Monstrilla mariaeugeniae Suárez-Morales & Islas-Landeros, 1993

Figs 21, 22, 23

Material examined.

Holotype

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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.

Type locality.

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.

Description of adult female holotype.

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. 21 A – C; Suárez-Morales and Islas-Landeros 1993: fig. 1 a). Oral cone weakly developed, papilla-like (Fig. 21 A, B, oc), located at 43 % along ventral surface of cephalothorax. Cephalic region with weakly produced, lightly rugose ‘ forehead’ (Fig. 22 C) with pilose integumental field between antennules insertion (Fig. 22 C, pvf); ventral preoral surface with single pair of protuberant nipple-like processes with adjacent integumental wrinkles (Fig. 22 C, nlp; Suárez-Morales and Islas-Landeros, 1993: fig. 1 k), and adjacent field of minute integumental wrinkles. Eyes weakly pigmented, comprising two lateral cups (Fig. 21 C, lec) and ventral medial cup (Fig. 21 C, 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. 22 A, B; Suárez-Morales and Islas-Landeros, 1993: fig. 1 e, 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. 22 A, B). Genital double-somite with weakly expanded lateral margins, dorsal surface ornamented with transverse integumental ridges (Figs 22 B, 23 B, arrowhead). Ovigerous spines short, sausage-like, inserted on ventral surface (Fig. 22 B, os), barely reaching distal margin of anal somite (Fig. 22 A, B); spines distally tapering into acute, parallel points. Caudal rami subquadrate (Fig. 23 B), ~ 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. 23 B).

Antennules relatively robust, slender, parallel, ~ 28 % of total body length and almost 43 % of cephalothorax length (Fig. 21 A, C); antennules 4 - segmented, segments 2–4 partly fused; proximalmost intersegmental division complete (Figs 22 D, 23 A); divisions between successive segments marked by strong constrictions (Fig. 22 D). 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 22 D, 23 A). Following Grygier and Ohtsuka’s (1995) setal nomenclature for female antennules, first segment unarmed, element 1 absent (Fig. 22 D, asterisk); second segment carrying complete armature comprising short, slender spiniform elements 2 v 1-3 and 2 d 1, 2, and setiform element IId (Figs 22 D, 23 A); third segment with slender, curved spiniform element 3 and setiform element IIId; element IIIv not observed (Fig. 22 D); third segment with disk-shaped integumental window (Fig. 22 D, iw 1). Fourth segment longest of antennule, showing two additional constrictions. Proximal 1 / 3 of segment armed with short, slender spiniform elements 4 v 1, 2, 4 d 1, 2 and ornamented with disk-like integumental window (Fig. 22 D, iw 2). Armature of fourth segment distal 1 / 2 reduced, comprising setiform element Vd only; third disk-like integumental window present on this segment (Fig. 22 D, iw 3). 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. 23 C – E), all with exopod longer than endopod. Legs 2–4: identical in size and setation, carrying outer basipodal seta (Fig. 23 C – E, bs); basipodal seta longest in leg 3 (Fig. 23 E). 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. 23 C – 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 22 A, B, E, 23 B) biramous, represented by large, smooth subrectangular exopodal segment armed with two equally long terminal setae (Figs 22 A, 23 B). Endopod represented by long digitiform, unarmed inner lobe reaching distal margin of exopodal lobe (Fig. 22 A). Fifth leg setae long, barely reaching distal margin of caudal rami (Fig. 23 B; Suárez-Morales and Islas-Landeros 1993: fig. 1 a, e, h).

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

Remarks.

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. 23 C – 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 1 C, E, F, 2 B). 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. 2 a; Suárez-Morales and McKinnon 2025: fig. 34 B). In M. hendrickxi the inner lobe is digitiform, slightly longer than the outer (Suárez-Morales and Velázquez-Ornelas 2024: fig. 4 A), but it is unarmed as in M. mariaeugeniae (Fig. 22 A, 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 37 B, 39 B).