Monstrilla annulata, Suárez-Morales, 2024

Suárez-Morales, Eduardo, 2024, Two new species of Monstrilla (Copepoda: Monstrilloida: Monstrillidae) from a protected reef system of the Mexican Caribbean, European Journal of Taxonomy 917, pp. 152-169 : 159-161

Monstrilla annulata sp. nov.

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Figs 4–6

Differential diagnosis

Medium- to large-sized female Monstrilla with long cephalothorax.Antennule 4-segmented, with reduced setal armature: segment 1 unmodified; segment 2 lacking all elements of ‘2v’ group (i.e., 2v 1-3); segment 3 with usual armature and 3 weak integumental constrictions; and segment 4 with 5–7 deep integumental constrictions making it appear multi-annulate, with apex modified into diagonally truncate tip forming flat, disc-like structure with reduced armature, i.e., lacking usual apical elements 6 1 and 6 2 but retaining reduced 6aes. Fifth legs weakly bilobed, with endopodal lobe represented by short, flat protuberance on proximal inner margin of exopodal lobe, latter represented by large, subrectangular plate with incised inner margin, distinctively pilose outer margin, and one distal and one subdistal seta. Ovigerous spines not reaching to distal margins of caudal rami. Surface of caudal rami and other appendages, including proximal parts of ovigerous spines and urosomites ( Fig. 6E ), densely sculptured with fine, thread-like undulate patterns. Caudal rami each bearing five setae.

Etymology

The specific name is a Latin adjective with a feminine suffix to match the gender of the genus, meaning ‘ringed’ in reference to the worm-like appearance of the integumental constrictions of antennular segments 3 and 4.

Type material

Holotype

MEXICO • ♀, undissected glycerin-preserved in vial; Xcalak reef lagoon; 18°16′04.05″ N, 87°49′44.24″ W; depth 2.5 m; 3 Mar. 2022; L. Vásquez-Yeomans, J.A. Cohuo-Colli, J. López-May, and F. Andrade leg.; plankton light-trap; ECO-CH-Z 11810 .

Paratypes

MEXICO • 3 ♀♀, 1 ♀ of them undissected, mounted in glycerine on slide, sealed with acrylic varnish; same collection data as for holotype; ECO-CH-Z 11811 • 2 ♀♀, two partly dissected, gold-coated, mounted on single SEM stub; same collection data as for holotype; ECO-CH-Z 11818 .

Other material

MEXICO • 2 ♀♀, undissected in vial with glycerol; same collection data as for holotype; author’s collection at ECOSUR .

Type locality

Reef lagoon of Xcalak (18°16′04.05″ N, 87°49′44.24″ W), southern part of the Mexican Caribbean coast.

Description (adult ♀)

MEASUREMENTS. Body length of holotype 1.96 mm measured from ‘forehead’ to posterior end of anal somite, three paratypes 2.00, 2.12, and 2.25 mm long, two non-type individuals 2.0 and 2.1 mm long. Body tagmosis as usual in female Monstrilla ( Isaac 1975; Suárez-Morales & Islas-Landeros 1993; Chang 2014).

CEPHALOTHORAX. Relatively robust, representing about 63% of total body length and fully incorporating first pedigerous somite. Oral cone prominent, located 30% of way back along ventral surface of cephalothorax. Cephalic region anteriorly subquadrate in dorsal view, ‘forehead’ flat, sensilla not observed. Nauplius eye as in M. xcalakensis sp. nov. except for lateral cups being almost unpigmented. Cuticular ornamentation of cephalothorax including medial cluster of minute, crater-like pores between antennule bases ( Fig. 5A ), two pairs of nipple-like processes between oral cone and antennule bases, and field of transverse striae adjacent to oral cone ( Fig. 6C ).

ANTENNULES. About 40% as long as body, almost 65% as long as cephalothorax, four-segmented as usual in female monstrilloids, with second and third segments separate from each other and fourth (longest) segment separated from third by well-defined suture ( Fig. 4A–B ). Length ratio of antennular segments (proximal to distal) 8.3: 22.2: 13.9: 56.1 = 100. Following Grygier & Ohtsuka’s (1995) setal nomenclature for antennules of female monstrilloids, first segment lacking spiniform setal element 1. Second segment bearing elements 2d 1-3 (dots in Fig. 4A ), but dorsal seta IId not observed. Third segment with short, spiniform element 3 and slender ventral seta IIIv ( Fig. 4A ), but dorsal seta IIId absent. Third and fourth segments with three and five to seven regular ring-like constrictions respectively; annulation more clearly expressed in fourth segment ( Fig. 4A–C ). Fourth segment armed with setal elements 4v 1-3 (arrowheads in Fig. 4A ), 4aes, IVd, IVv, Vm, 5 (broken), Vv, Vd, 6aes, 6 1, 6 2 (both vestigial) ( Fig. 4B, D ), and subapical ‘b’ setal group on outer margin comprising only four elements (b 1-4, thus lacking b 5 and b 6) ( Fig. 4A–B ). Antennular surface densely sculptured with fine, undulate thread-like patterns ( Fig. 4C ).

THORACIC SOMITES. Swimming legs 1–4 on posterior cephalothorax and first three free pedigerous somites armed as in M. xcalakensis sp. nov. ( Table 1 ).

