Monstrilla xcalakensis, 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 : 154-159

Monstrilla xcalakensis sp. nov.

urn:lsid:zoobank.org:act:0C50D553-D29D-499E-A16A-93EFBE8EAC9B

Figs 1–3

Differential diagnosis

Medium- to large-sized female Monstrilla with elongate, cylindrical cephalothorax constituting more than 60% of body length and prominent oral cone situated one-third of way along ventral surface of cephalothorax. Antennule four-segmented and segments 1–4 separate. Outer exopodal spines of legs 1–4 long and acute. Genital double-somite with anterioventral expansion carrying pair of posteriorly directed ovigerous spines, these not reaching to tips of caudal rami. Fifth legs bilobed, both lobes of similar length and breadth (endopodal lobe slightly shorter) and fused to each other in distal half; endopodal lobe armed with single seta, exopodal lobe with three setae. Caudal rami each bearing six setae, with proximal ends of apical setae III and IV much swollen.

Etymology

The specific name, an adjectival Mayan toponym (+ Latin suffix ‘- ensis ’ = ‘place’), refers to the reef system of Xcalak in the Mexican Caribbean, where this species was collected. The correct pronunciation of the species name starts with ‘sh’, followed by a strong ‘ka’. Gender is feminine.

Material examined

Holotype

MEXICO • 1 ♀, undissected and mounted in glycerin on slide; Xcalak reef lagoon; 18°16′04.05″ N, 87°49′44.24″ W; depth 1–4 m; 3 Mar. 2022; L. Vásquez-Yeomans, J.A. Cohuo-Colli, J. López-May, and F. Andrade leg.; ECO-CH-Z 11812 .

Paratypes

MEXICO • 4 ♀♀, undissected, mounted in glycerin on 4 slides; same collection data as for holotype; ECO-CH-Z 11813 to 11815 • 2 ♀♀, gold-coated and mounted together on one SEM stub; same collection data as for holotype; ECO-CH-Z 11817 .

Other material

MEXICO • 1 ♀, undissected, glycerin-preserved in vial; 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 (♀)

MEASUREMENTS. Body length of holotype 3.51 mm as measured from ‘forehead’ to posterior margin of anal somite, length range of paratypes 2.72–2.85 mm. Body tagmosis as usual in female Monstrilla ( Isaac 1975; Suárez-Morales & Islas-Landeros 1993; Chang 2014).

CEPHALOTHORAX. Long, cylindrical, with straight margins, representing about 60% of total body length and fully incorporating first pedigerous somite. Oral cone prominent (oc in Fig. 3B ), located 29% of way back along ventral surface of cephalothorax ( Figs 1B , 3B ). Cephalic region anteriorly subquadrate in dorsal view, ‘forehead’ flat with pair of small sensilla (arrowheads in Fig.1A ). Lateral cups of eye poorly developed, almost unpigmented (lc in Fig. 3B ). Ventral cup relatively small (vcu in Fig. 3B ), of nearly same diameter as lateral cups. Cuticular ornamentation of ‘facial’ region of cephalothorax consisting of fields of longitudinal and oblique striae between oral cone and bases of antennules ( Figs 1D , 3B ); two clusters of three protruding pores each, these being located on anterioventral medial surface between antennule bases ( Fig. 1D : white arrowheads); two pairs of nipple-like processes lateral and anteriolateral to oral cone, each with pore at apex, with additional indistinct pair between and slightly mesial to these ( Fig 1D ); and two pairs of minute pores at anteriolateral base of oral cone ( Fig. 1D ).

