Magelona japonica Okuda 1937
Creators: name: Taylor, Abbie
name: Mortimer, Kate
name: Jimi, Naoto
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Annelida
Polychaeta
Magelonidae
Magelona
Magelona japonica

description: Magelona japonica Okuda, 1937
[Japanese name: Morote-gokai]
Figs 6–9
Magelona japonica Okuda, 1937b
Type locality: Incheon, Korean Archipelago; Onagawa, Miyagi Prefecture
Material examined. SOUTH KOREA, Incheon, syntypes (ZIHU 2789, 9 af, 18 f, pf) from soft, muddy sediments, July 1936, collector unknown. JAPAN (NMW.Z.2022.001.0001, af), R. V. Toyoshiomaru Stn 4 (34.3098, 133.2260 to 34.3088, 133.2294), dredge, 05/11/2014, collected by NJ, sandy sediments, 13–14 m depth.
Diagnosis. Prostomium wider than long, horns present. Chaetigers 1–9 with slender triangular lamellae, neuropodia distinctly scoop-shaped on chaetigers 1–4. Distinct pigment band on posterior thorax. All thoracic chaetae capillary. Abdominal lateral lamellae roughly equal in size, triangular to foliaceous with pointed tips. Abdominal hooks tridentate, in two groups, vis-à-vis. No pouches observed.
Dimensions. A large, stout species; with a slight constriction between thorax and abdomen (Figs 6A; 7A, C), thorax dorsoventrally flattened, much thinner (when viewed laterally, Fig. 8C), but wider than the rounded abdomen (when viewed dorsally). Chaetigers 5–7 marginally wider than anterior and posterior thorax (when viewed dorsally). Japanese specimen, anterior fragment: prostomium 0.6 mm long, 1.1 mm wide; thorax 3.9 mm long (including prostomium), 0.98 mm wide (between chaetigers 6 and 7); total length 6.1 mm for 15 chaetigers (last chaetiger dissected and slide mounted). Twenty-eight fragments of suspected syntype material observed, including nine anterior fragments ranging from 3–34 chaetigers and 1.5–11.5 mm in length (N.B. syntype material has previously dried out, so measurements approximate given condition). Lateral margins of thoracic chaetigers characteristically rounded and bulbous (Figs 6A; 7A–C, E; 8E).
Description. Prostomium triangular (Figs 6B; 7D), wider than long (L:W ratio 0.55). Distinct prostomial horns, anterior margin triangular, not entirely smooth but not as marked as crenulations. Lateral prostomial margins slightly rounded, base clearly expanded. Prostomium with two pairs of prominent longitudinal dorsal muscular ridges, inners abutting for a third of the length, diverging at both ends. Outer pair of ridges abutting inner pair for majority of length. Markings either side of the ridges fairly weak, not as distinct as in other species. Burrowing organ partially everted (Figs 7C, F, G; 8C, F) on Japanese specimen and on four syntypes, oval (shape of full eversion unknown). Longitudinally ridged inferiorly, superior surface not observed. Palp stubs (showing signs of regeneration) on Japanese specimen, arising ventrolaterally from base of prostomium, thick (Figs 6A; 7) (now detached, present in vial). Palps reaching approximately chaetigers 1–2. A few short papillae present on distal portion of left-hand palp. Exact number of papillae and rows indeterminable. Palps retained partially on three syntype specimens (Fig. 8C) (one loose palp present in vial), reaching chaetigers 12–16, non-papillated region reaching chaetiger 2. Papillae long; one row either side of indistinct longitudinal line on distal portion, two rows either side medially, and three rows either side proximally.
