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

Monstrilla sekiguchii sp. nov.

urn:lsid:zoobank.org:act:90310D63-62DD-4C7B-BC77-6A4876820A99

( Figs 17–22 View FIGURE 17 View FIGURE 18 View FIGURE 19 View FIGURE 20 View FIGURE 21 View FIGURE 22 )

Material examined. Adult male holotype, undissected, mounted on slides in glycerine, (ECO-CHZ-12528). Adult male paratype partially dissected, mounted on slide in glycerine, not catalogued.

Diagnosis (based on the male holotype). Cephalothorax relatively short, about half of total body length; anterior margin flat except for medial protuberance containing part of medial eye cup. Urosome relatively short, about 1/3 or less of total body length. Oral cone protuberant. Antennules 5-segmented, with rounded tips and branched setae on outer margin, fourth segment longest. Fifth pedigerous somite with medial swelling; genital somite carrying short genital complex with robust, short shaft and pair of compact, thumb-shaped genital lappets distally, distally furnished with rows of small spinules; genital lappets medially connected by convex rounded process. Caudal rami with six caudal setae.

Type locality. Rhyll , Western Port Bay, Victoria, Australia (38°26.792’ S, 145°18.496’ E) sampled on 03 March 1984 GoogleMaps .

Additional locality. Corinella , Western Port Bay, Victoria, Australia (38°23.115’ S, 145°25.371’ E) sampled on 17 June 1985 GoogleMaps .

Description of male holotype. Body relatively slender.Total body length 1.13 mm in dorsal view.Cephalothorax almost 50% of total body length; anteriorly rounded, with moderately protuberant forehead ( Figs 17A View FIGURE 17 , 18D View FIGURE 18 , 22 A, B View FIGURE 22 ), latter with pair of hyaline bodies (sensu Suárez-Morales 2018) between lateral and medial eye cups (hb in Figs. 17C View FIGURE 17 , 18D View FIGURE 18 ); fused first pedigerous somite with posterodistal corners weakly expanded, reaching the proximal 1/3 of succeeding second pedigerous somite. Preoral anteroventral surface with medial rounded protuberance visible in lateral view (mp in Fig. 17C View FIGURE 17 ), field of integumental wrinkles, and adjacent pair of nipple-like processes on ventral surface (nlp in Fig. 17B View FIGURE 17 ) flanking medial cephalic protuberance. Oral cone protuberant, robust (oc in Fig. 17 A, C View FIGURE 17 ), strongly produced, located at 30% of way back along ventral surface of cephalothorax (oc in Fig. 17B, C View FIGURE 17 ). Eyes represented by two lateral cups and medial ventral cup at anterior end of cephalothorax, eyes pigmented, medial cup larger than lateral cups (mec, lec in Fig. 17B View FIGURE 17 ), medial cup protruding on frontal margin ( Fig. 17 A, B View FIGURE 17 ).

Urosome relatively short ( Fig. 22 C View FIGURE 22 ) 24% of total body length, comprising fifth pedigerous somite, genital somite, one free somite, preanal and anal somites, latter holding pair of caudal rami; relative length of urosomites, from proximal to distal: 34.8: 23.25: 16.27: 11.63: 13.93 = 100. Fifth pedigerous somite with straight lateral margins, lateral surface with longitudinal integumental wrinkles and bulging ventral process ( Figs 17E View FIGURE 17 , 18E View FIGURE 18 ). Genital somite with few integumental ridges on lateral surface, somite carrying genital complex. Succeeding preanal and anal somites with few lateral integumental ridges ( Fig. 18E View FIGURE 18 ); anal somite carrying caudal rami. Genital complex arising ventrally on genital somite ( Figs 18E View FIGURE 18 , 22C View FIGURE 22 ) with short thick shaft with smooth lateral margins; shaft branching into pair of short, thumb-like symmetrical, genital lappets ( Figs. 19A View FIGURE 19 , 20D View FIGURE 20 ); distally expanded into small subtriangular process (arrowheads in Figs. 20C View FIGURE 20 , 21D View FIGURE 21 ); lateral and distal margins furnished with minute spinules ( Figs. 19A–D View FIGURE 19 ) and medially conjoined by widely rounded convex process with integumental scars ( Figs. 19A, B View FIGURE 19 , 21D View FIGURE 21 ). Caudal rami armed with 6 caudal setae subequal in length and width: setae III and V weakly swollen proximally (asterisks in Fig. 20D View FIGURE 20 ).

