windows on the antennules (Suárez-Morales 2022: figs 1C, E, F, 2B). It is thus confirmed that M. mariaeugeniae belongs to the M. conjunctiva species group and it is likely that its male will have affinities with males of the other related species of this group.

Among the known species of Monstrilla, the longest fifth leg endopodal lobe of the female has been described in two Baja Californian species, M. gibbosa and M. hendrickxi, and also in two Australian species, M. janetgrieveae and M. latisetosa Suárez-Morales & McKinnon, 2025. In M. gibbosa and M. janetgrieveae the endopodal lobe carries one seta and is almost as long as the outer lobe (Suárez-Morales and Palomares-García 1995: fig. 2a; Suárez-Morales and McKinnon 2025: fig. 34B). In M. hendrickxi the inner lobe is digitiform, slightly longer than the outer (Suárez-Morales and Velázquez-Ornelas 2024: fig. 4A), but it is unarmed as in M. mariaeugeniae (Fig. 22A, enp), thus different from the structure exhibited by both Australian species, M. janetgrieveae with one endopodal seta, and M. latisetosa with two endopodal setae (Suárez-Morales and McKinnon 2025: figs 37B, 39B).