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

Mesonerilla dannyi sp. nov. urn:lsid:zoobank.org:act: AF47235F-7D0A-464E-904C-61D83C1E2AA3 Figs 13–14; Table 6 Diagnosis Hermaphroditic Mesonerilla with three antennae and equally short parapodial cirri on segments II–VIII. Segment I with short, ovoid cirri and lacking chaetae. Hermaphroditic with two pairs of spermioducts with separate openings in segments VI, VII, and one pair of oviducts opening in segment VIII. Etymology Named in honour of Danny Eibye-Jacobsen, in recognition of his valuable contributions to annelid taxonomy and systematics, as well as his highly appreciated mentorship of the first author. Type material Holotype SOUTH KOREA • adult; Jeju Island, Munseom Islet; 33.22779° N, 126.5675° E; shell gravel with mud at 25 meters depth; 24 May 2018; K. Worsaae and T. Park leg.; NIBRIV0000924486 as permanent whole mount. Paratypes SOUTH KOREA • 2 adults; Jeju Island, Seopseom Islet; 33.2304° N, 126.6015° E; shell gravel at 15 meters depth; 25 May 2018; K. Worsaae and T. Park leg.; NHMD 1842024, NHMD 1842025 mounted on SEM stubs. Representative DNA sequences GenBank accession numbers PQ149847 (nuclear 18S rRNA), PQ149827 (nuclear 28S rRNA) and PQ570598 (nuclear H3); from specimen with same collection data as holotype (Table 1). Description Measurements are based on LM of holotype, counts and ciliation from SEM; values for paratypes are given in parentheses. Body with nine segments (Figs 13A, 14A), total length about 875 µm long (810– 985 µm, n = 3). Trunk segments up to about 185 µm wide including parapodia (155–185 µm, n = 3), about 120 µm (115–135 µm, n = 3) excluding parapodia. Segment lengths of paratype 72, 69, 86, 102, 92, 112, 88, 77, 49 µm. Round prostomium with palps and antennae lost in all mounted specimens, but median antennal and lateral antennal scars observed in SEM (las, ma, Fig. 13A–B). Eyes absent. Nuchal organs visible with CLSM and SEM (no, Fig. 14B). Parapodial cirri in segments I–IX. Segment I with short cylindrical cirri, 35 µm long (33 µm long, n = 1) (bc, Fig. 13B). Segments II–VIII with cylindrical, slightly tapering cirri, up to 65 µm long (54–74 µm long, n = 3), similar in length, slightly shorter cirri on segment VIII (pc, Fig. 13C). Only regenerating pygidial cirri were observed (rpc, Fig. 13D). Compound chaetae in segments II–IX, with up to 12 chaetae per parapodium and maximum 180 µm long (128–155 µm long, n = 2) (cc, Fig. 14A). Prostomium with anterior and posterior fields of presumed sensory cilia (acf, Fig. 14B). Paired, densely ciliated nuchal organs on dorsolateral border between prostomium and segment I (no, Fig. 14B). Ventral ciliation consisting of i) densely ciliated mouth (mo, Fig. 14C), ii) midventral longitudinal ciliary band extending from mouth to pygidium (vc, Fig. 14C–D) and iii) transverse rows of up to about 10 small ciliary tufts at level of parapodia (tvc, Fig. 14D). Multiple single cilia scattered on dorsal, lateral, and ventral sides of body. Many cilia lost during preparation. Three lateral enteronephridia extending from posterior end of stomach along the hindgut until pygidium (Fig. 13F). Spermioducts present in segments VI and VII with posterior ventral opening. Two relatively long and slender oviducts opening ventrally in segment VIII (so, ovo, Figs 13E, 14D). Distribution and habitat Eastern coast of Seopseom Islet and southern coast of Munseom Islet, south of Jeju Island, South Korea. Collected from 25 meters depth from shell gravel with mud. Associated nerillids from Munseom Islet include Leptonerilla westheidei sp. nov., M. harubangi sp. nov. and Nerillidium sp. 1. Molecular information Mesonerilla dannyi sp. nov. is found as sister group to a clade composed of M. gamaglandulata sp. nov., Mesonerilla roscovita and Mesonerilla armoricana (PP/BS: 1/99). Mesonerilla dannyi sp. nov., like M. gamaglandulata sp. nov., has a high sequence similarity of 18S rRNA and 28S rRNA to three hermaphroditic Mesonerilla. Pairwise comparison of M. dannyi sp. nov. sequence similarity to other hermaphroditic Mesonerilla and Nipponerilla irabuensis: 18S rRNA – 98.88–99.04% similar to its sister group (clade comprising M. gamaglandulata sp. nov., M. roscovita and M. armoricana) – 99.11% similar to M. fagei – 97.52% similar to N. irabuensis; 28S rRNA – 89.42% similar to its sister group – 90.85% similar to M. fagei – 84.86% similar to N. irabuensis; COI – 72.86% similar to its sister group – 68.07% similar to N. irabuensis. Remarks (see also Table 6 for comparisons of morphologically similar nerillids) Mesonerilla dannyi sp. nov. is morphologically most similar to the hermaphroditic Mesonerilla (Levi 1953; Swedmark 1959; Worsaae et al. 2019a), due to its nine-segmented body, compound chaetae, three antennae, equally short and single parapodial cirri in segments II–VIII, and its configuration of gonoducts. It shares the absence of chaetae in segment I with M. roscovita and M. armoricana as well as the presence of short, ovoid cirri with M. fagei. Similar to M. armoricana, M. fagei and M. gamaglandulata sp. nov., cirri on the last segment are similar or nearly equal in length to cirri in the remaining trunk segments (Levi 1953; Swedmark 1959; Worsaae et al. 2019a).