Monstrilla elongata Suárez-Morales, 1994

Monstrilla elongata Suárez-Morales, 1994 View in CoL

Figs 12 View Figure 12 , 13 View Figure 13 , 14 View Figure 14 , 15 View Figure 15

Type material.

Holotype • adult female, undissected, ethanol-preserved, vial deposited in the collection of Crustacea, U. S. National Museum of Natural History, Smithsonian Institution, USNM 259488 About USNM . GoogleMaps

Type locality.

Puerto Escondido coastal system   GoogleMaps , eastern coast of the Baja California Peninsula Puerto Morelos, northern part of the Mexican Caribbean coast ( 20°51.40'N, 86°54.15'W). Date of collection 18 November 1993.

Description of adult female holotype.

Body length of holotype 4.2 mm. Cephalothorax robust, cylindrical, representing 58 % of total body length, with lateral expansions at distal end of cephalothorax (Fig. 12 C View Figure 12 ; Suárez-Morales 1994: fig. 1 A). Oral cone well developed, prominent, papilla-like (Figs 12 B View Figure 12 , 14 D View Figure 14 , oc), located 32–35 % along ventral surface of cephalothorax from head. Cephalic region of holotype with flat, smooth ‘ forehead’ (Fig. 12 C View Figure 12 ); ventral preoral surface with integumental ornamentation consisting of two pairs of nipple-like processes with adjacent integumental wrinkles in both the holotype and paratype (Figs 12 A, B View Figure 12 , 14 A View Figure 14 , nlp), field of minute wart-like integumental processes on outer surface of cephalic area adjacent to insertion of antennules (Fig. 15 A, B View Figure 15 , small arrowheads), and medial preoral pore with adjacent field of integumental wrinkles (Fig. 15 B View Figure 15 , pop). Eyes comprising two lateral cups (Fig. 15 A View Figure 15 , lec) and ventral medial cup (Fig. 15 A View Figure 15 , lec, mec), lateral cups slightly smaller in diameter than medial cup; lateral eye cups weakly pigmented.

Urosome consisting of four somites: fifth pedigerous somite (carrying fifth legs), genital double-somite with pair of ovigerous spines barely reaching distal margin of caudal rami in holotype (Fig. 12 A View Figure 12 ; Suárez-Morales 1994: fig. 1 F), slightly longer in paratype (Fig. 14 B, D View Figure 14 ), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) being: 34.91: 37.20: 16.27: 11.62 (Fig. 15 D, E View Figure 15 ). Genital double-somite longest of urosome, with weakly expanded proximal 1 / 2 ornamented with transverse ridges on dorsal surface; distal 1 / 2 ornamented with scattered integumental wrinkles in lateral and dorsal surfaces (Figs 14 B, F View Figure 14 , 15 D View Figure 15 ); pair of relatively short, corrugate ovigerous spines inserted on ventral surface (Figs 13 F View Figure 13 , 14 B View Figure 14 , os); spines equally long, both ending in acute, parallel points (Fig. 15 D, E View Figure 15 ). Caudal rami relatively large, subrectangular (Fig. 15 C View Figure 15 ), ~ 2.5 × as long as broad, each ramus armed with five caudal setae (I – V), lateral seta II reduced, being shortest and thinnest (Fig. 15 C, E View Figure 15 ).

