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

Monstrilla caromiobenelloides sp. nov.

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( Figs 45–47 View FIGURE 45 View FIGURE 46 View FIGURE 47 )

Material examined. Adult male holotype mounted on slide in glycerine, (ECO-CHZ-12540). Type locality. Scott Reef , Western Australia (- 14°2.500’ S, 121°52.800’ E), coll. on 6 June 2009 GoogleMaps .

Diagnosis. Male monstrilloid with cephalothorax relatively short, robust, about half of total body length; anterior margin with weakly produced, smooth forehead. Urosome relatively short, about 1/4 of total body length. Oral cone weakly protuberant, surrounded by field of transverse integumental wrinkles. Preoral surface with low medial protuberance visible in lateral view. Antennules 5-segmented, geniculate between fourth and fifth segments; fourth and fifth segments equally long; fifth segment with slender, simple setae on outer margin and with apical longitudinal rows of spinules and minute spinule integumental patch. Fifth pedigerous somite with ventral swelling and reduced pair of buds representing fifth legs. Genital somite carrying short genital complex with thick short shaft and pair of robust, weakly divergent genital lappets medially connected by deep invagination; lappets ornamented with spinule rows on inner margin and dorsal surface. Caudal rami armed with 5 subequally long caudal setae; setae III and IV unmodified.

Description of holotype. Body relatively robust, with short cephalothorax. Total body length 1.31 mm in dorsal view. Cephalothorax almost 50% of total body length ( Figs. 45A View FIGURE 45 , 46A View FIGURE 46 ), thick, anteriorly rounded, flat forehead ( Fig. 46A, B View FIGURE 46 ) with few integumental wrinkles and pair of hyaline bodies (sensu Suárez-Morales 2018) adjacent to lateral and medial eye cups (hb in Fig. 46A View FIGURE 46 ). Preoral anteroventral surface with field of integumental wrinkles, and three adjacent pairs of nipple-like processes on ventral surface (nlp 1-3 in Figs. 45 B View FIGURE 45 , 47D View FIGURE 47 ). Oral cone weakly protuberant, surrounded by field of transverse integumental wrinkles (oc in Figs. 45A View FIGURE 45 , 46B View FIGURE 46 , 47D View FIGURE 47 ); cone located at 30% of way back along ventral surface of cephalothorax ( Fig. 45A View FIGURE 45 ). Eyes pigmented, represented by two lateral cups and medial cup at anterior end of cephalothorax; medial cup slightly larger than lateral cups (mec, lec in Fig. 46A View FIGURE 46 ).

Urosome relatively slender ( Fig. 45E, D View FIGURE 45 ), almost 27% of total body length, comprising fifth pedigerous somite, genital somite, one free somite, preanal and anal somite, the latter carrying pair of caudal rami; relative length of urosomites, from proximal to distal: 28.5: 24.3: 17.1: 17.2: 12.9 = 100. Fifth pedigerous somite longest, with straight lateral margins and bulging ventral process ( Fig. 45E View FIGURE 45 ). Genital somite with genital complex on ventral surface. Succeeding preanal and anal somites with smooth lateral and dorsal surfaces; anal somite carrying caudal rami. Fifth pedigerous somite with ventral expansion carrying pair of poorly defined fifth legs buds (arrowhead in Fig. 45E View FIGURE 45 ). Genital complex arising from ventral surface of genital somite ( Fig. 45D, E View FIGURE 45 ); complex compact in ventral view, with straight lateral margins and short thick shaft branching distally into pair of short, symmetrical genital lappets, weakly divergent in ventral view ( Figs. 45D View FIGURE 45 , 46E View FIGURE 46 , 47B View FIGURE 47 ) and medially conjoined by wide A-shaped slit ( Figs. 46E View FIGURE 46 , 47B View FIGURE 47 ); lappets ornamented distally with spinule rows along inner margin ( Figs. 45E View FIGURE 45 , 46E View FIGURE 46 , 47B View FIGURE 47 ). Caudal rami armed with 5 caudal setae subequal in length and width ( Fig. 47C View FIGURE 47 ); setae I and II broken off in holotype but sockets indicate insertion points ( Fig. 47C View FIGURE 47 ).

Antennules 0.49 mm long, almost 37% of total body length; 5-segmented; segments 1–5 clearly divided, segments 4 and 5 longest ( Figs 45C View FIGURE 45 , 46C View FIGURE 46 , 47A View FIGURE 47 ). Following nomenclature by Grygier & Ohtsuka (1995), first segment with long, spiniform element 1 reaching distal margin of third segment ( Fig. 46B View FIGURE 46 ), second segment carrying spiniform elements 2d 1,2 and 2v 1,2, the former (2d 1) remarkably long, almost reaching distal end of fifth segment (asterisk in Figs. 46C View FIGURE 46 , 47A View FIGURE 47 ), second segment also carrying lightly setulated dorsal seta IId, third segment with long, setiform element 3 and lightly setulated setiform elements IIIv and IIId, fourth segment with short, spiniform elements 4d 1,2 and 4v 1,3 and aesthetasc 4aes on proximal half, distal half unarmed ( Fig. 46C View FIGURE 46 ). Following Huys et al.’s (2007) nomenclature for the setation of the male antennulary fifth segment, outer margin with minute aesthetascs AE, slender unbranched elements 2–7, and spiniform apical elements 1 and 2 ( Fig. 46C, D View FIGURE 46 ), inner margin with short uniserially pinnate element B and slender, unbranched seta C on inner subdistal position; distal 1/3 of fifth segment ornamented with subapical patch of minute spinules on ventral inner margin ( Fig. 46D View FIGURE 46 ) and adjacent inverted Ushaped row of larger spinules visible also in dorsal view ( Fig. 46D View FIGURE 46 , arrowheads in Fig. 47A View FIGURE 47 ).

