Monstrilla walteri, 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 : 43-47

Monstrilla walteri sp. nov.

rn:lsid:zoobank.org:act: 376BB21A-FC82-4C8C-96D7-A883D02630E2

( Figs. 26–28 )

Material examined. Adult male holotype, partly damaged, fifth segment of both antennules broken off, specimen undissected, mounted on slide in glycerine, (ECO-CHZ-12532).

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

Diagnosis. Male monstrilloid with cephalothorax relatively short, robust, about half of total body length; anterior margin flat, with few integumental ridges. Urosome relatively slender, about 1/3 of total body length. Oral cone protuberant; preoral ventral surface with medial keel-like process and pair of remarkably long nipple-like integumental processes with apical sensilla. Antennules 5-segmented, outer distal margin of fourth segment and proximal half of fifth segment distinctively undulated, fifth antennulary segment with simple setae only. Fifth pedigerous somite ventrally swollen, with pair of slender bud-like processes; genital somite carrying short, compact genital complex comprising robust, short shaft and pair of thick, thumb-shaped genital lappets curved inwards, coarsely ridged along outer and inner margins, unornamented, medially joined, with pair of unguiform opercular flaps. Caudal rami armed with 5 caudal setae subequal in length and width.

Description of holotype. Body relatively robust. Total body length 2.42 mm in dorsal view. Cephalothorax almost 50% of total body length, with flat forehead in lateral and ventral position ( Figs. 26C , 27D ), the latter with hyaline bodies anterior to medial keel-like process (hb in Fig. 26 C ). Fused first pedigerous somite with posterodistal corners weakly expanded, reaching proximal 1/3 of succeeding second pedigerous somite. Oral cone protuberant, robust, with integumental ridges on its surface (oc in Figs. 26C , 27B, D ). Eyes comprising large medial cup with smaller adjacent lateral cups; eyes weakly pigmented (mec, lec in Fig. 27D ). Preoral ventral surface comprising several characters including: (1) medial keel-like process (mk in Fig. 26C , 27B, D ), (2) pair of remarkably long nipple-like integumental processes with apical sensilla ( Fig. 27A , nlp in Figs. 26C , 27B, D ), (3) scattered crescent-shaped scars, and field of transverse wrinkles (csp in Fig. 27D ), and (4) pair of hyaline bodies (sensu Suárez-Morales 2018) between bases of antennules and medial keel (mk in Fig. 26C ).

Urosome 28% of total body length. Urosome comprising fifth pedigerous somite, genital somite, one free somite, preanal, and anal somites, the latter holding pair of caudal rami; relative length of urosomites, from proximal to distal: 38.3: 20.0: 24.0: 9.3: 8.3 = 100. Fifth pedigerous somite with straight lateral margins, with pair of small ventral buds reminiscent of fifth legs ( Figs. 26D , 27G ). Genital somite carrying genital complex. Succeeding preanal and anal somites smooth; anal somite carrying caudal rami.

Genital complex ventrally on genital somite ( Fig. 27G ), complex comprising short thick shaft with smooth lateral margins, with shaft distally branching into pair of short, thumb-like symmetrical, slightly divergent genital lappets; lappets weakly curved inwards ( Figs 27E , 28C ), reaching posterior margin of succeeding urosomite, with inner margins coarsely corrugate, medially joined, and pair of unguiform opercular flaps ( Fig. 28C , of in Fig. 27E ); lappets distal margin rounded, smooth. Caudal rami armed with 5 caudal setae (setae I–V) subequal in length and width ( Figs. 26D , 27F ).

Antennules 0.58 mm long, almost 36% of total body length, 5-segmented, segments 1–5 clearly divided, segment 4 longest ( Figs. 26A, B ). Following nomenclature by Grygier & Ohtsuka (1995), first segment with short, spiniform element 1, second segment carrying stout, spiniform elements 2d 1,2 and 2v 1–3 and long, lightly setulated dorsal seta IId, third segment with smooth setiform element 3 and slightly setiform elements IIIv and IIId, fourth segment with reduced armature including proximal elements IVd, 4d 1,2 and 4v 1,2, distal half of segment with straight margins, unarmed ( Fig. 26A, B ). Following Huys et al.’s (2007) nomenclature for the setation of the male antennulary fifth segment, inner margin with short, slender, lightly setulated seta A at midlength of inner margin, next to subdistal setae B and C, inner setae unbranched; outer margin with short, slender seta 7, followed by simple setae 6, 5, 4, 3 and short aesthetasc AE 1 in subdistal position ( Fig. 26A ). Antennules damaged, last segment broken off in both sides ( Fig. 28 A, B, D ), but recovered to describe the fifth segment armature and structure; the uneven distal outer margin of the fourth segment (indicated with arrows in Figs. 26A , 28A, B ) continues in the fifth segment (arrowheads in Fig. 26A ). Swimming legs 1–4 as in M. sekiguchii sp. nov.

Fifth pedigerous somite with pair of ventral bud-like appendages likely representing reduced male fifth legs (arrowed in Fig. 27G ).

Etymology. The species name is a masculine genitive eponym honouring our colleague Dr. Chad Walter (National Museum of Natural History, Smithsonian Institution), for his long-standing contributions and efforts to promote the knowledge of copepods through the development of accurate, reliable, and complete databases.

