Rhombonemertes, Hookabe & Moritaki & Jimi & Ueshima, 2022

Hookabe, Natsumi, Moritaki, Takeya, Jimi, Naoto & Ueshima, Rei, 2022, A new oerstediid discovered from wood falls in the Sea of Kumano, Japan: Description of Rhombonemertes rublinea gen. et sp. nov. (Nemertea: Eumonostilifera), Zoologischer Anzeiger 301, pp. 154-162 : 157-160

4.1. Systematic discussion for Rhombonemertes gen. nov.

Members of Oerstediidae Chernyshev, 1993 are morphologically circumscribed by a bilayered rhynchocoel-wall musculature, the cephalic vessels protruding into the rhynchocoel in cerebral ring, and the mid-dorsal vessel not penetrating the rhynchocoel (= lacking vascular plug) ( Kajihara, 2021). Although the cephalic vessels could not be traced in the cerebral region, the other two features were confirmed in R. rublinea gen. et sp. nov. ( Fig. 3H, L ), leading to our conclusion that the species belongs to Oerstediidae . This is further supported by the molecular phylogenetic analyses, where R. rublinea gen. et sp. nov. is nested in the Oerstediidae clade and more closely related to the monophyletic Oerstedia clade than to Argonemertes australiensis ( Dendy, 1892) in Plectonemertidae Gibson, 1990 ( Fig. 5 ).

Among 127 monostiliferan genera ( Kajihara 2021), comparatively few are known to possess a short rhynchocoel ( Gibson 1990b). In Table 2, free-living marine oerstediid genera with a bilayered rhynchocoel wall, rhynchocoel extending half the body length or shorter, and without a vascular plug are listed with selected morphological characters. R. rublinea gen. et sp. nov. is dissimilar to any known oerstediid species other than except Ischyronemertes Gibson, 1990 . This is especially due to: i) the presence of body-wall diagonal musculature, ii) the longitudinal muscle layer of body wallnot divided anteriorly, iii) the presence of a precerebral septum, iv) 4 eyes, and v) free-living, not parasitic or commensal lifestyle. Ischyronemertes was originally described from mid-to lower shore in Vancouver Peninsula (Western Australia) for three species, I. albanyensis Gibson, 1990 , I. erythrophleps Gibson, 1990 , and I. tetrophthalma Gibson, 1990 . Most conspicuous difference between R. rublinea gen. et sp. nov. and Ischyronemertes is the shape of head; the former possesses a lozenge-shaped head while the latter is characterized by a more rectangular, somewhat anteriorly bilobed head ( Gibson 1990b, figs. 17, 19, 21). The new species is also differentiated from I. albanyensis and I. erythrophleps in the number and arrangement of eyes—four large eyes in the new species ( Fig. 2A, B, D ) and 2–6 small eyes more or less irregularly arranged in four groups in I. albanyensis and I. erythrophleps ( Gibson 1990b) . Furthermore, accessory nerves are present in lateral nerve cords of Ischyronemertes ( Gibson 1990b) , which is another difference between R. rublinea gen. et sp. nov. and Ischyronemertes . Based on these morphological differences between the existing genera in Oerstediidae , we establish a new genus, Rhombonemertes gen. nov., for R. rublinea gen. et sp. nov.

In addition to the oerstediid genera in Table 2, Nemertopsis tetraclitophila Gibson, 1990 resembles R. rublinea gen. et sp. nov. in terms of the internal morphology; Hong Kong specimens of N. tetraclitophila exceptionally lack a mid-dorsal vascular plug in the genus ( Gibson 1990a), and thereby display a character state that fits morphological circumscription of Oerstediidae ( Kajihara 2021) . As was the case in Nemertopsis quadripunctata ( Quoy & Gaimard, 1833) ( Kajihara 2007) , however, the vascular plug might be hard to recognize in the Hong Kong specimens due to the small-sized body. Thus, the internal morphology of N. tetraclitophila needs to be revisited with additional materials. Apart from the internal morphology, R. rublinea gen. et sp. nov. is clearly distinguished from N. tetraclitophila in the external appearance—two longitudinal dark-green stripes on the dorsal surface and the rounded head in N. tetraclitophila .

