Sarsimonstrillus pseudantennullatus, 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 : 80-87

publication ID	https://doi.org/10.11646/zootaxa.5576.1.1
publication LSID	lsid:zoobank.org:pub:7A65A968-EF09-4F1A-AF0D-470BDF2EE1DD
persistent identifier	https://treatment.plazi.org/id/9D7DA92F-FFE2-853D-89AE-5B8D9552F8FC
treatment provided by	Plazi
scientific name	Sarsimonstrillus pseudantennullatus
status	gen. nov., sp. nov.

Treatment

Sarsimonstrillus pseudantennullatus gen. nov., sp. nov.

urn:lsid:zoobank.org:act:C7F7CF90-1EF9-446B-8A55-C0935F96B0D0

( Figs 50–54 View FIGURE 50 View FIGURE 51 View FIGURE 52 View FIGURE 53 View FIGURE 54 )

Material examined. Adult female holotype, undissected, mounted on slide in glycerine, (ECO-CHZ-12536).

Type locality. Port Phillip Bay , Black Rock, Victoria, Australia (37°58.066’ S, 145°.2705’ E), coll. on 17 March 1983 .

Diagnosis. Large (> 3.0 mm) female monstrilloid with robust cephalothorax widest at proximal half and tapering distally onto cephalic portion, cephalothorax about 66% of total body length; oral cone low, inconspicuous, located at anterior 30% of cephalothorax. Posterolateral corners of pedigers 2–4 weakly produced into rounded processes. Urosome relatively short, robust, 25% of total body length. Antennules 4-segmented, relatively short, almost 26% of total body length, anteriorly directed, moderately divergent in dorsal view; segments 1–4 divided; fourth segment longest, 52% of antennule, with reduced setal armature, apical and subdistal elements absent; anteroventral surface with strong triangular medial protuberance, anterodorsal surface carrying long, basally conjoined branched process appearing as a second pair of antennules; processes arising from common wide base tapering distally. Legs 1–4 with usual monstrilloid armature. Genital double-somite globose, with expanded lateral margins carrying pair of short ovigerous spines barely reaching beyond midlength of caudal rami; spines distally acute, curved. Fifth legs bilobed, exopodal lobe thick, subrectangular, armed with three subequally long setae; endopodal lobe thumb-like, with two setae, almost as long as exopodal lobe. Caudal rami subquadrate, armed with five caudal setae; apical setae III–V modified, proximally thickened.

Description of adult female holotype. Body length 3.11 mm. Body tagmosis as usual in females of Monstrilla . Cephalothorax long, robust with proximal half expanded, tapering distally onto narrow cephalic portion, about 65% of total body length and fully incorporating first pedigerous somite. Oral cone low, inconspicuous, located 15% of way back along ventral surface of cephalothorax (oc in Fig. 51C View FIGURE 51 ). Eyes comprising two lateral cups and medial ventral cup (mec, lec in Fig. 51D View FIGURE 51 ); relatively large hyaline bodies (sensu Suárez-Morales 2018) located anteriorly to eye cups (hb in Fig. 51D View FIGURE 51 ). Cephalic region highly modified, anteriorly projected in lateral and dorsal views, ‘forehead’ weakly produced, smooth, with field of transverse integumental wrinkles between antennule bases ( Fig. 50B View FIGURE 50 ); ventral preoral surface with reduced ornamentation, including strong medial keel-like subtriangular process (pmk in Figs 50B View FIGURE 50 , 51C View FIGURE 51 ) and field of transverse integumental wrinkles; anteriormost cephalic area carrying pair of long, wide-based, unsegmented, smooth processes tapering distally; processes arising from anteriormost dorsal surface, anteriorly directed, with inward curved distal half ( Figs. 50B View FIGURE 50 , 51C, D View FIGURE 51 , 53A, B View FIGURE 53 , 54C View FIGURE 54 ).

Antennules 0.77 mm long, about 25% of total body length; distinctly four-segmented, anteriorly directed, divergent ( Figs 50B View FIGURE 50 , 51D View FIGURE 51 , 53B View FIGURE 53 ), fourth segment largest, more than 50% of antennulary length ( Figs 50B View FIGURE 50 , 51A, C View FIGURE 51 , 53A View FIGURE 53 ). Length ratio of antennular segments (proximal to distal) 12.6: 12.6: 21.0: 53.8 = 100 ( Fig. 50B View FIGURE 50 ). Following Grygier and Ohtsuka’s (1995) setal nomenclature, first segment with strong setiform element 1 reaching about halfway of third segment ( Figs. 50B View FIGURE 50 , 51D View FIGURE 51 ); second segment bearing short, setiform lightly setulated dorsal element IId, and spiniform elements 2v 1–3, and 2d 1, 2, third segment with spiniform robust element 3 and adjacent setiform elements IIId and IIIv, fourth segment armed with setal elements 4v 1–3, Vm, Vd and Vv, subapical “b” setal group comprising three short, unbranched setal elements b 1, b 3 and b 5 inserted on outer margin ( Figs. 50A View FIGURE 50 , 51A View FIGURE 51 ); usual apical elements 6 1,2 and 6aes not observed, broken off in holotype ( Fig. 53A View FIGURE 53 ).

