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

Monstrilla pileata sp. nov.

urn:lsid:zoobank.org:act:CECA433E-4E9C-4E50-A1A1-57929702252B

( Figs 8–13 View FIGURE 8 View FIGURE 9 View FIGURE 10 View FIGURE 11 View FIGURE 12 View FIGURE 13 )

Material examined. Adult female holotype, undissected, mounted on slide in glycerine, (ECO-CHZ-12535). Adult female paratype undissected, mounted on slide in glycerine. Adult male allotype undissected, mounted on slide in glycerine, (ECO-CHZ-12526).

Type locality. Werribee , Port Phillip Bay, Victoria, Australia (37°57.085’ S, 144°47.128’ E), coll. on 20 March 1985 GoogleMaps .

Diagnosis. Medium sized (2 mm) female with elongate, cylindrical cephalothorax about 60% of total body length, oral cone prominent, located ventrally at anterior 1/3 of cephalothorax. Urosome relatively short, 20% of total body length. Antennules 4-segmented, 32% of total body length, anteriorly directed, straight or slightly curved inwards, divergent in some cases; segments 1–2 divided, segments 3–4 fused; distal segment with thick chitinous hyaline process covering the apical and subdistal margins of segment, process of variable thickness along subdistal margins, apical process sometimes forceps-like. Genital double-somite with rounded medial anteroventral protuberance of variable size, somite carrying pair of ovigerous spines barely reaching beyond distal margin of caudal rami. Fifth legs bilobed, outer exopodal lobe thick, subrectangular, armed with three subequally long setae; inner endopodal lobe unarmed, thumb-like or globose, reaching middle of inner margin of exopodal lobe. Caudal rami subrectangular, twice as long as wide, armed with five setae, apical setae III and IV proximally expanded. Male with slender body, cephalothorax about 50% of total body length; male genital complex with short thick shaft and smooth lateral margins; shaft branching into pair of digitiform, slightly asymmetrical, divergent genital lappets.

Description of adult female holotype. Body length of holotype 2.08 mm. Cephalothorax long, cylindrical, moderately robust with moderately expanded lateral margins ( Fig. 10A View FIGURE 10 ), about 58% of total body length. Oral cone strongly prominent, cylindrical (oc in Figs 8B View FIGURE 8 , 10D View FIGURE 10 ), located 15% of way back along ventral surface of cephalothorax. Cephalic region anteriorly subquadrate in dorsal view, ‘forehead’ weakly produced, smooth, with field of transverse integumental wrinkles between antennule bases ( Fig. 8B View FIGURE 8 ); ventral preoral surface with reduced ornamentation, including single pair of nipple-like processes (nlp in Figs. 8B View FIGURE 8 , 10D View FIGURE 10 ) and medial perioral pore (ppo in Fig. 10D View FIGURE 10 ). Eyes comprising two lateral cups and medial ventral cup (lec, hb in Fig. 9A View FIGURE 9 ); small hyaline bodies (sensu Suárez-Morales 2018) located anteriorly to eye cups.

Urosome consisting of four somites i.e. fifth pedigerous somite (with fifth legs), genital double-somite (ventrally carrying paired ovigerous spines reaching middle of caudal rami), free preanal somite, and anal somite carrying pair of caudal rami; length ratio of urosomites (from proximal to distal) 36.1: 36.5: 14.1: 13.3 = 100 ( Fig. 8A View FIGURE 8 ). Genital double-somite with weakly expanded lateral margins, with incomplete transverse suture visible in dorsal and lateral view ( Fig. 8A View FIGURE 8 ); pair of relatively short ovigerous spines on ventral surface, carrying eggs mass distally ( Figs 8A View FIGURE 8 , 10A View FIGURE 10 ). Caudal rami subrectangular ( Fig. 8A View FIGURE 8 ), 1.4 times as long as broad, each armed with five caudal setae I–V. Distalmost setae III and V proximally expanded (asterisks in Fig. 8A View FIGURE 8 ).