Achaetous region behind prostomium, roughly one and a half times the size of chaetiger 1 (Figs 6A; 7D, E; 8D). Chaetigers 1–8 similar (Figs 6A, C–J; 7A, E, F); parapodia biramous. Notopodia with low triangular, prechaetal lamellae confluent with slender, smooth-edged, triangular postchaetal lamellae. Lamellae decrease in size towards mid thorax, those of chaetiger 5 and 6 much smaller than preceding chaetigers, but lamellae increase again on chaetigers 7 and 8 (although not as large as anterior thorax). No superior dorsal lobes present on thoracic chaetigers. Neuropodial lamellae distinctly scoop-shaped on chaetigers 1–4 (Figs 6C–F; 7F), tips pointed, situated directly under chaetal bundle. Neuropodial lamellae of mid and posterior thorax (Fig. 6G–J) digitiform to slender triangular, becoming distinctly postchaetal by chaetiger 7 (Fig. 6I). Lamellae decrease in size to mid thorax, but increase from chaetiger 6, initially similar in size to notopodia, but becoming longer than notopodia by chaetiger 4.
Chaetiger 9 (Figs 6A; 8A, B): shorter and narrower than preceding chaetigers. Noto- and neuropodial lamellae similar, triangular postchaetal, smaller than on preceding chaetigers (Fig. 6K). No superior dorsal lobes. Notopodial lamellae below chaetal bundle, neuropodial lamellae above neurochaetae (Fig. 6K). Chaetae of chaetigers 1–9 simple, unilimbate winged capillaries with smooth blades, limbations slight (Fig. 6M). Neuropodial chaetae longer than notopodial ones, chaetal bundles of posterior thorax distinctly splayed (e.g., Fig. 6I–K). No thoracic ventral swellings observed.
Parapodia of abdominal chaetigers (Figs 6L; 8A, B) with approximately equal lateral lamellae in both rami. Lamellae marginally foliaceous to triangular in shape with pointed tips, slight basal constriction but with no obvious postchaetal expansion of lamellae behind chaetal rows. Minute to sporadic triangular dorsal (DML) and ventral (VML) processes present at inner margins of chaetal rows (Fig. 6L). Abdominal chaetae tridentate hooded hooks (Fig. 6N) of similar size, superior two fangs parallel, above, but not majorly distinct from main fang. Hooks in each ramus in two groups, main fangs vis-à-vis, group at inner margin with approximately twice as many hooks (Fig. 6A, L). Roughly ten hooks per ramus in anterior abdomen. No abdominal support chaetae (‘aciculae’) or abdominal pouches observed.
Posterior of Japanese specimen unknown. One posterior fragment observed in syntype material, pygidium conical and devoid of anal cirri (as noted by Okuda, likely damaged) (Fig. 8G). Several ovigerous syntype specimens observed. Sand balls and foraminifera additionally observed in the gut of syntypes, and evidence of a tube on two specimens, as noted by Okuda.
Colour. No living material observed, although a photograph of the freshly preserved specimen, provided by third author, shows a pale-yellow colouration with a dark reddish-brown pigment band (Fig. 7B) in the posterior thorax (now faded to a large extent, Fig. 7A, C). Band strongest between chaetigers 5–9, but with smaller pigmented regions adjacent to parapodia on chaetigers 3–4. Pigment band extends around the body from dorsal to ventral surface. Syntypes have darkened to a yellow-brown colour, perhaps as a consequence of drying out previously. Staining with Methyl Green (Fig. 7E, F) fairly indistinct, showing no clear pattern. Although, speckled areas from chaetigers 1–4, and abdominal interparapodial patches more distinct in stained specimens (Figs 6A; 8A, B). Thoracic staining retained on Japanese specimen for some time after staining. Transverse lines of white speckles apparent on chaetigers 1–4, and evenly spread across surface of chaetigers 5–9. Dense white speckles between chaetigers 8–10, particularly either side of mid ventral line, and as interparapodial patches in the abdomen present.
