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

Mesonerilla gamaglandulata sp. nov.

urn:lsid:zoobank.org:act:

Figs 10–11 View Fig View Fig ; Table 6 View Table 6

Diagnosis

Mesonerilla with similar-sized cylindrical parapodial cirri on segments II–I, and longer cylindrical pygidial cirri. Three antennae, median short and slightly swollen. Segment I with chaetae and short cirri. Three pairs of pigmented glands in segments I and II. Ventral transverse rows of ciliary tufts between parapodia and intersegmentally. About 14 enteronephridia spanning the hindgut; pairs of nephridia opening in segments III, IV, V. Hermaphroditic with spermioducts opening in segments VI and VII and gonoducts in segment VIII.

Etymology

‘Gama’, means ‘cave’ in Okinawan dialect and ‘glandulata’ refers to the relatively many glands associated with palps, peristomium and oesophagus. The Japanese name for this new species is given here as ‘Gama-usamimi-gokai’ (meaning ‘Cave dwelling-rabbit ear-bristle worm’ in English).

Type material

Holotype

JAPAN • adult; Okinawa Prefecture, Ie Island, ‘Unnamed Cave’ (or ‘ Sho-doukutsu’ ); 26.72518° N, 127.83107° E; 5–12 meters depth; 9 Jun. 2019; Y. Fujita, M.J. Hansen, M.C. Allentoft-Larsen and K. Worsaae leg.; NHMD 1842019 mounted on SEM stub.

GoogleMaps

Paratypes

JAPAN • 3 adults; same data as for holotype; 3 Jun. 2019; NHMD 1842021 to NHMD 1842023 (mounted on SEM stubs) GoogleMaps • 1 adult; same data as for holotype; 10–17 meters depth; 3 Jun. 2019; NHMD 1842020 as permanent whole mount. GoogleMaps

Representative DNA sequences

GenBank accession numbers PQ149843 (nuclear 18S rRNA), PQ149824 (nuclear 28S rRNA), PQ570593 (nuclear H3) and PQ421136 (mitochondrial COI) GoogleMaps ; from specimen with same collection data as holotype ( Table 1 View Table 1 ).

Description

Measurements are based on LM of holotype, counts and ciliation from SEM, and values for paratypes are given in parentheses. Hyaline body, about 785 µm long (670–785 µm, n = 4) with nine segments ( Figs 10A View Fig , 11A View Fig ). Segment lengths of holotype 93, 91, 92, 94, 101, 83, 78, 67, 60 µm. Trunk segments about maximum 135 µm wide, including parapodia (135–150 µm, n = 3), about 105 µm excluding parapodia (100–110 µm, n = 3).

Prostomium with club-shaped palps, 81 µm long (64–82 µm, n = 3) and 41 µm wide (36–40, n = 3) (pa, Figs 10A, D View Fig , 11B, F–G View Fig ) with unique lateral row of 4–6 glandular papillae interspersed by tufts of cilia (ppl, Fig. 11C View Fig ). Median antenna cylindrical and slightly swollen distally (ma, Figs 10C View Fig , 11B, F View Fig ), 84 µm long (75–80 µm, n = 3). Only scars from lateral antennae left but presence further documented by CLSM of lateral antennae muscles in prostomium (lam, mam, Fig. 10E View Fig ). Eyes absent. Paired nuchal organ between prostomium and segment I. Three groups of dark, paired glands: i) small, dorso-anterior on segment I and extending ventrally, ii) large, rounded, opening into mouth cavity, and iii) large group, dorsally in segment II (oesophageal?) (gl, Fig. 10D View Fig ).

Parapodial cirri in segment I short (bc, Figs 10D View Fig , 11A–B View Fig ), 15–20 µm long (16–22 µm, n = 3). Trunk segments with cylindrical cirri of similar length (pc, Figs 10A–B View Fig , 11A View Fig ), maximum 55 µm (49–59 µm, n = 3). Pygidium with 216 µm long cylindrical cirrus (pyc, Fig. 10A View Fig ).

Straight to slightly curved compound chaetae in all segments. Parapodia with up to eight chaetae, maximum 75 µm long in segment I and maximum 120 µm long in trunk segments.

Prostomium with anterior and posterior fields of sensory cilia extending between lateral antennae scars and palp insertions (acf, pcf, Fig. 11B, G View Fig ). Paired densely ciliated nuchal organs on dorsolateral border between prostomium and segment I. Palps with ventral longitudinal dense ciliary row from insertion points to tip of palps, lined by dorsolateral row of five ciliary tufts present posteriorly (each tuft with up to 15 cilia). Segment I with paired dorsolateral transverse rows of each two ciliary tufts (each tuft with>10 cilia) at the level of parapodia. Continuous transverse dorsolateral row with numerous cilia on all trunk segments, extending from basis of parapodia to about halfway to mid-dorsal line (tdc, Fig. 11A–B View Fig ). Ventral ciliation consisting of i) densely ciliated mouth (mo, Fig. 11G View Fig ), ii) midventral longitudinal ciliary band extending from mouth to pygidium (vc, Fig. 11E View Fig ) and iii) transverse rows of up to five tufts of cilia on each parapodia (tvc, Fig. 11D, H View Fig ). Additional ventral row intersegmentally with 2–6 tufts between segments I–VIII.

