Monstrilla reidae Suárez-Morales, 1993 b

Monstrilla reidae Suárez-Morales, 1993 b View in CoL

Figs 8 View Figure 8 , 9 View Figure 9 , 10 View Figure 10 , 11 View Figure 11

Type material.

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

Type locality.

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

Description of adult male holotype.

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

Urosome relatively slender (Fig. 10 B View Figure 10 ), 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 8 A View Figure 8 , 9 A View Figure 9 , 10 D View Figure 10 , 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 8 A View Figure 8 , 9 A View Figure 9 , 10 B View Figure 10 , GC), apparatus comprising long cylindrical shaft with smooth lateral margins; shaft distally branching into pair of short, divergent rounded genital lappets (Figs 10 B View Figure 10 , 11 F View Figure 11 , GL); lappets medially conjoined by rounded concave process with medial genital opening with everted spermatophore (Figs 9 A View Figure 9 , 10 B View Figure 10 , 11 F View Figure 11 , SPO). Caudal rami subrectangular, 1.6 × as long as wide (Fig. 10 C View Figure 10 ); rami armed with six caudal setae subequal in length and width; seta VI relatively shorter (Figs 8 B View Figure 8 , 10 C View Figure 10 , 11 A View Figure 11 ).

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. 10 A View Figure 10 ). 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 2 d 1, 2 and 2 v 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 4 d 1, 2 and 4 v 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. 10 A View Figure 10 ).

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. 11 B – E View Figure 11 , bs); seta on leg 3 lightly setulated, longer and thicker than in other legs (Fig. 11 D View Figure 11 ). Endopods and exopods of swimming legs 1–4 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.

Remarks.

In the original description ( Suárez-Morales 1993 b: 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 (1993 b) 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) .