Remarks. The preoral structures and integumental ornamentations of the Monstrilloida are usually ignored or overlooked, but they frequently become valuable, distinctive characters to recognize species (see Suárez-Morales & Gasca 1992). Males of Monstrilla species usually show more subtle preoral ornamentations or processes than females. Preoral processes comparable to the medial keel observed in M. walteri sp. nov., have been observed in several congeners like the Brazilian M. bahiana Suárez-Morales & Dias, 2001 , with strongly chitinized nipple-like elements located ventrally anterior to the oral cone. The Caribbean M. barbata Suárez-Morales & Gasca, 1992 has a distinctive preoral medial keel with coarse crenulate margins. A strong cephalic process is also present in two other species, namely M. pustulata from Brazil, and M. brevicornis from Bankga Strait, Java ( Suárez-Morales & Dias 2001). In these two species the cephalic process is prominent and conical, even more distinct in M. pustulata ( Suárez-Morales & Dias 2001, fig. 1B, C) than in M. brevicornis , a species originally described by Isaac (1974), who also depicted this process and described it as a tubercle. Its presence was confirmed when the holotype was reexamined ( Suárez-Morales & Dias, 2001, fig. 3B, C). A relatively weak process represented by a preoral swelling which is not particularly structured is found in the female of the Brazilian M. careli Suárez-Morales & Dias, 2000 , but in this case it is only a preoral swelling ( Suárez-Morales & Dias 2001 fig. 1B). A similar structure is found in the Caribbean M. ciqroi (Suárez-Morales, 1993) as a low protuberance (Suárez-Morales, 1993, fig. 3a). A low, wide-based rounded protruding process was reported in the male of the Caribbean M. chetumalensis Suárez-Morales & Castellanos, 2019 ( Suárez-Morales & Castellanos 2019, fig. 2B), differing from the subtriangular, keel-like process observed in M. walteri sp. nov. These species also differ in the antennule and in the structure, armature and ornamentation of the genital complex. Preoral protuberant processes are also present in M. globosa Suárez-Morales, 2003 (smaller, rounded integumental swelling covered by longitudinal wrinkles ( Suárez-Morales 2003, figs. 15, 20) and the male of the Korean species M. ilhoii with a medial bulging process covered by integumental wrinkles on the ventral surface between the antennules ( Lee & Chang 2016, figs. 4B, C, 5A). Monstrilla walteri sp. nov. has strongly developed nipple-like processes ornamented with 2 or 3 apical sensilla ( Fig. 22A , nlps in Fig. 23 B–D ). Strongly chitinized nipple-like processes were also found in the Brazilian M. bahiana ( Suárez-Morales & Dias 2001, fig. 17). Additionally, Suárez-Morales (2001) reported a low medial preoral protuberance in the Indo-Malayan species M. inserta Scott, A., 1909 , resembling the processes found in M. careli and in the male of M. globosa (Suárez-Morales, 2001, figs. 14, 17), but in M. inserta the medial process is variable and is part of a longitudinal row of similar globose integumental processes found in this species (Suárez-Morales, 2001, figs. 27–29). Considering the wide variation in development and shapes of preoral processes reported among species of Monstrilla , the presence of a keel-like process in M. walteri sp. nov. in combination with other cephalic characters like: (1) the presence of enlarged, sensilla-carrying nipple-like processes, and (2) crescent-shaped integumental processes form distinctive features to identify the species.

The type of male genital complex and the shape and size of the lappets are also distinctive amongst species. These characters, together with the presence or absence of fifth legs or fifth legs buds are taxonomically valuable to distinguish males of Monstrilla ( Suárez-Morales & Castellanos 2019) . Considering these comparative characters, the only Australian Monstrilla males with paired fifth legs buds are M. walteri sp. nov. and M. mammillata sp. nov. (description to follow), distinguishing them from the other Australian males i.e. M. longibrachiata sp. nov., M. pileata sp. nov. and M. sekiguchii sp. nov., all lacking fifth legs buds. Additionally, M. walteri sp. nov. is unique in possessing an indented outer margin of the fourth and fifth antennulary segments and a peculiar integumental ornamentation of the preoral ventral surface.

Figures

FIGURE 22. Monstrilla sekiguchii sp. nov., adult male holotype from Western Port Bay, Victoria, Australia. A) habitus, dorsal view; B) same, ventral view; C) same, lateral view.

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 26. Monstrilla walteri sp. nov., adult male holotype from Port Phillip Bay, Victoria, Australia. A) left antennule showing setation pattern with nomenclature of segments I–IV following Grygier & Ohtsuka (1995) and Huys et al. (2007) for fifth segment; B) right antennule, dorsal view; C) cephalic structures showing hyaline body (hb), medial keel-like process (mk), protuberant nipple-like processes (nlp), and oral cone (oc), lateral view; D) urosome showing fifth legs buds on fifth pedigerous somite (arrow), genital complex, and caudal rami with caudal setae I–V.

FIGURE 27. Monstrilla walteri sp. nov., adult male holotype from Port Phillip Bay, Victoria, Australia. A) protuberant nipplelike process with apical sensillum (nlps); B) additional cephalic structures, medial keel process (mk), paired protuberant nipplelike processes (nlps) and dorsal rows of spinules, and oral cone (oc); C) integumental ornamentation of preoral surface, lateral view; D) additional preoral surface ornamentation with crescent-shaped processes or scars (csp), ventral view; E) genital complex showing lappets (L) and opercular flaps (of), ventral view; F) urosome and caudal rami showing caudal setae I–V, dorsal view; G) detail of fifth pedigerous somite showing small fifth leg bud (arrow), lateral view.

FIGURE 28. Monstrilla walteri sp. nov., adult male holotype from Port Phillip Bay, Victoria, Australia. A) left antennule showing ridged outer distal margin on fourth segment (arrows), dorsal view; B) right antennule with corrugate outer margin (arrows), dorsal view; C) genital complex (GC) showing genital lappets (L), ventral view; D) antennules lacking fifth segment, dorsal view.