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

Suárez-Morales, Eduardo & P. M. B, Alexander David Mckinnon, 2025, The Australian Monstrilloida 3. Caromiobenella Jeon, Lee & Soh, 2018, Monstrilla Dana, 1849, and Sarsimonstrillus n. gen., Zootaxa 5576 (1), pp. 1-99 : 60-65

Monstrilla latisetosa sp. nov.

urn:lsid:zoobank.org:act:29DDFCA1-3B21-47BC-BB4A-1F9C2DB63956

( Figs 37–40 )

Material examined. Adult female holotype, undissected, mounted on slide in glycerine, (ECO-CHZ-12537). Three adult female paratypes of which two undissected, each mounted on one slide in glycerine, the other mounted on a SEM stub after SEM preparation, paratypes not catalogued.

Type locality. Western Port Bay , Victoria, Australia (38°26.153’ S, 145°13.327’ E), coll. on 26 February 1985 . Additional locality: Port Phillip Bay , Black Rock, Victoria, Australia (37°58.066’ S, 145°.2705’ E), coll. on 17 March 1983 .

Diagnosis. Large (> 3.5 mm body length), with robust cephalothorax about 55% of total body length; oral cone relatively small, located ventrally at anterior 1/4 of cephalothorax. Urosome relatively short, about 20% of total body length. Antennules 4-segmented, almost 32% of total body length, anteriorly directed; distal segment longest, tapering distally, with rounded tip, apical elements 6 1,2 (sensu Grygier & Ohtsuka 1995) reduced or modified into small, leaf-like elements. Genital double-somite with expanded anterior half, somite carrying pair of ovigerous spines almost reaching distal end of caudal setae. Third exopodal segments of legs 1 and 4 with modified, sickle-shaped outermost apical spine. Fifth legs bilobed, exopodal lobe thick, subrectangular, armed with three subequally long lightly setulated setae; endopodal lobe digitiform, as long as or slightly longer than exopodal lobe and armed with two equally long, lightly setulated setae. Caudal rami subrectangular, 1.5 times as long as wide, armed with five equally long caudal setae, of which apical setae III and IV proximally swollen.

Description of adult female holotype. Body length 3.56 mm. Body tagmosis as usual for females of Monstrilla ( Isaac 1975; Suárez-Morales & Islas-Landeros 1993; Chang 2014). Cephalothorax long, robust, about 58% of total body length and fully incorporating first pedigerous somite. Oral cone moderately prominent (oc in Figs. 37A , 39A ), located 18% of way back along ventral surface of cephalothorax. Eyes comprising two lateral cups and medial ventral cup (lec, mec in Fig. 37A ). Cephalic region anteriorly subquadrate in dorsal view, ‘forehead’ flat, with few integumental ridges between antennule bases ( Figs. 37A , 38A , 39A ); ventral preoral surface with reduced ornamentation, including one or two pairs of nipple-like processes (nlp 1, 2 in Figs. 37A , 39A , 40A ) and medial pore cluster (mpc in Figs. 39A , 40A ), as well as usual field of integumental wrinkles on ventral perioral surface.

Antennules relatively short, 0.75 mm long, about 20% of total body length and 30% of cephalothorax length, thick, distinctly four-segmented, anteriorly directed ( Fig. 37A ) or divergent ( Fig. 40A ), fourth segment longest; length ratio of antennular segments (proximal to distal): 17.2: 24.3: 25.71: 32.8 = 100 ( Fig. 37A ). Following Grygier and Ohtsuka’s (1995) setal nomenclature, first segment with reduced, spiniform setal element 1 ( Fig. 39A ), second segment bearing short setiform, setulated element IId, and short, robust spiniform elements 2d 1,2, and 2v 1–3, third segment with slender setiform element 3 and adjacent setiform elements IIId and IIIv, fourth segment longest, tapering distally, armed with setal elements IVd, IVv, 4v 1,2, Vm, Vd, and Vv, subapical “b” setal group on outer margin comprising four unbranched setal elements, namely b 1–3 and b 6; apical element 6 1 modified, represented by short spine or by small leaf-shaped element closely inserted to apical aesthetasc 6aes ( Figs. 37A , 38A , 40A ), element 6 2 not observed, aesthetasc 6aes short, inserted subapically next to element 6 1 ( Fig. 38A ).

First pedigerous thoracic somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs, all with exopod longer than endopod and 3-segmented. Setal armature pattern as in M. pileata sp. nov. All natatory setae slightly and biserially plumose.