Antennules remarkably long, slender, 0.64 mm in length, representing ~ 23 % of total body length and almost 44 % of cephalothorax length (Fig. 12 A – C View Figure 12 ); antennules 4 - segmented, but segments 2–4 partly fused; antennules weakly divergent in both holotype (Fig. 12 A – C View Figure 12 ) and paratype (Fig. 14 D, E View Figure 14 ) specimens. Intersegmental division between segments 1 and 2 complete (Fig. 13 E View Figure 13 ); length ratio of antennular segments (proximal to distal) 10.71: 25.71: 14.28: 49.3 (Fig. 13 E View Figure 13 ). Following Grygier and Ohtsuka’s (1995) setal nomenclature for female antennules, first segment unarmed, bearing thumb-like process on outer margin (Fig. 13 A, E View Figure 13 , arrows); putative second segment carrying relatively short spiniform elements 2 v 1-3 and 2 d 1, 2, setiform element IId absent, broken off, only sockets observed (Fig. 13 B View Figure 13 ); third segment with slender, stout spiniform element 3 and reduced setiform elements IIId and IIIv (Fig. 13 C View Figure 13 ), fourth segment longest of antennule, proximal 1 / 2 armed with short spiniform elements 4 v 1, setiform elements IVv and IVd, and aesthetasc 4 aes (Fig. 13 D View Figure 13 ). Distal 1 / 2 of fourth segment lacking apical armature except for reduced spiniform element 6 1 in subdistal position (Fig. 13 A View Figure 13 ). Antennules with set of two or three disc-like integumental windows of unknown function in segments 3 and 4 (Fig. 13 A – C View Figure 13 , iw).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs (Fig. 12 A – C View Figure 12 ), all with exopodite longer than endopodite. Swimming legs 1–4 slender, with setal armature pattern as described by Suárez-Morales 1994: fig. 2 J – L). 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 13 F View Figure 13 , 14 B View Figure 14 ) uniramous, represented by single oblong exopodal segment armed with two equally long terminal setae; legs with subtriangular process adjacent to insertion of setae (Fig. 13 F View Figure 13 , asterisk). Fifth leg setae long, reaching distal margin of caudal rami (Fig. 12 A – C View Figure 12 ; Suárez-Morales 1994: figs 1 F, 2 H).

Male. Unknown.

Remarks.

In the original description of this species, Suárez-Morales (1994) did not provide information about the eye cups morphology, which are indeed not easily discernible in the type specimens. Following Isaac’s (1975) criteria, the author incorrectly mentioned the absence of eyes as a character of the genus Monstrilla . The eye structure is described herein based on the re-examination of the type specimens.

According to Suárez-Morales (1994), the main character to distinguish this species from its congeners is the fifth leg structure and armature, consisting of a single segment armed with two setae, a character shared with several congeners like M. conjunctiva Giesbrecht, 1902 , Caromiobenella helgolandica Claus, 1863 (= M. helgolandica ), M. longipes Scott, 1909 (assigned to Maemonstrilla by Grygier and Ohtsuka 2008), and M. ghardaqensis Al-Kholy, 1963 , likely a synonym of C. helgolandica (pers. obs.). Additional species of Monstrilla with the same fifth leg armature pattern include the recently described M. annulata Suárez-Morales, 2024 , from a Mexican Caribbean reef system (see Suárez-Morales 2024), M. leucopis Sars, 1921 from Norway and the Gulf of California ( Suárez-Morales 2010; Suárez-Morales and Velázquez-Ornelas 2023), and M. wandelii from Greenland ( Park 1967). The antennule length is a helpful character to separate species of this group. In M. leucopis , M. annulata , and M. elongata antennules are long and slender, representing almost 50 % ( M. elongata ) or even> 60 % (i. e., M. leucopis , M. annulata ) of the cephalothorax length ( Suárez-Morales and Velázquez-Ornelas 2023; Suárez-Morales 2024). Among these three species, only M. elongata exhibits a distinctive set of disc-like integumental processes (Fig. 13 A – C View Figure 13 ). Furthermore, the recently described M. mahahualensis Suárez-Morales, 2022 , based on male specimens, shares with M. elongata two interesting characters of the antennules: the presence of disc-like integumental processes and the absence of antennular element 1 (sensu Grygier and Ohtsuka 1995). In males of M. mahahualensis the disc-like integumental processes are present in antennular segments 3 and 4 ( Suárez-Morales 2022: fig. 1 C, E, F), whereas in M. elongata , these processes are also found on the second segment (Fig. 13 B View Figure 13 ). The absence of antennular element 1 has been also reported in other species of Monstrilla like M. wandelii , M. leucopis , M. annulata , M. conjunctiva , and M. parki (see Suárez-Morales 2024; Suárez-Morales and McKinnon 2025), but only in M. elongata the first segment has a thumb-like outer process (Fig. 13 A, E View Figure 13 , arrows). Also, M. annulata has a distinctively annulated antennule and an hirsute fifth leg ( Suárez-Morales 2024), thus diverging from M. elongata .