ANTENNULES. Relatively short, about 22% as long as cephalothorax ( Fig. 1B ), distinctly four-segmented as usual in female monstrilloids, distal segment longest ( Figs 1A , 3E ). Length ratio of antennular segments (proximal to distal) 14.3: 21.4: 15.1: 49.3 = 100 ( Fig. 3A, C ). Following Grygier & Ohtsuka’s (1995) setal nomenclature for antennules of female monstrilloids, first segment with short, slender, spiniform setal element 1. Second segment bearing only four elements, including short elements 2d 1-3 plus long seta IId, with ventral setation reduced, elements 2v 1-3 being absent ( Fig. 1A ). Third segment with spiniform element 3 reaching to proximal third of succeeding fourth segment and lightly plumose setae IIId and IIIv. Fourth segment separated from third by well-defined suture, armed with setal elements 4d 1,2 (short and spiniform), 4v 1,2 (4v 1 shorter than 4v 2), 4aes, IVv, IVd, Vm, Vv, Vd, 6aes, equally long spiniform apical elements 6 1 and 6 2, and subapical ‘ b ’ setal group on outer margin comprising elements b 1-3 and b 6 but lacking subdistal elements b 4 and b 5 ( Figs 1A , 3E ).

THORACIC SOMITES. First pedigerous thoracic somite (incorporated into cephalothorax as noted above) and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs ( Figs 1C , 3D ), all with exopodite longer than endopodite. Setal armature pattern ( Table 1 ) as in M. elongata (see Suárez-Morales 2001). Basis of swimming legs 2–4 with short, slender outer seta, that on leg 3 longest and smooth; basipodal seta absent in leg 1. Outer apical spine of first and third exopodal segments of all legs noticeably long (arrows in Fig. 3D ); outer apical spiniform seta of each distal exopodal segment with denticles along outer margin, sparse setules along inner margin (arrowheads in Fig. 3D ); outer margin of endopodal segments 1 and 2 hirsute ( Fig. 3D ). All natatory setae lightly and biserially plumose.

UROSOME. Consisting of four somites ( Fig. 3A, C ): fifth pedigerous somite carrying fifth legs, genital double-somite ventrally carrying paired ovigerous spines, one short, free preanal somite, and anal somite carrying pair of caudal rami. Length ratio of urosomites (from anterior to posterior) 31.7: 36.8: 14.7:16.8 = 100 ( Fig. 3A, C ). Fifth legs ( Figs 2A , 3A, C ) well developed, with square, undivided, unarmed protopod and pair of long, distally widening and distally confluent ramal lobes, proximal halves of which separated by narrow, tapered gap or crevice, appearing as an integumental depression; endopodal lobe slightly shorter than exopodal lobe, former armed with one seta, latter with three setae. Anterior half of genital double-somite weakly expanded, with incomplete transverse suture visible in dorsal and lateral view; ovigerous spines relatively short, reaching to midlength of caudal rami ( Fig. 3C ). Caudal rami subrectangular in dorsal view ( Fig. 2B ), 1.7 times as long as broad, each armed with six caudal setae. Seta I situated on proximal lateral margin, seta II subdistally along outer margin. Distalmost setae (III and IV) proximally expanded ( Fig. 2B–C ). Ventral surface of ramus with one large pore and two adjacent indistinct integumental structures on distal half ( Fig. 2B ).

Justification for establishment of M. xcalakensis sp. nov.

