Mesonerilla harubangi, Worsaae, Gonzalez, Kerbl, Nielsen, Jorgensen, Armenteros, Iliffe & Martinez, 2025

Mesonerilla harubangi sp. nov.

urn:lsid:zoobank.org:act:

Fig. 12 View Fig ; Table 6 View 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.

GoogleMaps

Paratypes

SOUTH KOREA • 4 ♀♀ adults; same data as for holotype; NIBRIV0000924479 to NIBRIV0000924482 mounted on SEM stub GoogleMaps • 3 adults; same data as for holotype; NIBRIV0000924483 to NIBRIV0000924485 mounted on SEM stub. GoogleMaps

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 GoogleMaps ( Table 1 View 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 View Fig ), 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 View Fig ), and three antennae (las, mas, Fig. 12B–D View Fig ). Lateral antennae lost in holotype and in all but one paratype (148 µm long) (la, Fig. 12B View Fig ). Median antenna, 30 µm and possibly broken (ma, Fig. 12B View Fig ). Eyes absent. Paired nuchal organs dorsolaterally between prostomium and segment I (no, Fig. 12B View Fig ).

Parapodia uniramous in segment I ( Fig. 12B, D View Fig ), with short oval cirrus in paratype (36 µm long) (bc, Fig. 12D View Fig ). Parapodia biramous in segments II–IX, with cylindrical and tapering interramal cirrus (pc, Fig. 12A, F View Fig ); 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 View Fig ).

Compound chaetae in all segments. Chaetae with distal extension (es, Fig. 12G View Fig ) 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 View Fig ). Palps ciliated continuously on frontal side from insertion point to tip of palps (ct, Fig. 12C View Fig ), 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 View Fig ), ii) midventral longitudinal ciliary band extending from mouth to pygidium (vb, Fig. 12F View Fig ) and iii) short transverse ventrolateral band of at least 20 cilia on each parapodia (vt, Fig. 12F–G View Fig ). 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 View Fig ). Dense dorsolateral tuft of more than 20 cilia on each parapodium next to notochaetae insertion and cirrus in segments II–VII (dt2, Fig. 12E View Fig ). 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 View Fig ).

Gonochoristic. Females with round brooding hood, 103 µm long and 126 µm wide, in segment VII at parapodia level (bh, Fig. 12A, H View Fig ), 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 View 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.