Distribution and habitat. Type specimens were collected from soft, muddy sediments off Incheon, Korean Archipelago and Onagawa, Miyagi Prefecture. The current Japanese specimen was collected at a depth of 13–14 m off Innoshima Island, Seto Inland Sea, Japan (34.3098, 133.2260 to 34.3088, 133.2294) in sandy sediments (Fig. 1, Table 1). Further records from muds and sands from estuarine, intertidal flats, continental shelf and shallow sublittoral sediments exist from Korea (Okuda 1937a & b; Oh & Kim 1976; Choi & Koh 1986; Park 1991; Choi & Koh 1994; Hong & Yoo 1996; Lim & Hong 1997; Hong et al. 1999; Hong & Yoo 2001; Lim & Choi 2001; Hyun et al. 2002; An et al. 2006; Lim et al. 2006; Paik et al. 2007; Yoo et al. 2007; Cha et al. 2009; Yoon et al. 2009a –c; Seo et al. 2009; Choi et al. 2010; Yoon et al. 2010; Jung et al. 2011; Kim et al. 2011; Yoon et al. 2011; Yu et al. 2011; Yu et al. 2013; Park et al. 2014; Seo et al. 2014; Kim et al. 2016a & b; Lim et al. 2016; Seo et al. 2016; Kwon et al. 2017a & b; Jeong & Shin 2018; Seo et al. 2019; Kim et al. 2019; Khim et al. 2021; Kim et al. 2021; Youn et al. 2021), Japan (Imajima 1968; Kawabe 1975; Imajima & Takeda 1975; Yokoyama & Hayashi 1980; Imajima 1982; Lin 1983; Okanishi et al. 2016; Nishijima et al. 2015), South Viet Nam (Gallardo 1968; Paxton & Chou 2000), China (Wu et al. 1980; Yang & Sun 1988; Paxton & Chou 2000), South China Sea (Glasby et al. 2016), Andaman Islands, Bay of Bengal and Palk Bay, India (Tampi & Rengarajan 1964) (Fig. 9). Berkeley & Berkeley (1950, 1952) suggested the presence of M. japonica in the Canadian Pacific and off Washington, USA, however noted doubts of their identification suggesting M. longicornis as an alternative. Jones (1971) concurred with the authors, later synonymising the records with the latter species. The records of the species from the Indo-Pacific region (Andaman Islands, India, and South Viet Nam) do not mention the distinct pigment band which is present in M. japonica. Given the lack of such a distinctive character in the notes of both records, and their distance from the type locality, further verification is warranted to ascertain whether M. japonica really occurs in these regions. The authors suggest that M. japonica is predominantly a Temperate Northern Pacific (sensu Spalding et al. 2007) species (Fig. 9).
Many of the above records suggest the species dominates at shallower depths, however, several occurrences at more than 30 m have been noted (Choi & Koh 1986; Paik et al. 2007). The species has been reported as dominant in sediments with low organic content (Lim & Hong 1997) and a sensitivity to organic enrichment was also additionally noted by Nishijima et al. (2015).
Remarks. The original description of M. japonica does not disclose the holding institution of the type material, although does state that the specimens originally came from the Hokkaido Fishery Station. Brasil (2003) stated the holotype was originally deposited at the National Museum of Nature and Science, Tokyo (NSMT), however, reported the holotype to be lost. Discussions with NSMT, suggested the type material may have been deposited at the Hokkaido University Museum. Personal communications by the second author with Hiroshi Kajihara revealed putative syntypes held at the latter institution. The specimen label noting “ Magelona japonica ”, “Okuda”, and “Incheon”, fitting well with the type material. Unfortunately, the material has dehydrated at some point, although, the condition is sufficient to corroborate Okuda’s observations and many of the identifying characteristics. Examination revealed several specimens which have been previously dissected (Fig. 8D), possibly prior to drawing, and one specimen bears close resemblance to that originally drawn by Okuda (1937a: fig. 23) and herein imaged (Fig. 8C). A posterior fragment devoid of anal cirri (Fig. 8G) as described by Okuda is additionally present. Whilst slight discrepancies in the sizes of specimens exist between the original description and the redescription herein, this is likely due to desiccation of the syntype material. Okuda’s longest specimen was 42 chaetigers, and although the syntypes are marginally shorter, this may be due to dissection, or fragmentation. The current authors are content that the Hokkaido University Museum material represents the syntype material of the species.
