Monstrilla longibrachiata, 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 : 37-39

Monstrilla longibrachiata sp. nov.

urn:lsid:zoobank.org:act:846BDC7D-400A-4E83-8DB4-10634B3113A8

( Figs 23–25 )

Material examined. Adult male holotype, undissected, mounted on slide in glycerine, (ECO-CHZ-12529). Specimen damaged during mounting process, urosome missing.

Type locality. Werribee , Port Phillip Bay, Victoria, Australia (37°57.085’ S, 144°47.128’ E), coll. on 13 June 1985 .

Diagnosis. Male monstrilloid with cephalothorax robust, relatively short, about half of total body length; anterior margin weakly produced, with pair of sensilla. Medial eye cup strongly developed, partially hiding medially joined lateral cups. Urosome about 1/3 or less of total body length. Oral cone reduced. Antennules 5-segmented; fifth segment with tapering distal half, armed with unbranched setae only. Genital somite carrying genital complex with robust, short shaft and pair of elongate, digitiform genital lappets directed backwards, tips reaching proximal margin of anal somite; genital lappets with coarse inner margin, otherwise unornamented, connected medially by curved margin with medial bean-shaped opercular flap. Caudal rami armed with 6 caudal setae subequal in length and width.

Description of holotype. Body relatively slender. Total body length of holotype 1.50 mm in dorsal view. Cephalothorax almost 50% of total body length; anteriorly rounded, with moderately protuberant forehead furnished with pair of apical sensilla ( Fig. 23A ); preoral anteroventral surface with pair of nipple-like integumental processes (nlp in Figs. 23A , 25C ), adjacent field of integumental wrinkles ( Fig. 23A ). Oral cone small, not protuberant (oc in Fig. 23A ), weakly produced, located at 0.35% of way back along ventral surface of cephalothorax. Eyes represented by two lateral cups and medial ventral cup at anterior end of cephalothorax, moderately pigmented, medial cup (mec in Fig. 23A ) larger than lateral cups (lec in Fig. 25C ), medial cup lying medially between lateral cups (lec in Figs 23A , 25C ).

Urosome 31% of total body length, comprising fifth pedigerous somite, genital somite, one free somite, preanal and anal somites ( Figs 24B–D ); relative length of urosomites, from proximal to distal: 25.7: 24.2: 16.5: 17.0: 16.6 = 100. Fifth pedigerous somite with straight lateral margins. Genital somite with few transverse integumental wrinkles on dorsal surface, somite carrying genital complex. Succeeding preanal and anal somites subequal in length and shape, with smooth surfaces ( Fig. 24C, D ); anal somite carrying caudal rami. Genital complex ventrally on genital somite ( Fig. 24D ); complex with short thick shaft with smooth lateral margins; shaft branching into pair of remarkably long, digitiform curved arms (lappets), the latter medially conjoined, with medial bean-shaped opercular flap ( Fig. 23C ) reaching anterior margin of anal somite ( Fig. 24C ), strongly curved backwards ( Figs. 24B , 25B ), distal inner margin coarse, unornamented ( Fig. 25A, B ). Caudal rami armed with 6 caudal setae (setae I–VI), subequal in length and width ( Fig. 24C, D ).

Antennules 0.37 mm long, almost 25% of total body length, 5-segmented; segments 1–5 clearly divided, segment 4 longest ( Figs. 23B , 24A ). Following nomenclature by Grygier & Ohtsuka (1995), first segment with long, uniserially pinnate setal element 1 reaching distal margin of succeeding second segment (arrow in Fig. 23B ), second segment carrying spiniform elements 2d, 2v 1–3 and long, lightly setulated dorsal seta IId, third segment with long, slender spiniform element 3 and lightly setulated setiform elements IIIv and IIId, fourth segment with reduced armature including proximal elements 4v 1–3 and 4d 1,2, distal half with straight margins, armed with seta Vv ( Fig. 24A ). Following Huys et al.’s (2007) nomenclature for the setation of the male antennulary fifth segment, outer margin with short, slender, seta 6 on the middle of inner margin, followed distally by setae 5, 4, 3, 2 and short seta 1 on apical position; middle inner margin with simple setae A and B; apical elements AE and E, and spine 2 not observed; fifth segment with tapering distal half, tip rounded ( Figs 23B , 24D ).