UROSOME. As usual in Monstrilla , urosome consisting of four somites ( Fig. 3A, C ): fifth pedigerous somite carrying fifth legs, genital double-somite with anterior half expanded and bearing pair of posteriorly-directed ovigerous spines, one short free somite, and anal somite. Length ratio of urosomites (from anterior to posterior) 25.3: 26.6:31.4: 16.6 = 100. Fifth legs well developed, bilobed, with finely sculptured integument. Endopodal lobe reduced, unarmed, fused to outer lobe. Outer lobe large, subrectangular, plate-like, with hirsute outer margin ( Fig. 5B–C ), three to four diagonal incisions on inner margin ( Fig. 5C–D ), and one apical and one subapical seta. Ovigerous spines ( Fig. 5B ) relatively short, not reaching tips of caudal rami. Caudal rami subrectangular, 1.7 times as long as broad in dorsal view, armed with five caudal setae ( Fig. 6F ), with setae II, III and IV grooved along lateral surfaces ( Fig. 6D, F ).

Justification for establishment of M. annulata sp. nov.

The new species Monstrilla annulata sp. nov. has two distinctive features that make it readily distinguishable among its congeners and support its status as a new member of the genus Monstrilla . First is the modified antennule with integumental constrictions appearing as rings, particularly in segments 3 and 4. A generally similar pattern is known in Spinomonstrilla spinosa Suárez-Morales, 2019 , but in that species the annulated appearance is imparted by arcuate protruding scales, not by simple circular constrictions (cf. Suárez-Morales 2019: fig. 2d–e). Also, the antennules are remarkably elongate and narrow in S. spinosa , with the setal groups of segments 2–4 being more widely separated from each other than in M. annulata .

Other antennular modifications are found among species of Monstrilla , such as the absence of any intersegmental articulations in M. mariaeugeniae , with the segmental divisions only being marked by segmental constrictions (cf. Suárez-Morales & Islas-Landeros 1993: fig. 1b) and a similar pattern involving only segments 2–4, described in M. humesi from the Gulf of Mexico (cf. Suárez-Morales & Escamilla 2001: fig. 3b). An inflated margin of some antennular segments has been reported in M. brasiliensis (now in Caromiobenella Jeon, Soh & Lee, 2018 ; see Cruz Lopes da Rosa et al. 2021), M. grygieri Suárez-Morales, 2000a , and M. inserta A. Scott, 1909 (for both, see Suárez-Morales 2001).

The second distinctive feature of M. annulata sp. nov. is the uniquely modified exopodal lobe of the female’s fifth leg; no other member of Monstrilla has a fifth leg combining an indented inner margin and a heavily hirsute outer margin.A lightly hirsute outer margin is known only in the female Caromiobenella brasiliensis (Suárez-Morales & Dias 2000) , in which, however, the exopodal lobe carries three seta and the endopodal lobe is cylindrical and narrow (cf. Suárez-Morales & Dias 2000: fig. 3d), thus differing from M. annulata .A spinulose outer margin of the fifth leg is present in the poorly described M. dakinensis Davis, 1949 from Australia ( Dakin & Colefax 1940), which shares with M. annulata the same armature of two setae on the exopodal lobe and a reduced endopodal lobe, but differs in lacking the indentations in the exopodal lobe’s inner margin that help to define the new species. An exopodal lobe with two setae is also known in the Caribbean M. elongata , but the lobe is bulbous, not plate-like, and the antennules are unsegmented (cf. Suárez-Morales 1994: fig. 1c–d, g), unlike in the new species M. annulata .

Figures

Fig. 4. Monstrilla annulata sp. nov., paratype, ♀ (ECO-CH-Z 11818). A. Left antennule with some setal elements or setal groups (v.gr., 2v1-3, 2d 1-3) labeled following Grygier & Ohtsuka’s (1995) nomenclature, ventral view. B. Distal (fourth) segment of right antennule, ventral view, showing truncate, disc-like apical modification. C. Detail of ring-like constrictions of fourth antennulary segment, ventral view. D. Truncate, disc-like apical process of antennule, showing reduced aesthetasc 6aes and absence of usual apical elements 6 1 and 6 2 (sensu Grygier & Ohtsuka 1995).

Fig. 5. Monstrilla annulata sp. nov., paratype, ♀ (ECO-CH-Z 11818). A. Cluster of three crater-like pores (arrow) between the antennule bases, ventral view. B. Urosome, showing fifth leg and short ovigerous spines, semi-lateral view. C. Fifth leg showing fine integumental sculpturing and hirsute outer margin, ventral view. D. Detail of inner margin of fifth leg’s exopodal lobe showing diagonal incisions, ventral view.

Fig. 6. Monstrilla annulata sp. nov., paratype, ♀ (ECO-CH-Z 11818). A. Leg 2, outer spine of first exopodal segment, showing ornamentation. B. Leg 1, outer spine of third exopodal segment and outer apical spiniform seta, both showing ornamentation. C. Anterior half of cephalothorax showing oral cone (oc) and preoral ornamentation, ventral view. D. Detail of caudal setae II and III, lateral view, showing grooves. E. Last urosomites showing dense pattern of fine, undulate thread-like integumental sculpturing, ventral view. F. Caudal rami showing large ventral pore (CP) and modified caudal setae II, III, and IV, with lateral grooves.

Fig. 3. Monstrilla xcalakensis sp. nov., holotype, ♀ (ECO-CH-Z 11812). A. Urosome showing fifth legs and insertion of ovigerous spines, ventral view. B. Cephalic morphology and ornamentation, lateral view, showing eye complex with naupliar eye medial cup (vcu), lateral cup (lc), cluster of three minite conical pores (large arrowhead), oral cone (oc), and pair of oral pores (op). C. Urosome, lateral view. D. Swimming leg 2 showing setation pattern with long exopodal spines (arrowheads) and hirsute outer margins of endopodal segments. E. Antennular armature with setal elements labelled following Grygier & Ohtsuka’s (1995) nomenclature, dorsal view.