Antennules ( Figs 8B View FIGURE 8 , 9A–C View FIGURE 9 , 10A, B View FIGURE 10 ) 0.66 mm in length, about 32% of total body length and almost 55% of cephalothorax length ( Fig. 10A View FIGURE 10 ); distinctly 4-segmented as usual in female monstrilloids, anteriorly directed, not completely straight but slightly curved inwards ( Fig. 10A View FIGURE 10 ); segments 1–3 divided, segments 3–4 partly fused ( Figs 8B View FIGURE 8 , 9A, C View FIGURE 9 ); length ratio of antennular segments (proximal to distal) 13.3: 16.6: 20.0: 50.1 = 100 ( Fig. 9A, C View FIGURE 9 ). Following Grygier and Ohtsuka’s (1995) setal nomenclature for antennules, first segment with reduced, slender setal element 1, second segment bearing reduced setiform element IId, and slender setiform elements 2d 1,2, and 2v 2; third segment with setiform, lightly setulated element 3 and adjacent setiform elements IIId and IIIv, fourth segment separated from third by poorly-defined suture, armed with setal elements IVd, 4v 3, 5, and Vv, aesthetasc 4aes present in medial position, subapical “ b ” setal group comprising two branched setal elements b 1 and b 2 inserted on outer margin, usual apical elements 6 1,2 and 6aes not clearly observable, except for part of aesthetasc 6aes ( Figs 9C View FIGURE 9 , 10A View FIGURE 10 ), with distinctive chitinous thickening resembling an apical helmet covering the subdistal and apical margins of antennules ( Fig. 9B View FIGURE 9 ), helmet thickest apically on both antennules ( Figs 9B, C View FIGURE 9 , 10A View FIGURE 10 ).

First pedigerous somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs ( Fig. 10A View FIGURE 10 ), all with exopodite longer than endopodite. Setal armature pattern as in M. elongata (see

Suárez-Morales, 2001b). All natatory setae lightly and biserially plumose. Outer margin of outer apical exopodal seta of legs 1–4 smooth. 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

Fifth legs ( Fig. 8A View FIGURE 8 ) well-developed, bilobed. Outer lobe subrectangular, armed distally with three subequally long lightly setulated setae. Inner lobe wide, globose, reaching halfway of outer lobe inner margin; inner lobe unarmed.

Intraspecific variability of females

Description of adult female paratype. Body length of paratype 2.05 mm. Cephalothorax as in holotype except for exhibiting corrugate flat forehead instead of smooth, weakly produced forehead as in holotype ( Fig. 11C View FIGURE 11 ). Ventral preoral surface as in holotype, paratype with tighter integumental wrinkles pattern, higher medial bulging protuberance (mcp in Fig. 11B View FIGURE 11 ), and single pair of nipple-like processes ( Figs 11B View FIGURE 11 , 13B View FIGURE 13 ).

Antennules as in holotype, except for lack of curved position, paratype antennules straight, divergent ( Fig. 11C View FIGURE 11 ) and with division between segments 3 and 4 represented by weak constriction ( Fig. 11C View FIGURE 11 ), segment 4 longest. Following Grygier and Ohtsuka’s (1995) setal nomenclature, paratype setation as in holotype except for more complete setation in segment two (with elements 2v 1–3, 2d 1,2, IId), third segment with setiform, lightly setulated element 3 and adjacent setiform elements IIId and IIIv, fourth segment separated from third by complete intersegmental division, armed with setal elements IVd, IVv, 4d 1,2, 4v 1,2, Vm, Vd, and Vv, slender aesthetasc 4aes present in medial position, subapical “b” setal group comprising single unbranched setal element b 3 on outer margin, thus differing from the holotype having two branched “b” group setae, usual apical elements 6 1,2 not clearly observable in paratype, except for subapically inserted aesthetasc 6aes ( Fig. 11C View FIGURE 11 ); fourth segment exhibiting distinctive integumental thickening of subdistal and apical margins, but showing a weaker development, with process being narrower subdistally and thicker apically, forming a thick high element resembling a short, robust forceps ( Fig. 11D, E View FIGURE 11 , arrow in Fig. 13A View FIGURE 13 ). Swimming legs 1–4 as in holotype, with intercoxal sclerites with transverse wrinkles, that of leg 1 being smallest ( Fig. 12A View FIGURE 12 ).

