Monstrilla janetgrieveae, Suárez-Morales & P.M.B, 2025

Monstrilla janetgrieveae sp. nov.

urn:lsid:zoobank.org:act:6BECA32C-6949-4AC6-8516-E3A7C77FBFFF

( Figs 34–36 View FIGURE 34 View FIGURE 35 View FIGURE 36 )

Material examined. Adult female holotype undissected, mounted on slide in glycerine, (ECO-CHZ-12538).

Type locality. Port Phillip Bay , Black Rock, Victoria, Australia (37°58.066’ S, 145°.2705’ E) on 17 March 1983 .

Diagnosis. Large (> 3 mm body length) female monstrilloid, with elongate, cylindrical cephalothorax about 55% of total body length; oral cone moderately prominent, located ventrally at anterior 1/3 of cephalothorax. Urosome relatively short, 20% of total body length. Antennules anteriorly directed, 4-segmented, thick, relatively short, almost 32% of total body length; segments 1–2 divided, distal segment longest, tapering distally, with rounded tip; apical elements 6 1,2 (sensu Grygier & Ohtsuka 1995) thick, equally long. Genital double-somite with expanded lateral margins, somite carrying pair of ovigerous spines barely reaching beyond distal margin of caudal rami. Fifth legs bilobed, exopodal lobe thick, subrectangular, armed with three subequally long, lightly setulated setae; endopodal lobe unarmed, thumb-like or globose, reaching slightly beyond distal margin of exopodal lobe, armed with single seta. Caudal rami subrectangular, 1.5 times as long as wide, armed with five equally long caudal setae, apical setae III and IV proximally swollen.

Description of adult female holotype. Body length 3.27 mm. Body tagmosis as usual in females of Monstrilla ( Isaac 1975; Suárez-Morales & Islas-Landeros 1993; Chang 2014). Cephalothorax long, cylindrical, moderately robust; cephalothorax about 58% of total body length and fully incorporating first pedigerous somite. Oral cone moderately prominent (oc in Figs. 34A View FIGURE 34 , 35A View FIGURE 35 ), located 18% way back along ventral surface of cephalothorax. Eyes comprising two lateral cups and medial cup (lec, mec in Fig. 34A, E View FIGURE 34 ); small bean-shaped hyaline bodies (sensu Suárez-Morales 2018) located anteriorly to eye cups. Cephalic region anteriorly subquadrate in dorsal view, ‘forehead’ flat, with few integumental indentations between antennule bases ( Figs. 34E View FIGURE 34 , 36A View FIGURE 36 ); ventral preoral surface with reduced ornamentation, including single pair of nipple-like processes (nlp in Figs. 34A View FIGURE 34 , 35 A View FIGURE 35 ) and medial pore cluster (mpc in Fig. 34A View FIGURE 34 , 35A View FIGURE 35 ), as well as usual field of integumental wrinkles on ventral perioral surface.

Antennules robust, about 32% of total body length and almost 55% of cephalothorax length, distinctly four-segmented, anteriorly directed ( Fig. 34A, E View FIGURE 34 ). Segments 1–3 divided, segments 3–4 partly fused ( Figs. 34E View FIGURE 34 , 35A View FIGURE 35 ); length ratio of antennular segments (proximal to distal) 13.3: 16.6: 20.0: 50.1 = 100 ( Figs. 34A View FIGURE 34 , 35A View FIGURE 35 ). Following Grygier and Ohtsuka’s (1995) setal nomenclature, first segment with reduced, spiniform setal element 1, second segment bearing short setiform element IId, and short, robust spiniform elements 2d 1,2, and 2v 1,2, third segment with spiniform element 3 and adjacent setiform elements IIId and IIIv, fourth segment longest, almost 50 % of the total antennule length, armed with setal elements IVd, IVv, 4v 1,2, Vm, Vd, and Vv, subapical “ b ” setal group comprising single slender unbranched element b 3 inserted subapically on outer margin; apical elements 6 1,2 equally long, closely inserted next to each other ( Fig. 35 B, C View FIGURE 35 ); aesthetasc 6aes short, inserted subapically next to elements 6 1,2 ( Fig. 35C View FIGURE 35 ).

