Mesonerilla harubangi Worsaae, Hansen, Defourneaux, Olesen, Park, Park & Fujita, 2025, sp. nov.

Mesonerilla harubangi sp. nov. urn:lsid:zoobank.org:act: 32DE4DE5-C0FB-4AE8-BAE5-D108C2E8114C Fig. 12; Table 6 Diagnosis Mesonerilla with three antennae. Segment I uniramous, with oval cirri; trunk segments biramous with cylindrical parapodial cirri. Pygidial cirri cylindrical. Row of seven dorsolateral ciliary tufts on palps. Trunk segments with short continuous ventrolateral band of cilia at each parapodia. Paired dorsolateral rows of cilia at basis of parapodia, not medially connecting. Dorsolateral ciliary tufts next to chaetae in segments II–VII. Gonochoristic. Females with rounded brooding hood extruding from the dorsal epidermis of segment VIII. Etymology The species name refers to the characteristic stone statues and deities (Dol-Hareubang, which means ‘Stone Grandfather’) of Jeju Island in South Korea, a large island geographically close to the islets where the species was collected. Type material Holotype SOUTH KOREA • ♀; Jeju Island, Munseom Islet; 33.22772° N, 126.56330° E; sand and shell gravel at around 28 meters depth; 21 Oct. 2015; K. Worsaae and T. Park leg.; NIBRIV0000924478 mounted on SEM stub. Paratypes SOUTH KOREA • 4 ♀♀ adults; same data as for holotype; NIBRIV0000924479 to NIBRIV0000924482 mounted on SEM stub • 3 adults; same data as for holotype; NIBRIV0000924483 to NIBRIV0000924485 mounted on SEM stub. Representative DNA sequences GenBank accession numbers PQ149844 (nuclear 18S rRNA) and PQ570594 (nuclear H3); from specimen collected near type locality: SOUTH KOREA • Jeju Island, Seopseom Islet; 33.2294° N, 126.6027° E; shell gravel at around 33 meters depth, 19 October 2015, same collectors as holotype (Table 1). Description Measurements are based on holotype and values for paratypes are given in parentheses, all measurements obtained from SEM. Body with nine chaetigerous segments (Fig. 12A), total length about 590 µm excluding appendages (455–725 µm, n = 7). Trunk segments similar in size and up to about 130 µm wide including parapodia (120–125 µm, n = 2), about 80 µm excluding parapodia (75–80 µm, n = 2). Prostomium with club-shaped palps, 56 µm long (50–52 µm, n = 2) and 23 µm wide (26–28 µm, n = 2) (pa, Fig. 12A, C–D), and three antennae (las, mas, Fig. 12B–D). Lateral antennae lost in holotype and in all but one paratype (148 µm long) (la, Fig. 12B). Median antenna, 30 µm and possibly broken (ma, Fig. 12B). Eyes absent. Paired nuchal organs dorsolaterally between prostomium and segment I (no, Fig. 12B). Parapodia uniramous in segment I (Fig. 12B, D), with short oval cirrus in paratype (36 µm long) (bc, Fig. 12D). Parapodia biramous in segments II–IX, with cylindrical and tapering interramal cirrus (pc, Fig. 12A, F); increasing in length posteriorly up to 123 µm (56–152 µm, n = 7). Pygidium with 70 µm long cirri similar in shape to parapodial cirri, (pyc, Fig. 12H). Compound chaetae in all segments. Chaetae with distal extension (es, Fig. 12G) and increasing in length towards pygidium. Parapodia with up to 10 chaetae in segment I, maximum 109 µm long (42–98 µm, n = 7). Up to 13 neuro- and 13 notochaetae in segments II–IX, maximum 184 µm long (127–191 µm, n = 7). Prostomium with anterior and posterior fields of cilia and paired lateral ciliary bands extending between lateral antenna and palps insertion (an, Fig. 12D). Palps ciliated continuously on frontal side from insertion point to tip of palps (ct, Fig. 12C), lined by dorsolateral row of ciliary tufts present posteriorly (each tuft with up to seven cilia). Ventral ciliation consisting of i) densely ciliated mouth (mo, Fig. 12D), ii) midventral longitudinal ciliary band extending from mouth to pygidium (vb, Fig. 12F) and iii) short transverse ventrolateral band of at least 20 cilia