Three pairs of discontinuous ciliated nephridia extending along segments and opening latero-ventrally in segments III, IV, V (n3–n5, Fig. 10F View Fig ). About 14 enteronephridia extending along the hindgut (en, Fig. 10H–I View Fig ), on the dorsal (4), lateral (6) and ventral (4) sides. Hermaphroditic with two pairs of slightly curved dorsoventrally extending spermioducts opening separately in segments VI and VII (sp, Figs 10G View Fig , 11E, H View Fig ) and one pair of relatively straight oviducts opening in segment VIII (ov, Figs 10G View Fig , 11H View Fig ).

Distribution and habitat

Unnamed Cave ‘Sho-doukutsu’, eastern reef of Ie Island, Okinawa Prefecture, Ryukyu Islands, Japan. Submarine cave located on the reef slope. Specimens collected from plankton tow and sediment samples from the anchialine right hall of the cave, at 5–17 meters of depth; see Osawa & Fujita (2019) for the description of the cave. Associated nerillids from the cave include Cirrinerilla sulcipalpata gen. et sp. nov., Leptonerilla sp. 3 and Trochonerilla sp. 1 .

Molecular information

Mesonerilla gamaglandulata sp. nov. is found in both analyses ( Fig. 2 View Fig ) as a sister taxon to M. armoricana Swedmark, 1954 and M. roscovita Levi, 1953 , albeit with low support (PP/BS: 0.51/69). Together, this clade has a sister relationship to M. dannyi sp. nov. (PP/BS: 1/99). The hermaphroditic Mesonerilla spp. within clade C does not form a monophyletic sub-clade, as Nipponerilla irabuensis groups with the previously mentioned species of Mesonerilla , although with low support (PP/BS: 0.68/79), and this clade is itself sister to the hermaphroditic Mesonerilla fagei Swedmark, 1959 (PP/BS: 1/96). Morphologically, M. gamaglandulata sp. nov. has a strong resemblance to the three hermaphroditic Mesonerilla spp. and exhibits a high degree of similarity in 18S rRNA and 28S rRNA sequences.

Pairwise comparison of M. gamaglandulata sp. nov. sequence similarity to other hermaphroditic Mesonerilla spp. and Nipponerilla irabuensis : 18S rRNA – 98.83–98.89% similar to its sister group (clade comprising M. armoricana and M. roscovita ) – 98.88–99.14% similar to remaining hermaphroditic Mesonerilla spp. – 96.92% similar to N. irabuensis ; 28S rRNA – 90.52% similar to its sister group – 89.42–91.37% similar to the remaining hermaphroditic Mesonerilla spp. – 84.89% similar to N. irabuensis ; COI – 72.86% similar to the remaining hermaphroditic Mesonerilla spp. – 70.36% similar to N. irabuensis ; H3 – 90.76–92.32% similar to its sister group – 86.72% similar to the remaining hermaphroditic Mesonerilla spp. – 91.97% similar to N. irabuensis .

Remarks (see also Table 6 View Table 6 for comparisons of morphologically similar nerillids)

With its nine segments, compound chaetae, spoon-shaped palps, three antennae, equally long parapodial cirri and configuration of gonoducts M. gamaglandulata sp. nov. bears most resemblance to the three hermaphroditic species of Mesonerilla ( M. roscovita , M. fagei and M. armoricana ) and Mesonerilla dannyi sp. nov. ( Levi 1953; Swedmark 1959; Westheide 2008; Worsaae et al. 2019a, 2021a; Worsaae 2021). Mesonerilla gamaglandulata shares different characteristics with the three hermaphroditic Mesonerilla including the presence of chaetae and shorter cirri in segment I (also seen in M. fagei ), ventrolateral rows of cilia, oesophageal glands and cylindrical pygidial cirri (also seen in M. roscovita and M. fagei ) and a swollen median antenna and spoon-shaped palps (also seen in M. armoricana ) ( Levi 1953; Swedmark 1959; Westheide 2008; Worsaae et al. 2019a). The main difference between M. gamaglandulata and Mesonerilla dannyi is the presence or absence of chaetae in segment I, respectively. Mesonerilla gamaglandulata , Mesonerilla dannyi and N. irabuensis were monophyletic in the Bayesian and maximum likelihood analyses and they all have compound chaetae, nine segments and three antennae. Nipponerilla irabuensis differs from the two others by having a cigar-shaped body, spermioducts in segments VII and VIII and being gonochoristic. In contrast to M. gamaglandulata , N. irabuensis also has long palps and a relatively long median antenna ( Worsaae et al. 2021b), traits that could not be determined from Mesonerilla dannyi .

Apart from M. gamaglandulata sp. nov., oesophageal glands have also been recorded in M. laerkae , Mesonerilla katharinae Worsaae, Mikkelsen & Martínez, 2019 , M. roscovita , M. fagei and M. xurxoi Worsaae, Mikkelsen & Martínez, 2019 , with two lateral glands also being observed in segment I of the latter species ( Worsaae et al. 2019a). Furthermore, M. minuta Jouin, 1970 has about 10 yellowish dorsal epidermal glands on each side of segment I ( Swedmark 1959). Three independent groups of glands in the head region, as seen in M. gamaglandulata , have, to our knowledge, not been recorded previously in Nerillidae and the anteriormost pair of small glands seems unique to the species.