Leptonerilla westheidei Worsaae, Hansen, Defourneaux, Olesen, Park, Park & Fujita, 2025, sp. nov.

Leptonerilla westheidei sp. nov. urn:lsid:zoobank.org:act: 756B9FFA-2D1E-49B3-AF75-7558EF2FA291 Fig. 7; Table 4 Diagnosis A Leptonerilla diagnosed by a combination of following characteristics: three antennae, two club-shaped palps. Segment I with relatively long, unpaired, cylindrical cirri and few chaetae. Following trunk segments with long, double parapodial cirri. Gonochoristic; females with one pair of oviducts in segment VIII, males not observed. Etymology The species is named in recognition of the annelid researcher Wilfried Westheide, who erected the genus Leptonerilla and described the morphologically highly similar L. diplocirrata. Type material Holotype SOUTH KOREA • adult; Jeju Island, Munseom Islet; 33.22772° N, 126.56330° E; sand and shell gravel around 25 meters depth; 21 Oct. 2015; K. Worsaae and T. Park leg.; NIBRIV0000927415 mounted on SEM stub. Paratype SOUTH KOREA • 1 adult; same data as for holotype; NIBRIV0000927416 mounted on SEM stub. Representative DNA sequences GenBank accession numbers PQ149836 (nuclear 18S rRNA), PQ570586 (nuclear H3) and PQ421134 (mitochondrial COI); from specimen collected near type locality: SOUTH KOREA • Jeju Island, Beomseom Islet; 33.22115° N, 126.51809° E; sand and shell gravel at around 20 meters depth; 23 Oct. 2015 (Table 1). Description Measurements are based on LM of holotype, counts and ciliation from SEM; values for paratype are given in parentheses. Body with nine chaetigerous segments (Fig. 7A–B), total length about 705 µm (565–640 µm, n = 2). Segment lengths 54, 75, 91, 83, 81, 78, 74, 65, 63 µm. Maximum width about 180 µm including parapodia (140–145 µm, n = 2), about 130 µm excluding parapodia (90–95 µm, n = 2). Prostomium with median and two lateral antennae (only scars visible) and two, ventrally inserted, short, club-shaped palps (47–50 µm long and 35–36 µm wide, n = 2) (las, mas, pa, Fig. 7A, D, F). Presence of eyes difficult to discern in fixed material. Parapodium of segment I uniramous with long, single, cylindrical (cirriform) cirrus, up to 120 µm long (85 µm, n = 1) and inserted ventral to the chaetal bundle (bc, Fig. 7B, D, F). All trunk segments with long, cylindrical, double, interramal cirri (Fig. 7A), up to 263 µm long (186–192 µm, n = 2). Parapodial cirri with few evenly scattered cilia. Pygidium with scars from two lost cirri and midterminal short, cirriform lobe (pyl, Fig. 7A). Compound chaetae in segments I–IX plus 1–2 short, dorsal, bent, simple chaetae in most trunk segments (n = 2). Up to nine posteriorly pointing compound chaetae in segment I (n = 3), maximum 135 µm long (Fig. 7A, D, F); up to 18 compound chaetae in segments II–IX (n = 3), maximum 195 µm long (blade up to 150 µm, shaft up to 54 µm, extension of shaft at chaetal joint up to 8 µm) (Fig. 7A); length of chaetae increasing posteriorly. Prostomium with anterior and posterior fields of presumed sensory cilia, lateral rows of cilia between lateral antenna and palp insertion, paired densely ciliated nuchal organs on lateral border between prostomium and segment I (Fig. 7D, F). Palps with ventral longitudinal dense ciliary row from insertion point to tip of palps (pvc, Fig. 7F), dorsal row of minimum 3 ciliary tufts on distal half of palp (each tuft with>15 cilia) (ct, Fig. 7F), and lateral row of small tufts of cilia (sct, Fig. 7F). Segment I with paired dorsolateral transverse rows of each two ciliary tufts at the level of parapodia. Following trunk segments with dorsal transverse continuous row of up to 12 ciliary tufts at level of parapodia (tdc, Fig. 7A, C, E); tufts increasing in numbers, ciliary length, and density in posterior segments. Segment II with an additional lateral pair of longitudinal ciliary bandlets (ldc, Fig. 7E). Ventral ciliation not examined. Gonochoristic. Females with mature eggs (up to 110 µm long) and oocytes in segments VI–VIII (eg, Fig. 6B). Nephridia and gonoducts not examined. Distribution and habitat Western coast of Munseom Islet and northern coast of Beomseom Islet, south of Jeju Island, South Korea. Collected between 20–25 meters depth from sediment consisting of sand and shell gravel. Associated nerillids from the Munseom and Beomseom islets include Meganerilla sp. 2, Mesonerilla dannyi sp. nov. and Nerillidium sp. 1. Molecular information Leptonerilla westheidei sp. nov. always groups together with L. purschkei sp. nov. and L. sp. 1 in all phylogenetic analyses. Pairwise comparison of L. westheidei sp. nov. sequence similarity to other Leptonerilla spp.: 18S rRNA – 99.66% similar to its sister group (L. sp. 1) – 97.25–99.89% similar to the remaining Leptonerilla spp.; COI – 96.53% similar to its sister group (L. sp. 1) – 81.29–89.57% similar to the remaining Leptonerilla spp.; H3 – 98.66% similar to its sister group – 89.69–96.59% similar to the remaining Leptonerilla spp. Remarks (see also Table 4 for Leptonerilla spp. comparisons). The small sized Leptonerilla westheidei sp. nov. shows the greatest morphological and molecular similarity to the two geographically closest species L. purschkei sp. nov. from Japan and L. diplocirrata from Hainan, China. It differs, however, from both by its half number of chaetae in segment I, and from these and all other species of Leptonerilla by having much longer cirri on segment I and molecular differences (see also remarks of L. purschkei). Specimens of L. westheidei sp. nov. used for morphological examination were collected from Munseom Islet, while the specimen used for DNA analysis was obtained from Beomseom Islet and identified in the field prior to DNA extraction.