Monstrilla rebis Suárez-Morales, 1993 a

Monstrilla rebis Suárez-Morales, 1993 a View in CoL

Figs 6 View Figure 6 , 7 View Figure 7

Type material.

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

Type locality.

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

Description of adult female holotype.

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. 6 B View Figure 6 , oc), located 22 % of way back along ventral surface of cephalothorax. Cephalic region anteriorly expanded in dorsal view, ‘ forehead’ rounded, smooth, weakly produced (Fig. 6 A View Figure 6 ); ventral preoral surface with integumental ornamentation including two pairs of nipple-like processes with adjacent integumental wrinkles in both the holotype and paratype (Fig. 6 B View Figure 6 , nlp). Eyes comprising two relatively large lateral cups (Fig. 6 A View Figure 6 , lec) and ventral medial cup (Fig. 6 A View Figure 6 , 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. 6 A View Figure 6 ).

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 1993 a: fig. 1 a), 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. 6 C, D View Figure 6 ). Genital double-somite longest of urosome, with weakly expanded lateral margins (Fig. 6 D, E View Figure 6 ); pair of slender ovigerous spines on ventral surface; spines equally long, both ending in thin, straight parallel points (Fig. 6 F View Figure 6 ). Caudal rami subrectangular (Fig. 6 D View Figure 6 ), ~ 1.4 × as long as broad, each armed with five caudal setae (I – V), seta V being shortest (Fig. 6 D View Figure 6 ).

Antennules 0.64 mm in length, representing ~ 23 % of total body length and almost 44 % of cephalothorax length ( Suárez-Morales 1993 a: fig. 1 a); as usual in female monstrilloids, antennules distinctly 4 - segmented, relatively slender, anteriorly directed, weakly divergent; intersegmental divisions segments 1–3 complete (Fig. 7 E View Figure 7 ); length ratio of antennular segments (proximal to distal) 10.71: 25.71: 14.28: 49.3 (Fig. 7 E View Figure 7 ). 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 2 v 1-3 and 2 d 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 4 v 1, 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 6 aes (Fig. 7 E View Figure 7 ).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs ( Suárez-Morales 1993 a: fig. 2 b, c, d), all with exopodite longer than endopodite. Swimming legs 1–4 slender, with setal armature pattern as shown in Fig. 7 A – D View Figure 7 . 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. 6 E, P 5 View Figure 6 ) 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. 6 E View Figure 6 ).

Male. Unknown.

Remarks.

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 1993 a); 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. 2 A, 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. 1 A) vs the anterior ¼ of the cephalothorax in M. rebis ; and the number of caudal setae, six in M. grygieri ( Suárez-Morales 2000: fig. 2 D), vs five in M. rebis .