There are several differences noted between the original description and the redescription given herein, particularly in relation to the parapodia. The original description notes “the eight anterior chaetigers bear slender dorsal and ventral postchaetal lobes”, however, the figures show distinctly ventral neuropodia for the first and fifth parapodia, which is in agreement with the current findings, in which they only become postchaetal at chaetigers 7 to 9. Okuda (1937a) made no mention of scoop-shaped thoracic neuropodia; however, they can be observed on the syntype material, even in their dried state (Fig. 8C–E). Okuda also noted the lamellae of chaetiger 9 to be subequal, however, they are relatively similar in length in both rami, as he originally drew. The pigment bands of the type specimens have now faded to a large extent, but this is consistent with that seen in other magelonid specimens after more than ten years in preservatives (pers. obs. of second author).
Okuda noted scattered brown spots on the lateral sides of the thorax, just behind the parapodia. Although not observed by the current authors, white spots scattered over the thorax can be seen. Six to eight abdominal hooded hooks per rami were described by Okuda, although there are approximately ten for the Japanese specimen. On examination of one of Okuda’s syntypes eight hooks were seen per ramus, not dissimilar from the current specimen.
The subspecies Magelona japonica var. koreana Okuda, 1937 was elevated to species level (Magelona koreana) by Jones in 1971. However, it has been largely ignored in the literature outside of a few sporadic records (e.g., Gallardo 1968; Glasby et al. 2016). The two species share many morphological similarities, but M. koreana differs in possessing small ventral lamellae on chaetiger 9, and sub-equal abdominal lamellae. The original description of M. koreana is ambiguous in terms of pigmentation, stating “no marked pigmentation”, this requires further verification. It is entirely possible that some records of M. japonica may actually represent M. koreana. The type material of Magelona koreana is believed lost and no further information can be added at this time.
In their phylogenetic hypotheses under Magelonidae, Mortimer et al. (2021a) highlighted a group containing the species Magelona fasciata Mortimer, Kongsrud & Willassen, 2021 and M. cincta. The group (Group F) was characterised by the presence of scoop-shaped thoracic neuropodial lamellae, and a pigmented band in the thorax. Magelona japonica was not included in that analysis, but it is highly likely to belong to this ‘ Magelona cincta ’ group of species. Magelona fasciata, a species described off Western African, differs from M. japonica in possessing bidentate, not tridentate, hooded hooks as in the latter species. The prostomial horns of M. fasciata are rudimentary, and pigmentation runs throughout the entire body, whilst in M. japonica prostomial horns are more distinct and pigmentation is limited to the posterior thorax. Similarly, M. cincta differs from M. japonica in possessing rudimentary prostomial horns.
A further eight species are recorded to possess a thoracic pigment band: M. alleni; Magelona equilamellae Harmelin, 1964; Magelona sp. I of Uebelacker & Jones (1984); Magelona variolamellata Bolívar & Lana, 1986; Magelona guineensis Mortimer, Kongsrud & Willassen, 2021; Magelona picta Mortimer, Kongsrud & Willassen, 2021; Magelona nanseni Mortimer, Kongsrud & Willassen, 2021; and Magelona mackiei Mortimer, Kongsrud & Willassen, 2021. They all differ from M. japonica in not possessing scoop-shaped neuropodial thoracic lamellae, and in lacking distinct prostomial horns. Magelona alleni further differs in possessing sub-equal abdominal lamellae (as seen in M. koreana), M. mackiei in possessing bidentate hooded hooks, and lastly Magelona sp. I in possessing polydentate hooded hooks.
Two morphologically similar species known from an adjacent ecoregion are Magelona mickminni Nateewathana & Hylleberg, 1991, and Magelona petersenae Nateewathana & Hylleberg, 1991. Magelona mickminni differs from M. japonica in lacking scoop-shaped lamellae. Whilst M. petersenae shares many morphological similarities with M. japonica the thoracic lamellae are slightly less scoop-shaped and the prostomium frontal margin is crenulate.
description: Published as part of Taylor, Abbie, Mortimer, Kate & Jimi, Naoto, 2022, Unearthing the diversity of Japanese Magelona (Annelida: Magelonidae); three species new to science, and a redescription of Magelona japonica, pp. 451-491 in Zootaxa 5196 (4) on pages 460-462, DOI: 10.11646/zootaxa.5196.4.1, http://zenodo.org/record/7235610