Inermosyllis asteriakavalaris, Martin & Antokhina & Zvonareva & Britayev, 2025

Inermosyllis asteriakavalaris View in CoL sp. nov.

Figs. 1–7 View FIGURE 1 View FIGURE 2 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7

Inermosyllis sp. – Britayev & Antokhina (2012): 28 –29, Pl. 6E–F.

ZooBank: urn:lsid:zoobank.org:act:

Material examined. Holotype: ZM_ MSU Pl-5024, complete specimen in two fragments, Nha Trang city beach ( N 12°14.22′, E 109°11.89′), 3 m depth, on L. maculata , fixed in 96 ° ethanol, coll. TIA, 03.09.2024 GoogleMaps ; Paratypes: ZM_ MSU Pl-5025, complete

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specimen, north of Tre Island ( N 12°15.00′, E 109°17.75′), Nha Trang Bay, 19–25 m depth, on L.maculata , fixed in 70 ° ethanol, coll. TAB, 30.04.2003 GoogleMaps ; ZM_ MSU Pl-5026, complete specimen, Dam Bay , ( N 12°11′42″ E 109°17′25″), Tre Island, Nha Trang Bay 4–12 m depth, on L. maculata , fixed in 96 ° ethanol, coll. E.S. Mekhova, 30.08.2024 GoogleMaps ; ZM_ MSU Pl-5027, complete specimen, Dam Bay , ( N 12°11′42″ E 109°17′25″), Tre Island, Nha Trang Bay 4–12 m depth, on L. maculata , fixed in 96 ° ethanol, coll. K.A. Dudka, 16.09.2024 GoogleMaps .

Additional material. Fragment without head and anterior segments, Dam Bay , ( N 12°11′42″ E 109°17′25″), Tre Island, Nha Trang Bay 4–12 m depth, on L. maculata , fixed in 96 ° ethanol, coll. K.A. Dudka, 16.09.2024 GoogleMaps .

Description. Holotype complete specimen in two fragments, 46 (24 and 22) mm long, 0.6 mm wide (without parapodia) and 2.0 mm wide (with parapodia), with 161 chaetigers. Atokous region with 85 chaetigers, epitokous region with 75 chaetigers (fig. 2B). Body long, cylindrical, wid- est at proventricle level, gradually tapering posteriorly. Uniformly orange in vivo with brown pigmentation of epitokous region, yellowish to whitish when preserved. Prostomium oval to subpentagonal, twice wider than long; eyes absent. Antennae whip-shaped; central antenna inserted on posterior end of prostomium, with about 23 articles, 1/4 longer than prostomium and palps together; lateral antenna about 2/3 shorter than median antenna, similar in shape, inserted on anterior margin of prostomium, with about 12–13 articles. Palps broad, triangular, as long as prostomium, only fused at base, lacking sen- sory cilia. Nuchal organs as dorso-lateral

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FIGURE 3

Inermosyllis asteriakavalaris sp. nov.

SEM micrographs. (A) anterior end of specimen with everted pharynx, dorsal view; (B) anterior end, dorsal view; (C) Detail of the anterior end, dorsal view. (D) Light micrographs of the dissected pharynx. White arrow: location of nuchal organ

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ciliated patches between prostomium and peristomium (fig. 3B). Pharynx with a crown of 10 large papillae and an inner ring of cilia (fig. 3), lacking teeth or trepan, extending through six chaetigers (fig. 2C). Proventricle long, cylindrical, extending through five chaetigers (8–13) when alive (6, 8–14 when preserved), with about 62 muscular cell rows (fig. 2C). Peristomium distinct, about half as long as following

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segments. Dorsal tentacular cirri similar in shape to central antenna but slightly shorter, with about 13–20 articles; ventral tentacular cirri similar in shape to lateral antennae but slightly shorter, with 7–14 articles. Dorsal cirri similar in shape to antennae and tentacular cirri, longest ones as long as body width, alternating in length as between 19–23 (long) and 9–14 (short), then non- alternating in number of articles but slightly alternating in length due to middle articles being longer than remaining ones in long dorsal cirri (fig. 4). Parapodial lobes conical, 2.5–3 times shorter than body width. Ventral cirri digitiform, short, reaching tips of parapodial lobes (fig. 4C–D). 5–7 chaetae on each parapodium, similar in all observed chaetigers, large, thick, appear- ing simple by partial fusion of blade and shaft, articulation still visible; shaft with a long curved tip, sub-distally swollen to form a triangular fang with slightly marked serration or crevices on upper side; blades short, 11–14 µm long, smooth, bidentate, with subdistal tooth distinctly smaller and well- separated from distal one (figs. 5C–F, 6). One–two aciculae on each parapodia, with distally truncate or slightly oblique tips (fig. 5A–B). Dorsal and ventral simple chaetae absent. Pygidium with two long cirri located above anal opening.

Measurements. 0.6–0.8 mm wide (without parapodia) and 1.2–2.0 mm wide with parapodia, 34–53 mm long, for 154–220 chaetigers (table 2). 85–164 and 43–75 chaetigers in atokous and epitokous regions, respectively. 4–7 chaetae per bundle. 1–2 acicula per parapodium.

Reproduction. All studied specimens have developing stolons. Holotype female, with the stolon still attached to the atokous region (fig. 1B), with segments wider and thicker than those in the atokous

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region and dark violet in vivo, likely due to the developing intracolelomic oocytes (53–70 µm in diameter). The anterior region of the stolon apparently lacked appendages (although these may have not yet been developed). However, there were two ventro-lateral dark stripes that seemed to be eyespots. Therefore, we are tentatively considering the stolon as cephalous.