Swimming legs 1–4 as in M. sekiguchii sp. nov. and M. huysi sp. nov. 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 derived from the monstrilloid genus name Caromiobenella ending with the Greek suffix oides meaning similar or looking alike. Both the root Caromiobenella and the ending oides are feminine (see ICZN art. 30. 1.2) and thus agree in gender with the genus name. A different criterion classifies oides as neuter, but it is applied for Prokaryotes ( Parker et al. 2015). The species epithet refers to the resemblance of the antennulary ornamentation observed in this species of Monstrilla with that known in Caromiobenella .

Remarks. The new species M. caromiobenelloides was assigned to the genus Monstrilla despite several affinities with Caromiobenella . Firstly, the fifth antennulary segment is not modified as in members of Caromiobenella , which have 5 transverse serrated ridges consisting of numerous minute spinules (Jeon et al. 2018). In M. caromiobenelloides sp. nov. the antennulary ornamentation is relatively weak, with a single longitudinal spinule row, and not arranged in transverse ridges which is typical in species of Caromiobenella . Furthermore, M. caromiobenelloides sp. nov. shows a unique patch of minute spinules on the apical surface of the fifth segment, a character not reported in members of Caromiobenella . Secondly, a fifth leg or traces of it is absent in males of Caromiobenella species (Jeon et al. 2018) and M. caromiobenelloides sp. nov., like at least two other males of Monstrilla , has a pair of fifth leg buds on the ventral surface of the fifth pedigerous somite. Additionally, M. caromiobenelloides lacks the typical crater-like integumental processes on the dorsal surface of the cephalothorax that are distinctive of Caromiobenella .

This species of Monstrilla exhibits characters suggestive of clear affinities with members of Caromiobenella , i.e. the structure of the genitalia, mainly including the shape and arrangement of the genital lappets. Monstrilla caromiobenelloides’ genital complex closely resembles that of Caromiobenella castorea (Jeon et al. 2018, fig. 2D) except for the presence of a deeper slit in the new species. As discussed for C. jeoni sp. nov., Jeon et al. (2019) recognized two distinct groups of species based on the type of genital complex, one with a deep medial notch on the posterodistal position (type I) and the other with a homologous medial distal protrusion (type II) (see Jeon et al. 2019). The genital complex of C. jeoni was thus classified as type I, with a distinctive medial slit, a character clearly present in M. caromiobenelloides sp. nov. as well.

The antennulary setation pattern found in M. caromiobenelloides sp. nov. shows additional similarities with the pattern of Caromiobenella , including the strong development of the setal elements 1 and 2d 1,2 (sensu Grygier & Ohtsuka 1995). In C. castorea , the type species of Caromiobenella , element 1 is strongly developed, represented by a stout seta almost reaching the distal margin of the succeeding second segment (Jeon et al. 2018, fig. 2B). This element is also well-developed in the new species, and even longer, almost reaching the distal margin of the third antennulary segment. Element 2d 2 is strongly developed in species of Caromiobenella , and described as a stout, biserially setulate seta in C. castorea and C. polluxea , reaching the distal margin of the fourth antennulary segment (Jeon et al. 2018, figs. 3A, 7A). This element is also stout and even longer in the new species M. caromiobenelloides sp. nov., almost reaching the fifth segment’s distal end. An elongate setulated element 2d 2 has been reported widely among species of Caromiobenella , including females of C. helgolandica ( Park 1967) and males of the C. pygmaea (Suárez-Morales 2000) . Similar observations were reported for males of C. helgolandica ( McAlice 1985) , C. serricornis ( McAlice 1985) , and C. arctica ( Davis & Green 1974) , but some of these reports lack detailed data on the type of element and its attributes, or can be differently interpreted by its position, like in C. patagonica males ( Suárez-Morales et al. 2008), in which this seta was reported as seta 2d 1 due to its insertion point relative to other elements. As emphasized by Jeon et al. (2018), the variability of the setal armature details in these reports does not allow to determine if there is a sexual dimorphism pattern applying to the attributes of these elements. Based on these shared characters, we decided to emphasize the resemblance of the new species with members of Caromiobenella . Pending a deeper, detailed phylogenetic analysis of the genus, it is suggested that M. caromiobenelloides sp. nov. or a related species could potentially be the intermediate taxon linking Monstrilla and Caromiobenella , whose ancestor likely shares several of these characters.