First pedigerous thoracic somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs, all with exopodite longer than endopodite and usual monstrilloid segmentation. Setal armature pattern as in female of M. pileata sp. nov.

Armature of swimming legs 1–4:

Leg Basis Endopod Exopod

1 1-0 0-1; 0-1;1-2-2 I-1; 0-1; I-2-2

2–4 1-0 0-1; 0-1;1-2-2 I-1; 0-1; I-2-3

Urosome consisting of four somites; fifth pedigerous somite (with fifth legs), genital double-somite (ventrally carrying paired ovigerous spines), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) 49.3: 25.5: 13.7: 11.5 = 100 ( Fig. 34D View FIGURE 34 ). Fifth legs well-developed, bilobed; exopodal lobe robust, subrectangular, armed distally with three subequally long, lightly setulated setae; endopodal lobe thick, digitiform, reaching slightly beyond distal margin of exopodite, armed with single seta ( Fig. 34B View FIGURE 34 ). Genital double-somite with expanded lateral margins, with incomplete transverse suture visible in dorsal and lateral view ( Figs. 34C, D View FIGURE 34 , 35D View FIGURE 35 ) and proximal half exhibiting integumental corrugation on dorsal surface (arrowhead in Fig. 34D View FIGURE 34 ). Ovigerous spines arising ventrally from genital somite (os in Fig. 36D View FIGURE 36 ), relatively short, barely reaching beyond distal margin of caudal rami (os in Fig. 34D View FIGURE 34 ) carrying eggs mass distally. Caudal rami subrectangular in dorsal view ( Fig. 34C View FIGURE 34 ), 1.4 times as long as broad, each armed with five caudal setae I–V; distalmost setae III and IV proximally expanded (arrowheads in Figs. 34C View FIGURE 34 , 36 C View FIGURE 36 ).

Remarks. Monstrilla janetgrieveae sp. nov. can be easily recognized by its possession of several distinctive characters, the most important being the possession of a bilobed female fifth leg with the outer lobe carrying three seta, and inner lobe carrying one seta and being as long as or longer than the outer lobe; this character is shared with M. latisetosa sp. nov., but in M. janetgrieveae the fifth leg armature consists of 3 exopodal setae and 1 endopodal, vs. a pattern of 3 and 2 setae in M. latisetosa . Monstrilla barbata has the same armature pattern but the inner lobe is very reduced ( Suárez-Morales & Gasca 1992, fig. 2f). The same applies to M. careli (see Suárez-Morales & Dias 2000, fig. 1D) and for M. careloides Suárez-Morales, 2001 , in which the inner lobe is only represented by a seta (Suárez-Morales 2001, fig. 16). Among the known species of Monstrilla , the longest inner lobe of the female fifth leg has been observed in M. gibbosa from the southern Gulf of California, and in M. hendrickxi collected off the eastern coast of the Baja, California peninsula. In M. gibbosa the inner lobe carries one seta and is almost as long as the outer lobe ( Suárez-Morales & Palomares-García 1995, fig. 2a, b). In M. hendrickxi the inner lobe is digitiform, slightly longer than the outer ( Suárez-Morales & Velázquez-Ornelas 2024, fig. 4A), but it is unarmed, thus different from the structure exhibited by M. janetgrieveae sp. nov. The new species can also be recognized among its Australian congeners by the proximally swollen caudal setae III and IV, a character only known in M. pileata sp. nov. and the males of M. fisgata sp. nov. It can be distinguished from M. pileata sp. nov. by the lack of a modified apical section of the antennules, distinctive of M. pileata sp. nov. (see description).