on each parapodia (vt, Fig. 12F–G). Continuous transverse dorsolateral row with numerous cilia on all trunk segments, extending from basis of parapodia to about halfway to mid-dorsal line (dt1, Fig. 12E). Dense dorsolateral tuft of more than 20 cilia on each parapodium next to notochaetae insertion and cirrus in segments II–VII (dt2, Fig. 12E). Dorsal ciliation patterns of last two segments not observable. Few individual cilia scattered on ventral and dorsal surfaces. Sixteen ciliary tufts observed on ridge of the brooding hood (ct, Fig. 12H). Gonochoristic. Females with round brooding hood, 103 µm long and 126 µm wide, in segment VII at parapodia level (bh, Fig. 12A, H), partially covering up to two embryos attached dorsally. One embryo typically more developed. Juveniles attached to female with six chaetigers maximum and conspicuous parapodia. Male gonopores not found. Nephridia and gonoducts not studied. Distribution and habitat Eastern coast of Seopseom Islet and the western coast of Munseom Islet, south of Jeju Island, South Korea. Collected from 22–33 meters depth from sediment consisting of sand and shell gravel. Associated nerillids from the Seopseom and Munseom Islets include Leptonerilla westheidei sp. nov., Mesonerilla dannyi sp. nov. and Nerillidium sp. 1. Molecular information Only the maximum likelihood analysis resolved M. harubangi sp. nov. as a sister group to Mesonerilla sp. 2 and M. laerkae (albeit poorly supported, BS: 78) within the large clade of gonochoristic Mesonerilla spp. Pairwise comparison of M. harubangi sp. nov. sequence similarity to other gonochoristic Mesonerilla spp.: 18S rRNA – 96.28–97.42% similar to its sister group (clade comprising M. laerkae and M. sp. 2) – 96.22–97.38% similar to the remaining gonochoristic Mesonerilla spp.; COI – 65.28–65.86% similar to its sister group – 56.57–71.65% similar to the remaining gonochoristic Mesonerilla spp. Remarks (see also Table 6 for comparisons of morphologically similar nerillids) Mesonerilla harubangi sp. nov. has nine segments, compound chaetae and club-shaped palps, and thereby conforms to the main characteristics of Mesonerilla. Furthermore, it resembles M. intermedia and M. laerkae by possessing a brooding hood and being gonochoristic (Worsaae 2005a, 2021; Worsaae et al. 2019a). Mesonerilla laerkae has a pointed tip on the brooding hood and equally long parapodial cirri in the trunk segments, which is not seen in either M. harubangi or M. intermedia. Mesonerilla harubangi differs from both species by being smaller in size and by not possessing continuous dorsolateral transverse ciliary rows (found in M. intermedia). The dorsal and ventral ciliation patterns of M. harubangi are most comparable to those of M. xurxoi, but this species does not possess a brooding hood (Worsaae et al. 2019a). Specimens of M. harubangi sp. nov. used for morphological examination were collected from Munseom Islet, while the specimen used for DNA analysis was obtained from Seopseom Islet and identified in the field prior to DNA extraction. The unresolved phylogenetic position of M. harubangi sp. nov. among the other gonochoristic Mesonerilla could be explained by the absence of two of the four genes, as only 18S rRNA and COI fragments were successfully obtained. Individual maximum likelihood analysis of COI sequences found M. harubangi as sister group to Mesonerilla, Meganerilla, Speleonerilla, N. irabuensis, C. sulcipalpata gen. et sp. nov., Micronerilla, Leptonerilla, Trochonerilla and Aristonerilla. This is also reflected in the low pairwise sequence similarity of COI between M. harubangi and all other gonochoristic Mesonerilla (around 60%). Inclusion of additional sequence data is expected to clarify the phylogenetic position of this species within the clade of gonochoristic Mesonerilla spp.