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

Monstrilla mammillata sp. nov.

urn:lsid:zoobank.org:act:75C58B78-2683-49B4-97B3-B7C7BE8DDE84

( Figs. 32 View FIGURE 32 , 33 View FIGURE 33 )

Material examined.— Adult male holotype, undissected, mounted on slide in glycerine, (ECO-CHZ-12534).

Type locality. Hamelin Pool Channel, Shark Bay , Western Australia (26°19.770’ S, 114°03.354’ E), coll. on 17 June 1983 GoogleMaps .

Diagnosis. Small (total body length ≈ 1 mm); cephalothorax relatively short, slender, about half of total body length; forehead flat in ventral view, weakly projected anteriorly, ornamented with integumental ridges and field of wrinkles. Urosome relatively short, robust, about 1/3 of total body length. Oral cone moderately protuberant; ornamentation of the preoral ventral surface comprising pair of medial bulging protuberance, pair of nipple-like integumental processes, and adjacent field of deep integumental wrinkles.Antennules 5-segmented; fifth antennulary segment geniculate, with simple setae only and apical oval element 1 (sensu Huys et al. 2007). Fifth pedigerous somite with straight lateral margins, ventral surface carrying pair or short buds representing reduced fifth legs. Genital somite with transverse striae on dorsal and lateral surfaces, genital complex comprising wide based, short shaft and pair of thick, mammiform, divergent genital lappets with apical spermatophore spines; inner and outer margins of lappets smooth, medially connected by low, smooth rounded process. Caudal rami armed with 6 caudal setae subequal in length and width.

Description of holotype. Body relatively slender, small. Total body length 1.14 mm in dorsal view. Cephalothorax almost 45% of total body length, with flat forehead in dorsal and ventral position, forehead lightly corrugate ( Fig. 32A, B View FIGURE 32 ). Eyes comprising large medial cup (mec in Fig. 32C View FIGURE 32 ) and smaller lateral cups (lec in Fig. 32C View FIGURE 32 ). Integumental ornamentation of preoral ventral surface comprising: (1) medial bulging process (mbp in Fig. 32 A, C View FIGURE 32 ), (2) pair of nipple-like processes (nlp in Fig. 32A View FIGURE 32 ), and (3) dense field of integumental wrinkles ( Fig. 32A View FIGURE 32 ). Fused first pedigerous somite with posterodistal corners weakly expanded, reaching proximal 1/3 of succeeding second pedigerous somite. Oral cone moderately protuberant, robust, with constricted base and adjacent integumental wrinkles (oc in Figs. 31A, C View FIGURE 31 ). Urosome relatively short, 26% 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 as: 28.6: 22.8: 18.6: 15.7: 14.3: = 100. Fifth pedigerous somite longest, with straight lateral margins, ventral surface smooth except for pair of small buds on ventral posterior surface (arrow in Fig. 32E View FIGURE 32 ). Succeeding genital somite carrying genital complex. Preanal and anal somites smooth; anal somite carrying caudal rami.

Genital complex ventrally on genital somite ( Fig. 32E View FIGURE 32 ), complex comprising posteriorly directed short thick shaft with smooth lateral and anterior margins; shaft branching into pair of long, symmetrical mammiform genital lappets, latter curved backwards in lateral view ( Fig. 32E View FIGURE 32 ), divergent in ventral view ( Figs. 32D View FIGURE 32 , 33C, D View FIGURE 33 ), and reaching posterior margin of preanal somite ( Fig. 33C, D View FIGURE 33 ), with inner margins smooth, medially conjoined ( Fig. 33D View FIGURE 33 ), connected by small rounded convex process, with acute spermatophore spines on apical position (ss in Figs. 32F View FIGURE 32 , 33C, D View FIGURE 33 ). Caudal rami armed with 6 caudal setae (setae I–VI) subequal in length and width ( Fig. 32B View FIGURE 32 ).