Several species of Monstrilla have a well-developed, bilobed female fifth leg armed with one seta on the inner lobe and three on the outer, including some from the western Caribbean: M. careli , M. careloides , and M. humesi Suárez-Morales & Escamilla, 2001 (Suárez-Morales 2001; Suárez-Morales & Escamilla 2001). Such setation is a plesiomorphic character state that is seen in M. grandis Giesbrecht, 1891 , M. orcula A. Scott, 1909 , and M. cymbula A. Scott, 1909 as well (Scott 1909; Suárez-Morales 2000; Chang 2014). The new species differs from both M. careli and M. careloides in the strong development of the fifth leg’s endopodal lobe, which is very reduced in M. careloides (cf. Suárez-Morales 2001: fig. 16) and weakly developed in both M. careli and M. humesi , barely reaching to the midlength of the exopodal lobe (cf. Suárez-Morales, 2001: fig. 6; Suárez-Morales & Escamilla, 2001: fig. 4d). The Caribbean M. barbata Suárez-Morales & Gasca-Serrano, 1992 also has a reduced endopodal lobe (cf. Suárez-Morales & Gasca-Serrano, 1992: figs 1d, 2f). Two western Caribbean species, M. ciqroi Suárez-Morales, 1993 and M. rebis Suárez-Morales, 1993 , share with the new species the same armature of the fifth leg (i.e., 1, 3), but in both of them the endopodal lobe is much reduced, being represented only by a small protuberance (cf. Suárez-Morales 1993: figs 1d, 3d). In the Brazilian M. brasiliensis Suárez-Morales & Dias, 2000 , the endopodal lobe is almost as long as the outer lobe, but is nonetheless more weakly developed and more slender (cf. Suárez-Morales & Dias, 2000: fig. 3c–d) than the strong, broad lobe of M. xcalakensis sp. nov. The fifth leg’s endopodal lobe is wide and well-developed in M. gibbosa Suárez-Morales & Palomares-García, 1995 from the Gulf of California, but is not fused with the exopodal lobe (cf. Suárez-Morales & Palomares-García 1995: figs 2a, 3b). In addition, the antennular armature differs between these two species; in M. gibbosa the armature of the second segment includes elements 2v 1-3 and 2d 1,2, and element IId is absent, whereas in M. xcalakensis elements 2v 1-3 are absent and IId is present. Also, M. gibbosa has branched setae in the ‘b’ setal group versus simple, unbranched ‘b’ setae in the new species ( Fig. 3E ).

Overall, the Korean species M. ilhoii Lee & Chang, 2016 most closely resembles M. xcalakensis sp. nov. It shares with the new species some important features: a large body size, exceeding 3 mm in total length; the same peculiar structure of the fifth leg, with a strong, broad endopodal lobe that largely is fused with the exopodal lobe; and proximally expanded terminal caudal setae (cf. Lee & Chang 2016: figs 1E, 2D). The two species nonetheless differ in several respects. Nearly the entire body surface of M. ilhoii , including the antennules, caudal rami, and legs 1–4, is finely sculptured with a polygonal pattern (cf. Lee & Chang 2016: fig. 4b–d), which M. xcalakensis wholly lacks. The cephalothorax of M. ilhoii constitutes 53% of the total body length, versus slightly more than 60% in the new species. The oral cone is more strongly protuberant in M. ilhoii (cf. Lee & Chang 2016: figs 1b, 4b) than in M. xcalakensis ( Fig. 3B ). Antennular segments 3 and 4 are completely fused in M. ilhoii , together forming a long distal segment that represents 69% of the antennular length (cf. Lee & Chang 2016: fig. 2a), whereas all four antennular segments are distinct and separate in the new species ( Fig. 1A ). Finally, according to Lee & Chang (2016), the ovigerous spines of M. ilhoii are long, reaching well beyond the tips of the caudal setae, while they are much shorter in the new species, not reaching the distal end of the caudal rami ( Fig. 3A ). The differences observed from the other known species of Monstrilla , and especially from M. gibbosa and M. ilhoii , appear to be sufficient to warrant the proposal of a new species for the present specimens.

Figures

Fig. 1. Monstrilla xcalakensis sp. nov., paratype, ♀ (ECO-CH-Z 11814). A. Anterior cephalic region showing pair of sensilla, ventral view. B. Habitus, ventral view. C. Pedigerous somites 1–4 showing swimming legs, ventral view. D. Cephalic region showing oral cone (oc), paired nipple-like processes (np1,2), and integumental wrinkles. Arrowheads indicate sensilla (A) or pores (D).

Fig. 2. Monstrilla xcalakensis sp. nov., paratype, ♀ (ECO-CH-Z 11813). A. Fifth legs, ventral view. B. Caudal rami and distal part of ovigerous spines, ventral view showing inflated caudal setae III and IV and large adjacent pores (p). C. Detail of proximally inflated caudal setae III and IV, ventral view. D. Preoral area, another specimen, ventral view, showing medial field of transverse striae, adjacent pairs of nipple-like integumental processes (np1,2), pair of pore clusters (arrowheads) at antennule bases, and oral cone (oc) with two pairs of preoral pores.

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.