Coloration. Atokous region orange or yellowish; epitokous region brown or dark violet in vivo (fig. 2B). Whitish to yellowish when preserved (fig. 7A–B), with two dark brown ventro-lateral pigment stripes on the first epitokous segment (fig. 7B).

Ecology. Found at 3–25 m depth on L. maculata . Examined sea stars showed radii ranging from 75–220 mm, with symbionts found on hosts within 50– 10 mm ( 1 specimen), 101–150 mm ( 2 specimens), and 201–250 mm ( 1 specimen) (fig. 8). Each host individual harbored always one specimen of I. asteriakavalaris sp. nov., which often shared the host with two other species of polychaetes – the polynoid Pottsiscalisetosus praelongus (Marenzeller, 1902) and the hesionid Oxydromus sp. – and the porcellanid crab Lissoporcellana polybioides Adams & White, 1849 . As Inermosyllis sp. , the species was previously reported living in association with A. angulatus ( Britayev & Antokhina, 2012) ; however, as this sea star was purchased in a fish market, the association with this host specimen requires further confirmation. The specimens of I.a steriakavalaris sp.nov. were found hidden within the ambulacral grooves or even inside the mouth opening of their sea star hosts. In 2003, two out of four examined sea stars harbored the symbiotic syllid (prevalence = 50%). In 2024, the prevalence was 20% for the Dam Bay hosts (n = 10) and 20% for the Dam Bay hosts (n = 10). We also examined seven specimens of the asteroid A. monocanthus found inhabiting the same environment as L. maculata , which proved to be not infested (table 1).

Distribution. Known only from Nha Trang Bay ( Vietnam, South China Sea), found there at Nha Trang city beach, Tre Island, and Dam Bay.

Etymology. The specific name derives from kavalaris (καβαλΆΡΗς, meaning “rider” in Greek), and asterias ( ασΤΕΡΊαs, meaning “sea star” in Greek), referring to the symbiotic association between the syllid and its asteroid hosts.

Remarks. Inermosyllis , replacing Pseudosyllides Augener, 1927 as homonymous with Pseudosyllides Czerniavsky, 1882 , resembles Syllis Lamarck, 1818 , but lacks pharyngeal tooth ( Augener, 1927; Czerniavsky, 1882; Lamarck, 1818; San Martín, 2003). The Vietnamese new species represents the fifth in the genus, together with I. curacaoensis ( Augener, 1927) , from Curaçao Island, Caribbean Sea; I. balearica ( San Martín, 1982) , from the Mediterranean, I. mexicana (Góngora-Garza & de León-González, 1993) , from the Pacific coasts of México, and I. pseudohaploides San Martín, Hutchings & Aguado, 2008 from Western Australia ( Augener, 1927; Góngorra-Garza & León-Gonzalez, 1993; San Martín, 1982, 2003; San Martín et al., 2008). In addition to a clearly distinct biogeographical distribution, I. curacaoensis , I. balearica , and I. mexicana differ from I. asteriakavalaris sp. nov. in having capillary chaetae and compound chaetae with blades well-separated from shafts. Contrary, the Australian I. pseudohaploides shares with our new species the blades being partially fused to shafts, but its blades are unidentate, shaft tips are conical and short, and there are no traces of serration of the shaft fang ( San Martín et al., 2008). Moreover, although the holotype of I. pseudohaploides is a ripe female with intracoelomic oocytes in the last body segments, no information on their size was provided in the original description, neither the possible starting of stolo- nization was mentioned ( San Martín et al., 2008), thus preventing comparisons with I. asteriakavalaris sp. nov. The reproduc- tive process in the other three species of the genus is currently unknown.

Simple chaetae apparently originated by blades being fused to shafts are also known to occur in different species of Haplosyllis Langerhans, 1879 , such as H. anthogorgicola Utinomi, 1956 from Japan, associated to octocorals ( Fourreau et al., 2024; Martin et al., 2002; Utinomi, 1956) or H. spongiphila ( Verrill 1885) from the NW Atlantic and Caribbean, H. loboi ( Paola, San Martín & Martin 2006) from SW Atlantic and H. chaetafusorata Latig & Martin, 2011 from Caribbean, associated to sponges ( Lattig & Martin, 2009, 2011; Paola et al., 2006; Verrill, 1885). However, all these species show a pharynx with a pharyngeal tooth, instead of unarmed, as well as different biogeographic distributions and hosts.

Molecular analyses. A nucleotide BLAST comparison against the GenBank database identified the closest match as Syllis magdalena Wesenberg-Lund, 1962 ( KX792211 View Materials ) ( Álvarez-Campos & Verdes, 2017), with

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80.43% identity and an E-value of 1e-122. However, the phylogenetic analyses did not resolve the evolutionary relationships of our sequences within the Syllidae , as the sequences obtained from I. asteriakavalaris sp. nov. formed an individual branch within a largely polytomic tree including mainly sequences of Syllis , but also other genera of Syllinae (fig. 9). Nevertheless, the position of the clade formed by the sequences of Inermosyllis clearly differs from that of T.asterobia , the other asteroid-associated Syllinae , which belongs to well-defined “ribbon-clade” (sensu Aguado et al., 2015).

Nomenclatural acts. The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for theICZ N.The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the pre- fix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:. The electronic edition of this work is published in a journal with an ISSN, and has been archived and is available from the following digital repositories: ResearchGate, DigitalCSIC and SIEE RAS.