Family Polynoidae Kinberg, 1856
Subfamily Aphroditiformia Pettibone, 1984
Genus Branchipolynoe Pettibone, 1984
Brnachipolynoe onnuriensis n. sp.
Six specimens. Holotype (B_S_MA_00031740) and five paratypes (B_S_MS_00031741-3), collected from the OVF on the northern Central Ridge of India (st. GTV1906 - 11°24.96’ S, 66°25.397’ E, 2064m).
Etymology and Host
Named in honor of the discoverer of the OVF, the host is Gigantidas vrijenhoeki Jang & Won, 2020.
Body long, slightly tapered anteriorly and posteriorly, arched dorsally, and flattened ventrally, with 21 segments, including the first achaetous segment (Fig. 1A, B), and 10 pairs of elytra and elytrophores, on segments 2, 4, 5, 7, 9, 11, 13, 15, 17, and 19. Elytra moderately large, oval, smooth, without border papillae (Fig. 2C-F), covering anterior and posterior dorsal ends, but leaving the middle of the body partially covered by the dorsum (Fig. 1A, B). Non-elytra-bearing segments with short, smooth dorsal cirri on short, cylindrical cirrophores. Dorsal cirri with short slender tips, tapering gradually, longer than anterior and ventral cirri, not extending beyond the tips of the neurochaetae.
Prostomium ellipsoidal, bilobed with almost rounded anterior lobes. Short, conical median antenna inserted between two anterior lobes and pair of short conical palps (Fig. 1C). Median antenna and palps smooth and tapering to the slender tip. Palps extending beyond the prostomium. Prostomium lacking frontal filaments, eyes, and lateral antennae. First segment fused to prostomium with two pairs of short anterior cirri. Anterior cirri smooth and slightly slender, not exceeding the prostomium length (Fig. 1C). Thick, extended muscular pharynx with five pairs of dorsal and ventral small, sac-like terminal papillae surrounding the mouth (Fig. 2A).
Branchiae on segments 3–21 dense and arborescent, with short terminal filaments (Fig. 2B), not extending beyond the elytral border. Branchiae separated into two types, showing dorsal and ventral emergence, respectively. No discernible dorsal tubercles. Branchiae gradually decreasing in size anteriorly and posteriorly.
Parapodia subbiramous. Notopodia smaller than neuropodia, with few notochaetae projecting beyond notopodia (Additional file 4). Neuropodia large, rounded, enclosing numerous neurochaetae with rounded lobes. Notochaetae smooth, stouter, and shorter than neurochaetae (Fig. 3A). Notochaetae few, more abundant on the middle and posterior than anterior segments, slightly tapered with serrated distal part; tip rounded, shaft with inconspicuous rows (Fig. 3B-D). Neurochaetae numerous, more abundant in the middle than anterior and posterior segments, arranged as a vertical fan; tapered, with subdistal swelling and small spines along edge, serrations starting at the midpoint on only one side and extending distally.
Neurochaetae divided into supraacicular and subacicular neuroseatae. Supraacicular neurochaetae long, stout, slender tips, each with a minute hook; serrated distally and flattened on one side. Subacicular neurochaetae with slightly hooked tip, serrated parts shorter than supraacicular neurochaetae (Fig. 4).
Ventral cirri small, smooth, without papillae, and attached to the middle regions of neuropodia; projecting anteriorly (Fig. 1D). Elongated ventral papillae of female on segments 11 and 12; projecting posteriorly, reaching the subsequent segment. Elongated ventral papillae of males on segment 12, not extending beyond half the length of the subsequent segment (Fig. 1D). Pygidium small, with pair of short, thick, tapered conical anal cirri.
The holotype 28 mm long and 13 mm wide, including parapodia. Paratypes vary in size, 23–31 mm long and 9–15 mm wide. All specimens with elongated ventral papillae on segments 11 and 12, suggesting that males were not found.
Nine species have been described in the genus Branchipolynoe [4,13,19,20,32,33]. The diagnostic characteristics of the genus were determined from Branchipolynoe symmytilida and amended by Pettibone for B. seepensis [13,33]. This latter revision included the first position of the branchiae, the presence of the frontal filament, and form of the parapodium. Subsequently, Zhou (2017) published a description of B. longqiensis in the Indian Ocean and Lindgern (2019) published descriptions of five new species from the Pacific Ocean (Additional file 3). Members of the genus have 21 segments and 10 elytra; the elytra partially cover the dorsal region. Only B. symmytilida has frontal filaments, and all have a bilobed prostomium, except B. kajsae. In Branchipolynoe n. sp, the branchiae start at the third segment, and the parapodium is subbiramous, forming a very similar shape to B. longqiensis and B. tjiasmantoi, but with a short, rounded notopodial acicular lobe, inconspicuous pharynx papillae, and different shapes of the tips of sub-acicular neurochaetae (Additional file 5).
Among Branchipolynoe species, the mitochondrial COI genetic distance ranged from 0.056 to 0.237, with an average of 0.175 (Table 1). The species with the closest COI genetic distance to that of B. onnuriensis n. sp. was B. tjiasmantoi from the western Pacific, with a value of 0.056. This value is similar to those of B. halliseyae and B. kajsae (0.059), which are referred to as sister species in previous studies due to their similar morphology . The second closest COI genetic distance belonged to B. longqiensis (0.099) found in the Indian Ocean.
The ML tree and BI analyses inferred from Branchipolynoe mitochondrial COI sequences produced a single topology for each region (Fig. 5). The phylogenetic trees isolated the Branchipolynoe species and supported a clade comprising B. onnuriensis n. sp. and B. tjiasmantoi (ML: 78%, BI: 1).
General features of mitochondrial genomes
The complete mitochondrial genome of B. onnuriensis n. sp. was 16,217 bp in length, comprising 15 protein-coding genes (PCGs) (Fig. 6), 7 NADH dehydrogenase subunits (nad1–6 and nad4L), 4 cytochrome oxidase subunits (cob and cox1–3), 2 ATP synthase subunits (atp6 and atp8), and 2 small and large ribosomal RNA genes (rns and rnl). We identified 22 transfer RNA (tRNA) genes and an A+T-rich region. Among these genes, ND5 was the longest (1,525 bp) and atp8 was the shortest (160 bp). The tRNA length ranged from 64 (trnC) to 71 (trnQ), with an average length of 66.41 bp (Table 2). No gene rearrangement was detected (Additional file 6).
To assess the mitochondrial genome, we calculated its nucleotide composition (A%, C%, G%, T%, A+T%, C+G%), AT skew, and GC skew. AT and GC skew were calculated as follows: AT skew = (A – T%) / (A% + T%) and GC skew = (G – C%) / (G% + C%). The overall nucleotide composition of the complete mitochondrial genome was 28.45% A, 24.44% C, 8.99% G, and 38.12% T. The proportion of AT content (66.57%) was ~1.99 times higher than that of GC content (33.43%) (Additional file 7). Most genes showed positive AT skew, except rns and rnl. All genes also showed negative GC skew, indicating that PCGs in B. onnuriensis n. sp. contained a higher percentage of T and C than A and G, except rns and rnl.