Urosome as in holotype ( Fig. 12B, C View FIGURE 12 ), but paratype with integumental wrinkles on the dorsal surface of genital double-somite ( Fig. 12B View FIGURE 12 ). Fifth legs endopodal lobes thumb-like, not globose ( Fig. 12C View FIGURE 12 ). Genital double-somite with anteroventral protuberance (arrow in Fig. 12B View FIGURE 12 ); somite with weakly expanded lateral margins, with incomplete transverse suture visible in dorsal and lateral views ( Fig. 12B View FIGURE 12 ); pair of relatively short, slender ovigerous spines on ventral surface. Caudal rami subrectangular in dorsal view ( Fig. 2B View FIGURE 2 ), rami twice as long as broad, each armed with six caudal setae I– VI in the paratype ( Fig. 12D View FIGURE 12 ). Distalmost caudal setae III and IV with weak proximal expansion (asterisks in Figs 12D View FIGURE 12 , 13D View FIGURE 13 ) .

Description of male allotype. Body relatively slender. Total body length 1.63 mm in dorsal view. Cephalothorax almost 50% of total body length, anteriorly rounded, with moderately protuberant forehead ( Figs 14A View FIGURE 14 , 16C View FIGURE 16 ) with pair of hyaline bodies (sensu Suárez-Morales 2018, hb in Fig. 14B View FIGURE 14 ), with posterolateral expansions of incorporated first pediger reaching about half-length of succeeding second pedigerous somite ( Fig. 14A View FIGURE 14 ); preoral anterior surface with field of transverse integumental wrinkles and pair of nipple-like processes on ventral surface (nlp in Fig. 14B View FIGURE 14 ). Eyes represented by two strongly pigmented lateral cups and medial ventral cup at anterior ¼ of cephalothorax (mec, lec in Fig. 14B View FIGURE 14 ), medial cup larger than lateral cups, lateral cups rounded, slightly smaller than medial cup in dorsal view (mec, lec in Fig. 14B View FIGURE 14 ). Oral cone weakly produced, located at 30% of way back along ventral surface of cephalothorax (oc in Fig. 14B View FIGURE 14 ).

Urosome 24% of total body length; 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: 20.0: 20.3: 14.6: 11.4: 33.6 = 100. Fifth pedigerous somite with straight lateral margins, ventral surface with integumental wrinkles and reduced bud-like paired processes representing male fifth legs (arrowhead in Fig. 14C View FIGURE 14 ). Genital somite with genital complex. Anal somite longest ( Fig. 14A, C, D View FIGURE 14 ), carrying caudal rami. Genital complex arising from ventral surface of genital somite ( Fig. 14C, D View FIGURE 14 ), with posteriorly projected short, thick shaft with smooth lateral margins; shaft branching into pair of digitiform, slightly asymmetrical, divergent genital lappets; right lappet straight, reaching distal margin of anal somite ( Figs. 15C View FIGURE 15 , 16D View FIGURE 16 ), left lappet curved inwards, both with low inner swellings, and with smooth distal margins ( Figs. 15C View FIGURE 15 , 16D View FIGURE 16 ); medial opercular flap ( Fig. 15C View FIGURE 15 , of in Fig. 13D View FIGURE 13 ) present distally on ventral surface of complex covering genital opening. Caudal rami armed with 5 caudal setae (setae I–V) ( Fig. 15D View FIGURE 15 ); setae I–II and, V subequal in length and width; setae III and IV slightly shorter, with proximal swelling ( Figs. 14A View FIGURE 14 , 15C, D View FIGURE 15 , 16A View FIGURE 16 ).