In the resulting tree, R. rublinea gen. et sp. nov. was a sister clade of Monostilifera sp. Genroku Seamount ( Fig. 5 ); uncorrected p- distance based on COI between the two species was higher than interspecific COI divergences (e.g., Chen et al., 2010; Sundberg et al., 2016; Hookabe et al., 2022). Morphological differences between the two species also warrant the recognition of R. rublinea as a species distinct from Monostilifera sp. Genroku Seamount—the latter species lacking eyes and diagonal muscles of the body wall, and having rhynchocoel extending 3/4 of the body length. The sister-taxon relationships with the clade formed by R. rublinea gen. et sp. nov. and Monostilifera sp. Genroku Seamount were not well resolved in our ML and BI analyses ( Fig. 5 ). More extensive taxon sampling including apparently polyphyletic, and yet-to-be-discovered deep-sea oerstediid lineages will be required to obtain a more robust phylogeny of these taxa.

Figures

Fig. 3. Rhombonemertes rublinea gen. et sp. nov. (paratype). Photomicrographs of transverse (A, B, D–M) and frontal (C) sections. (A) Body wall in precerebral region; (B) Body wall in foregut region; (C) diagonal muscle fibers (arrowheads) between outer circular and inner longitudinal muscle layers of body wall, intestinal region; (D) frontal organ; (E, F) precerebral vessels (arrowheads); (G) cephalic glands; (H) mid-dorsal vessel (arrowhead); (I) proboscis, arrowheads pointing to proboscis nerves; (J) brain; (K) nephridial canals (arrowhead); (L) rhynchocoel musculature and dorsoventral muscles; (M) ovaries. Abbreviations: br, brain; co, cerebral organ; dvm, dorso-ventral muscle of body wall; ep, epidermis; ic, intestinal caecum; ilm, body-wall inner longitudinal muscle; ocm, body-wall outer circular muscle; pe, proboscis epithelium; pic, proboscis inner circular muscle; pml, proboscis middle longitudinal muscle; poc, proboscis outer circular muscle; py, pylorus, rcm, rhynchocoel circular muscle; rlm, rhynchocoel longitudinal muscle; ov, ovary. Scale bars: 45 μm (A, B); 50 μm (C–E); 100 μm (F–K), 75 μm (L, M).

Fig. 5. A maximum likelihood (ML) tree based on concatenated sequences of 16S (400 bp), COI (602 bp), 18S (1750 bp), and 28S (1043 bp). Numbers near each node are support values generated by a separate partitioned ML bootstrap analysis with 1000 replicates and posterior probability by the BI analyses. Solid triangles indicate full support values (100% of bootstrap value/1.00 of posterior probability).

Fig. 2. Rhombonemertes rublinea gen. et sp. nov, living specimens. (A) Whole body, dorsal view, holotype (male) and paratype (female). (B) Magnification of head, dorsal view, holotype. (C) Magnification of head, ventral view, paratype. (D) Microphotograph of a living specimen, squeezed with a coverslip after anesthetization, holotype. (E) Magnification of stylet apparatus in everted proboscis, a black arrowhead pointing to a central stylet and white arrowheads to accessory stylet pouches. Abbreviations: af, anterior cephalic furrows; cc, cerebral canal, cg, cerebral ganglia; co, cerebral organ; ic, intestinal caecum; id, intestinal diverticulum; pb, proboscis; pf, posterior cephalic furrows; pp, proboscis pore; rc, rhynchocoel, te, testis. Scale bars: 5 mm (A), 500 μm (B, C), 750 μm (D), 100 μm (E).