Swimming legs 1–4 biramous, with 3-segmented rami; endopods slightly smaller than exopods. Outer basipodal seta present in all legs, longest on leg 3. Outer spines on first and third exopodal segments short, about half length of bearing segment. Apical spiniform seta on third exopodal segment slightly 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

Etymology. The species name is composed of two Latin terms, namely longis, meaning long, and brachium, meaning “arm” or appendage; the combined name, an adjective substantive term refers to the possession of remarkably long arms (e.g. lappets) of the genital complex, a distinctive character of this species. Gender is feminine.

Remarks. The structure and ornamentation of the male genitalia have been reported as taxonomically valuable characters to distinguish species of monstrilloids (Suárez-Morales 2000), but it should be noticed that the intraspecific variation of the genital complex could lead to confusion ( McAlice 1985) seen in different illustrations of closely similar genitalia of the male of C. helgolandica , which likely included males of at least three distinct species. Therefore, a male with a highly distinctive genital complex can more easily be recognized as belonging to a specific species as e.g. the male of M. longibrachiata sp. nov., whose genital complex is not comparable to any other reported among species of Monstrilla .

Relatively long, slender genital lappets are found in a few congeneric species e.g. the Indian M. lata Desai & Bal, 1962 , exhibiting long, sausage-like lappets with a coarse inner margin ( Desai & Bal 1962, fig. 2,5), but in this species lappets are straight in lateral view, not curved backwards as observed in M. longibrachiata sp. nov. Furthermore, the cephalothorax ornamented with fields of coarse wrinkles ( Desai & Bal 1962, fig. 2.2), the poorly developed eyes, and the different antennule proportions and armature ( Desai & Bal, 1962 fig.2,3) clearly differentiates M. lata from M. longibrachiata sp. nov. Other species sharing a genital complex with relatively long, curved lappets are the widespread M. longiremis Giesbrecht, 1893 and M. longicornis Thompson, 1890 , with genital lappets reaching the proximal margin of the preanal somite ( Huys & Boxshall 1991, fig. 2.5.7.,; Suárez-Morales 2010, figs. 5G, 6A, B). However, the genital lappets of these two morphologically similar species have a pair of long distal spermatophore spines that point backwards. Furthermore, in M. leucopis Sars, 1921 and the South African M. papilliremis Isaac, 1975 the genital lappets have apical spiniform structures (spermatophore spines) that are absent in M. longibrachiata sp. nov. Also, the male of M. longiremis exhibits a pair of fifth legs ( Suárez-Morales 2010), whereas fifth legs are absent in the new species. The Caribbean species Monstrilla marioi Suárez-Morales, 2003 exhibits distinctively long genital lappets ( Suárez-Morales 2003, figs. 1, 4, 8), longer than those found in M. longibrachiata sp. nov., reaching the distal margin of the anal somite, whereas in the new species they only reach the distal margin of the preanal somite. Furthermore, the genital lappets of M. marioi taper distally into slender filaments and they are coarsely corrugated along and around its surface ( Suárez-Morales 2003, fig. 5), which differ from those of M. longibrachiata sp. nov. Additional differences between the two species include the body proportions, antennule armature, the caudal rami armature (with only 4 setae in M. marioi vs. 6 in M. longibrachiata sp. nov.), and the presence of protuberant fifth legs buds in M. marioi (vs. none in M. longibrachiata sp. nov.).