Antennules 0.56 mm long, almost 35% of total body length, 5-segmented; segments 1–5 clearly divided; segment 4 longest ( Fig. 15A View FIGURE 15 ). Following antennule armature nomenclature by Grygier & Ohtsuka (1995), first segment with short, spiniform element 1, second segment carrying spiniform elements 2d 2 and 2v 1,2 and 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 uniserially spinulate elements 4v 1–3, with inner swelling on proximal half ( Fig. 15A View FIGURE 15 ). Following Huys et al.’s (2007) nomenclature for male antennulary setation, short, slender, unbranched seta 3 on outer margin, setae A–C on inner medial margin; apical elements 6 1, 6 2, and 6aes not observed ( Fig. 15A View FIGURE 15 ); distal margin of antennules with helmet-like chitinous thickening as described in female ( Figs. 15A View FIGURE 15 , 16B View FIGURE 16 ).

Swimming legs 1–4 as in female.

Etymology. The species name is derived from the Latin noun pileus, a conical hat used in Ancient Rome. It refers to the distal thickened hyaline chitinous process of the antennules, distinctive of this species. Gender is feminine.

Remarks. The new species M. pileata sp. nov., can be easily recognized by several distinctive characters, including: (1) the bilobed female fifth leg with the outer lobe carrying three setae and an unarmed, globose inner lobe. A similar fifth leg structure and armature is known only in females of M. brevicornis Isaac, 1974 from Indonesia, in which the outer lobe has three setae, and the inner lobe is smooth; it differs from the pattern observed in M. pileata sp. nov. by the shape and size of the endopodal lobe, which is reduced, beak-like in M. brevicornis ( Isaac 1974, fig. 1D) vs. globose, well-developed in M. pileata sp. nov. Isaac (1974) recognized this reduced spinous inner process as the endopodal lobe, but Suárez-Morales & Dias (2001) could not confirm the actual structure and shape of the inner lobe because the fifth leg of the holotype was damaged; (2) the presence of a helmet-like integumental process covering the tip of the antennules, a structure present in both the male and female of M. pileata sp. nov. that has not been described in any other congener or in other monstrilloids, (3) the symmetrically inward curvature of the antennules found in the female of M. pileata sp. nov. a unique character not previously reported in other monstrilloid copepods with only the right antennule of the Indonesian M. brevicornis depicted as having a similar pattern ( Isaac 1974, fig. 1A), (4) the proximally protuberant caudal setae III and IV is also a rare character among species of Monstrilla which has been reported previously in females of two species, the Korean M. ilhoii Lee & Chang, 2016 ( Lee & Chang 2016, fig. 1E), and the Caribbean M. xcalakensis Suárez-Morales, 2024 (Suárez-Morales 2024, fig. 2B, C). This character is also reported in M. janetgrieveae sp. nov., described later in this account of the Australian Monstrilla . In these cases, two terminal caudal setae (III, IV) are modified, so females of M. pileata sp. nov. can be easily distinguished from M. xcalakensis and M. ilhoii by the structure and armature of the fifth legs, the new species displaying a smooth inner lobe vs. an armed inner lobe in M. ilhoii ( Lee & Chang 2016, fig. 1E) and M. xcalakensis (Suárez-Morales 2024, fig. 2A). The differences found in the male and female M. pileata sp. nov. with respect to all other known species of Monstrilla lead to the proposal of a new species. Monstrilla pileata sp. nov. and M. janetgrieveae sp. nov. differ in the structure and ornamentation of the antennules, the latter lacking the helmet-like apical process on the antennules.

Matching both sexes of M. pileata sp. nov. One of the main challenges in the current study of monstrilloid copepods lies in reliably matching the sexes of a species ( Suárez-Morales 2011). The use of molecular tools seems to be the most effective and definitive method for matching male monstrilloids to their corresponding females. It has recently been applied in Monstrillopsis ( Jeon et al. 2018b) and Caromiobenella (Cruz Rosa et al. 2021) . However, comparative morphology provides some guidance, particularly when both sexes share distinctive autapomorphies, like a modified antennulary segment e.g. in C. brasiliensis (Cruz Rosa et al. 2021) . In our opinion, this is also applicable to males and females of the new species M. pileata sp. nov., as both sexes share the following distinctive characters: (1) the helmet-like integumental process on the tip of the antennules, and (2) the proximally swollen caudal setae III and IV. We consider these characters unique for the species and their combined presence in both male and female individuals seems to be reliable morphological evidence to present these specimens as the male and female of M. pileata sp. nov.