Antennules 0.39 mm long, less than 40% of total body length, 5-segmented, segments 1–5 clearly divided, segment 4 longest ( Fig. 32B View FIGURE 32 ), first segment shortest ( Fig. 32B View FIGURE 32 ). Following nomenclature by Grygier & Ohtsuka (1995), first segment with strongly developed element 1, element long, setulated, almost reaching distal end of third segment ( Fig. 32B View FIGURE 32 , arrow in Fig. 32A, C View FIGURE 32 ), second segment carrying short setiform elements 2v 1,2 and 2d 2, this element modified, with thickened proximal half (2d 2 *in Fig. 32B View FIGURE 32 ), and long, lightly setulated dorsal seta IId, third segment with slender, slightly curved spiniform element 3 and lightly setulated setiform elements IIIv and IIId, fourth segment longest, with armature including proximal elements 4v 1,2, 4d 1,2 and IVd ( Fig. 32B View FIGURE 32 ), distal half with outer margin expanded; fourth and fifth segments connected by bilobed neck (arrowhead in Figs. 32B View FIGURE 32 , 33A View FIGURE 33 ); fifth segment geniculate ( Fig. 33A View FIGURE 33 ). Following Huys et al.’s (2007) nomenclature for the setation of the male antennulary fifth segment, inner margin with short, slender seta A on subdistal inner margin, next to subdistal setae B and C; inner setae unbranched; outer margin with short, slender seta 5, followed by simple setae 4, 3, 2, and apical oval element 1 (black arrowhead in Fig. 33A View FIGURE 33 ), and short aesthetasc AE 1 (LLE in Fig. 32B View FIGURE 32 ).

Swimming legs 1–4 as in M. sekiguchi sp. nov.

Etymology. The specific epithet is derived from the nominal Latin term mamilla = breast, nipple; the adjective suffix - ata was added to denote possession of. The epithet refers to the shape of the genital lappets of the new species, including the distal nipple-like spermatophore spine. Gender is feminine.

Remarks. There are only two other species of this genus exhibiting a comparable set of genital lappets, i.e. the Caribbean M. chetumalensis and the Korean M ilhoii . Both share with M. mammillata sp. nov. the following characters of the male genitalia: (1) very short genital complex shaft, (2) distally tapering, widely divergent genital lappets, (3) spermatophore spines present, and (4) poorly ornamented inner medial margin. There are, however, several morphological details by which these species can be distinguished. In M. ilhoii , the spermatophore spines are very small and inconspicuous ( Lee & Chang 2016, fig. 4E, 5D) and clearly longer and thicker in M. chetumalensis ( Suárez-Morales & Castellanos 2019, fig. 2E) than in M. mammillata sp. nov. The latter two species differ in the ornamentation of the inner medial margin connecting both lappets; it is deeply corrugate and concave in M. chetumalensis ( Suárez-Morales & Castellanos 2019, fig. 2E) vs. a smooth convex margin in M. mammillata sp. nov. ( Fig. 31 D View FIGURE 31 ). Also, M. ilhoii has well-defined opercular flaps ( Lee & Chang 2016, fig. 5D), but these structures are not observable in M. chetumalensis ( Suárez-Morales & Castellanos, 2019, fig. 2E) or in M. mammillata sp. nov.

These three species, however, can also be separated by other characters, including the number of caudal setae (only 4 setae are present in M. chetumalensis ( Suárez-Morales & Castellanos 2019, fig. 2A) vs. 6 caudal setae in both M. ilhoii ( Lee & Chang 2016) and M. mammillata sp. nov. (see Fig. 32E View FIGURE 32 )).

It is clear that there is more morphological resemblance between the Korean M. ilhoii and the Australian M. mammillata which share several important characters: (1) the presence of a medial cephalic bulging protuberance on the preoral ventral surface ( Lee & Chang 2016, figs. 4B, C, 5A), (2) a long, well-developed antennulary setal element 1 (sensu Grygier & Ohtsuka 1995) ( Lee & Chang 2016, fig. 5C), (3) a modified, proximally thickened element 2d 2 on the second antennulary segment ( Lee & Chang 2016, fig. 5C), and (4) an apical leaf-like element 1 (sensu Huys et al. 2007) on the fifth antennulary segment. These species, however, can be easily separated by other important details, namely the size of the spermatophore spines (already discussed), the polygonal reticulation on the body surface of M. ilhoii vs. the non-reticulated body in M. mammillata sp. nov., the presence of male fifth leg buds in M. mammillata sp. nov. vs. fifth legs vestiges entirely lacking in the male of M. ilhoii ( Lee & Chang 2016, fig.4D), and their body size ( M. mammillata is a small species (male 1.14 mm) and the male of M. ilhoii is about three times larger (2.76 mm) ( Lee & Chang 2016).