Remarks on the antennules of Monstrilla . The antennule armature of Monstrilla species is quite conservative, but comparison of some setal elements is now possible after the publication of Grygier & Ohtsuka’s (1995) setal nomenclature. The unusual development of some of these elements has been used to compare species of Caromiobenella (Jeon et al. 2018) , particularly regarding the traits and ornamentation of elements IId and 2d 2, both inserted on the second antennulary segment. Elements of the “ b ” group (sensu Grygier & Ohtsuka 1995) typically inserted along the distal half of the last segment, can be simple or branched as in species of the widespread species group linked to M. grandis Giesbrecht, 1891 , i.e., M. bahiana Suárez-Morales & Dias, 2001 , M. bernardensis Davis & Green, 1974 , and M. gibbosa Suárez-Morales & Palomares-García, 1995 . Element 2v 3 has been described as unusually long (i.e., reaching beyond midlength of succeeding fourth segment ( Suárez-Morales & Dias 2000, fig. 4A, B) or ornamented (spinulate) as in the female of C. brasiliensis ( Suárez-Morales & Dias 2000) . It was expected to have similar traits in the male, but it is only shorter (see Cruz Lopes da Rosa et al. 2021, fig. 3A). A setal element identifiable as 2v 1 in the male of M. papilliremis is extremely long, reaching beyond the distal end of the antennule ( Isaac 1975, fig. 1C) and in the same species the setal elements 4v 2 or 4v 3 can also be remarkably long. The antennules of Monstrillopsis reticulata ( Davis, 1949) also appear to have long elements of the 4v group ( Davis 1949, fig. 3).

The most proximal element of the antennulary armature is element 1 (sensu Grygier & Ohtsuka 1995), usually represented by a short (i.e., shorter than the carrying segment), slender seta or spine, like in M. lata ( Desai & Bal 1962, fig. 3), M. pustulata Suárez-Morales & Dias, 2001 ( Suárez-Morales & Dias 2001, fig. 2B), M. longa ( Suárez-Morales & Gasca 2004, fig. 7), and M. xcalakensis (see Suárez-Morales 2024, fig. 3E). It can also be absent as in M. humesi ( Suárez-Morales & Escamilla 1997, fig. 3B), M. satchmoi Suárez-Morales & Dias 2001 , fig. 2), M. leucopis Sars, 1921 , and in members of the M. conjunctiva species group, described in this account. The absence of this element has been used to separate species with elongate, poorly segmented antennules ( Suárez-Morales & Velázquez-Ornelas 2024). A long setal element 1 has been reported in M. marioi ( Suárez-Morales 2003, fig. 10) and in M. hendrickxi Suárez-Morales & Velázquez-Ornelas, 2024 (see Suárez-Morales & Velázquez-Ornelas 2024, fig. 3A), and in both species it is simple, lacking any ornamentation, thus differing from its uniserially pinnate condition observed in M. longibrachiata sp. nov. ( Fig. 23B ).

Overall, the new species can be readily distinguished from its congeners by a combination of two unusual characters: (1) the structure of the genital complex including long, digitiform lappets curved backwards, and (2) the antennules armature, particularly with the modified, uniserially strongly pinnate setal element 1. It is likely that this character is present in the unknown female of the new Australian species M. longibrachiata sp. nov.

Figures

FIGURE 23. Monstrilla longibrachiata sp. nov., adult male holotype from Port PhillipBay, Victoria,Australia.A) cephalothorax showing cephalic structures, medial eye cup (mec), lateral eye cups (lec), frontal sensilla, nipple-like processes (nlp), and oral cone (oc), ventral view; B) right antennule showing setation pattern with nomenclature of segments I–IV following Grygier & Ohtsuka (1995) and Huys et al. (2007) for fifth segment, with long, uniserially pinnate element 1 (arrow), dorsal view; C) genital complex showing genital lappets (L), ventral view.

FIGURE 24. Monstrilla longibrachiata sp. nov., adult male holotype from Port Phillip Bay, Victoria, Australia. A) urosome showing genital complex, lateral view; B) urosome and caudal rami showing protuberant corners of genital somite (arrows) and caudal setae I–VI, dorsal view; C) pedigerous somites 1–5 (legs omitted) and urosome, ventral view; D) right antennule showing setation pattern with nomenclature of segments I–IV following Grygier and Ohtsuka (1995) and Huys et al. (2007) for fifth segment.

FIGURE 25. Monstrilla longibrachiata sp. nov., adult male holotype from Port Phillip Bay, Victoria, Australia. A) genital complex with genital lappets, lateral view; B) same, semi-ventral view; C) preoral surface with medial eye cup, lateral eye cups (mec, lec), and nipple-like processes (nlp), ventral view; D) caudal rami, dorsal view.