Armature of swimming legs 1–4 with modified, sickle-shaped outer apical spines on exopods of legs 1 and 4 ( Figs. 37 C, D ):

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) 36.6: 43.13: 13.1: 7.2 = 100 ( Fig. 37B ). Fifth legs well-developed, bilobed; outer lobe robust, subrectangular, armed distally with three subequally long lightly setulated setae; inner lobe digitiform, reaching distal margin of or slightly longer than outer lobe, armed with two subequally long, lightly setulated setae ( Figs 37B , 38B, D , 40B ). Genital double-somite with weakly expanded proximal half, with incomplete transverse suture visible in dorsal view. Anal somite short, with integumental ridges on dorsal surface (arrowhead, Fig. 40C ). Ovigerous spines arising ventrally from genital somite (os in Figs. 37B , 38D ), relatively long, almost reaching distal end of caudal setae, only one spine carrying eggs distally. Caudal rami subquadrate in ventral view ( Figs. 37B , 40B ), 1.1 times as long as broad, each ramus armed with five caudal setae (I–V); distalmost setae III and IV proximally swollen (shadowed in Figs 37B , 40B , indicted by arrowheads in Fig. 39 C, D ).

Intraspecific variability. The holotype and paratype specimens differ in several minor details, including: (1) size (the paratype is smaller (2.6 mm vs. 3.56 mm of holotype); (2) relative length of the antennule (relatively shorter in the holotype (15% of total body length) than in the paratype (29%)); (3) antennulary setation pattern (holotype with uniformly short, robust setal elements 4d 1,2 and 4v 1–3 ( Fig. 40A ), whereas these elements are relatively longer, slenderer in the paratype; additionally, element 3 is long, setiform in the holotype ( Fig. 37A ) but short, spiniform in the paratype; (4) relative length of the fifth leg endopodal lobe (as long as the exopodal lobe ( Fig. 37B ) in the holotype, but inner lobe longer than the outer lobe in one paratype ( Fig. 39B ); (5) the paratype shows a relatively smooth preoral integumental surface ( Fig. 39A ) vs. a wrinkled surface in the holotype ( Fig. 37A ).

Remarks. Monstrilla latisetosa sp. nov. can be easily identified by its possession of several distinctive characters, the most important being a bilobed female fifth leg with the exopodal lobe armed with three setae and endopodal lobe carrying two setae and being as long as or longer than the outer lobe ( Figs. 37B , 39B , 40C ). The length and development of the fifth leg endopodal lobe is an important character and only a few species have strongly developed fifth leg endopods, namely M. gibbosa , the Australian M. janetgrieveae sp. nov., and M. hendrickxi . They can be readily differentiated from M. latisetosa sp. nov. in the endopodal lobe armature, consisting of a single seta in the Australian M. janetgrieveae sp. nov. and in M. gibbosa ( Suárez-Morales & Palomares-García, 1995, fig. 2a, b), and being unarmed in M. hendrickxi ( Suárez-Morales & Velázquez-Ornelas, 2024, fig. 4A). There are several Australian species of Monstrilla with proximally swollen caudal setae (i.e., M. pileata sp. nov., M. fisgata sp. nov., M. janetgrieveae sp. nov., M. latisetosa sp. nov.), but M. latisetosa sp. nov. can also be recognized among its Australian congeners by the proximally swollen caudal setae III and IV combined with a 3, 2 setation formula (exopod, endopod) of the female fifth legs.

Figures

FIGURE 37. Monstrilla latisetosa sp. nov., adult female holotype from Western Port Bay, Victoria, Australia. A) cephalic structures and antennules showing setation pattern following Grygier & Ohtsuka (1995), medial and lateral eye cups (mec, lec), nipple-like processes (nlp), oral cone (oc), and adjacent field of integumental wrinkles, ventral view; B) urosome and caudal rami showing bilobed fifth legs, egg-carrying ovigerous spines (OS), and caudal setae I–IV with proximally thickened setae III and IV (shadowed); C) leg 1 third exopodal segment showing sickle-shaped outer apical spiniform element (arrowhead); D) same, on leg 4, with reduced outer spine on first exopodal segment.

FIGURE 38. Monstrilla latisetosa sp. nov., adult female holotype from Western Port Bay, Victoria, Australia. A) forehead and antennules, ventral view; B) urosome showing fifth legs (P5) and insertion of ovigerous spines (OS), ventral view; C) left caudal ramus showing caudal setae I–V, showing proximal swellings of setae III and IV (arrows), ventral view.

FIGURE 39. Monstrilla latisetosa sp. nov., adult female paratype from Port Phillip Bay, Victoria, Australia, SEM-prepared individual. A) cephalic structures showing nipple-like processes (nlp), medial pore cluster (mpc), oral cone (oc), and adjacent field of integumental wrinkles, ventral view; B) fifth leg (P5), ventral view; C) caudal rami showing proximally thickened caudal setae III and IV (arrowheads), ventral view; D) detail of caudal setae proximal swelling (arrowheads), ventral view.

FIGURE 40. Monstrilla latisetosa sp. nov., adult female paratype from Western Port Bay, Victoria, Australia. A) cephalic structures and antennules showing setation pattern following Grygier & Ohtsuka (1995), nipple-like processes (nlp1,2) and adjacent field of integumental wrinkles, ventral view; B) urosome with caudal rami showing fifth legs and proximally thickened caudal setae III and IV (shadowed), ventral view; C) urosome with fifth legs showing dorsal corrugation on anal somite (arrowhead), lateral view.