Antennules 0.43 mm long, almost 36% of total body length ( Figs 22A–C View FIGURE 22 ), 5-segmented; segments 1–5 clearly divided, segment 4 longest ( Figs 20A View FIGURE 20 , 18A–C View FIGURE 18 ). Following nomenclature by Grygier & Ohtsuka (1995), first segment with short, spiniform element 1; second segment carrying spiniform elements 2d 1,2 and 2v 1 and long, lightly setulated dorsal seta IId; third segment with long, stout spiniform element 3 and lightly setulated setiform elements IIIv and IIId; fourth segment with reduced armature including proximal elements 4d 1,2 and 4v 1–3, distal half with straight margins, unarmed ( Figs 18C View FIGURE 18 , 20A View FIGURE 20 ). Following Huys et al.’s (2007) nomenclature for the setation of the male antennulary fifth segment, short, slender, lightly setulated seta 5 on the middle of inner margin, with longer setulated seta 6 on subdistal position; outer margin with short, slender seta A, and branched setae B–E on outer subdistal position, apical elements 6 1, 6 2, and 6aes not observed, distal part of fifth segment rounded, with a few wrinkles ( Fig. 18A, B View FIGURE 18 ).

Swimming legs 1–4 biramous, with three-segmented endopods and exopods; endopods slightly smaller than exopods. Outer basipodal seta present in all legs, longest in leg 3 ( Fig. 21C View FIGURE 21 ). Outer spines on first and third exopodal segments short, about half as long as segment. Outer margin of outer apical exopodal seta smooth; apical spiniform seta on third exopodal segment lightly spinulose along outer margin, inner margin setulose. 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

Variability. The two type specimens examined share the body proportions (holotype 1.13 mm, paratype 1.14 mm), similar cephalic structure with anteriorly protuberant medial eye cup and smaller lateral cups, prominent oral cone, similar genital complex, with robust shaft ending in pair of short lappets furnished with rows of spinules, small distal conical process and lappets connected by a medial convex protuberance with integumental scars. Both specimens have six caudal setae with the same distribution and size, i.e. setae I and II are very closely inserted in both specimens (see Figs 20D View FIGURE 20 , 21A View FIGURE 21 ). However, the individuals show some differences which were not deemed enough to consider them as separate species: (1) in the holotype spinules are present only on the distalmost margin of both lappets (arrowheads in Fig. 17F View FIGURE 17 ), whereas in the paratype the spinule rows are present also on the distolateral surface of the genital shaft ( Fig. 19C View FIGURE 19 ), (2) the lappets are slightly shorter in the paratype ( Fig. 19C View FIGURE 19 ) than in the holotype ( Fig. 19B, D View FIGURE 19 ), (3) the fifth antennulary segments of these specimens differ slightly in the apical structure and armature, in the holotype individual the segmental tip is uniformly rounded ( Figs 18A, B View FIGURE 18 , 21A View FIGURE 21 ), vs. a conical apical tip in the paratype ( Fig. 18A, B View FIGURE 18 ) and the armature of this segment is reduced in the holotype ( Figs. 18A, B View FIGURE 18 , 21A View FIGURE 21 ) only carrying 2–4 setae, whereas the paratype has a more complete set comprising seven setae, some of them branched ( Fig. 18C View FIGURE 18 ).

Type locality. Rhyll , Western Port Bay, Victoria, Australia (38°26.792’ S, 145°18.496’ E) GoogleMaps .

Other localities. Corinella , Western Port Bay, Victoria, Australia (38°23.115’ S, 145°25.371’ E) GoogleMaps .

Etymology. The species name is an eponym to honour the memory of the Japanese Professor Emeritus Dr. Hideo Sekiguchi, who passed away on January 17, 2023. He was well known for decades of biological and oceanographic work on larvae of spiny lobsters and authored early taxonomical studies on Japanese Monstrilloida . Gender is masculine.

Remarks. There are only a few species of Monstrilla showing male genitalia like that observed in M. sekiguchii sp. nov., with a short shaft and compact lappets medially connected by a convex rounded process. Although now placed in Monstrillopsis , M. bernardensis ( Willey, 1920) was originally described as a species of Monstrilla and it has a genital complex comparable to that of M. sekiguchii sp. nov., but in M. bernardensis the lappets are connected by a flat medial structure (see Davis & Green 1974, fig. 13), thus different from the pattern observed in the new species. Another species with a similar genital complex is Monstrilla rugosa Davis, 1947 , from Florida, its lappets are short but not thumb-like as in M. sekiguchii sp. nov., but tapering distally, and medially conjoined ( Davis 1947, fig. 4), thus different from the new species.

A similar case is that of the Korean M. ilhoi whose male genitalia comprise short lappets that are not rounded as in the new species but are triangular, tapering distally, connected medially by a flat rugose margin ( Lee & Chang 2016, fig. 4E), and with medial opercular flaps, also different from the structure of M. sekiguchii ’s sp. nov. genitalia. This type of genital complex, with a short shaft and reduced lappets is also frequently found among species of Caromiobenella (Jeon et al. 2018) , including the Australian C. jeoni sp. nov. Considering Jeon et al.’s (2019) criteria to classify the male genitalia among species of Caromiobenella , the genital complex observed in the Australian M. sekiguchii sp. nov. is more similar to the type II genital complex, mainly because of the absence of a deep medial triangular notch, but not by having a medial distal protrusion ( Jeon et al. 2019).