First pedigerous thoracic somite and succeeding three free thoracic somites each bearing well-developed pair of biramous swimming legs, all with exopodite longer than endopodite. Setal armature pattern as in M. elongata (see Suárez-Morales 2001). 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 lightly spinulose along outer margin, inner margin setulose. Third leg first endopodal segment with reduced fan-like seta on outer margin (arrow in Fig. 50C View FIGURE 50 ) 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

Urosome consisting of four somites: fifth pedigerous somite (with fifth legs), genital double-somite (ventrally carrying paired ovigerous spines reaching midlength of caudal rami), free preanal somite, and anal somite carrying pair of caudal rami. Length ratio of urosomites (from proximal to distal): 36.3: 47.5: 7.7: 8.5 = 100 ( Figs. 51E View FIGURE 51 , 52A View FIGURE 52 ). Fifth legs well-developed, bilobed; outer lobe subrectangular, armed distally with three subequally long lightly setulated setae; inner lobe thumb-like, reaching distal margin of outer lobe, armed with two equally long setae ( Fig. 52C View FIGURE 52 ). Genital double-somite with expanded lateral margins, with incomplete transverse suture visible in dorsal and lateral view ( Figs. 51E View FIGURE 51 , 52A View FIGURE 52 ), ornamented with pair of nipple-like integumental processes on dorsal surface (arrowheads in Fig. 52B View FIGURE 52 ); pair of slender, relatively short ovigerous spines on ventral surface (OS in Figs. 52A–C View FIGURE 52 , 53C View FIGURE 53 ). Anal somite with ridged dorsal surface ( Fig. 51 E View FIGURE 51 ). Caudal rami subrectangular in dorsal view ( Fig. 51E View FIGURE 51 ), 1.3 times as long as broad, each ramus armed with five caudal setae I–V; distalmost setae III, IV, and V thickened proximally ( Figs 51E View FIGURE 51 , 54D View FIGURE 54 ).

Etymology. The species name is a substantivated adjective using the Greek prefix pseudo (ψευδής) meaning “false”, and the substantive antenna in reference to the distinctive preantennulary paired processes mimicking a second pair of antennules. The masculine ending component “ us ” is added. The species name is masculine.

Remarks. Sarsimonstrillus pseudantennulatus gen. nov., sp. nov. can easily be distinguished from any other known monstrilloid by the conspicuous presence of paired appendages on the dorsal anteriormost area, next to the insertion of the antennules. Because of their position and size, these structures could give the impression of a second pair of antennules. In most crustaceans, including copepods, antennules are primarily sensory appendages and as such determine the individual relation with the environment, including feeding, mating, and detection of predators ( Boxshall & Jaume 2013). Antennules are the anteriormost paired limb on the head of arthropods, linked during the development with the deuterocerebral cephalic somite in the crustaceans. Its typical structure is monoaxial and in female copepods it is ancestrally 28-segmented ( Huys & Boxshall 1991). Antennules exhibit variant forms comprising a secondary axis, the flagellate patterns, either with terminal or intercalate annulation along the antennulary axis; the former type is present in malacostracan crustaceans and the latter in some crustacean larvae (Boxshall 2004). Malacostracans exhibit a biramous antennule including the flagellum which typically lacks an intrinsic musculature. The primitive Remipedia, discovered in caves of the Bahamas ( Yager 1981), have true biramous antennules, comprising dorsal and ventral rami and a basal peduncle. Only the main antennulary axis, the dorsal one, shows intrinsic musculature; the ventral ramus lacks any musculature, and its movement is produced by the peduncle musculature (Boxshall 2004).

The pair of preantennulary appendages found in S. pseudantennulatus gen. nov., sp. nov. cannot be interpreted as an antennulary basal flagellum because the appendage is unsegmented, whereas the malacostracan flagellum is always annulated. These structures are not homologous to the primitive remipede pattern of a biramous antennule with ventral and dorsal rami because in this case the main dorsal ramus would have an intrinsic musculature and the ventral one would lack muscles; the pattern shown by S. pseudantennulatus gen. nov., sp. nov. is the opposite, with the ventral ramus being segmented and muscle-bearing and the dorsal ramus being simple, unsegmented, lacking intrinsic muscles ( Fig. 53B View FIGURE 53 ). Also, these rami do not share a common muscular peduncle. Overall, we consider that the structures shown by the new species are likely an adaptive process linked to the parasitic stage as they are not movable and lack any traces of sensory organs or segmentation. It could also be a functional adaptation to the planktonic stage of this species or even for mate recognition. In terms of structure and morphology, it is clear to us that these processes are unrelated to any known crustacean antennule type.

Pictorial account (habitus) of the Australian Monstrilla and Caromiobenella

We provide digital photographs of the morphotypes representing each species examined; we include dorsal, ventral and lateral views of each to allow a better recognition of these taxa from field samples. All scales = 0.5 mm.