Notomastus sp.

Notomastus sp. TVG12

( Fig. 5)

Material examined: RSIO POLY49004 : non-vent site, organic mater-rich sediment on the Southwest Indian Ridge ( 48°43.38′E, 38°0.6′S), depth 3082 m GoogleMaps , R / V Xiangyanghong 10, Cruise DY 49, DY49II-SWIR-S030 - TVG12 , 2018 .

Description: Incomplete specimen, anterior fragment, 4 mm long and 4.8 mm width, with ~42 chaetigers. Alcohol-preserved specimen pale yellow. Prostomium conical, anterior rounded, without palpode ( Fig. 5A, D). Proboscis with sof papillae ( Fig. 5C). Eyespots and nuchal organs not observed. Peristomium and thoracic chaetigers with tessellated epithelium ( Fig. 5A–D). Torax with 11 chaetigers, biannulate, with only capillaries. Chaetigers increasing in width to chaetiger 7, tapering discretely in chaetigers 9–11 Fig. 5A–C). Genital pores or lateral organs not observed from thoracic chaetigers. Transition between thorax and abdomen marked abruptly by chaetal change from capillary in the thorax to hooded hooks in the abdomen, and constriction with the last thorax chaetiger distinctly more slender than the rest ( Fig. 5A, C). Lateral organs occurring from first abdominal chaetiger as small protuberances, closer to notopodial lobes ( Fig. 5E). Abdominal segments with smooth epithelium. Abdominal notopodia and neuropodia greatly reduced as a low ridge ( Fig. 5C, E, F); notopodia lobes atached dorsally ( Fig. 5E), with 20–25 hooded hooks per fascicle; neuropodial lobes extended to the lateral end, ventrally connected by a medial ridge ( Fig. 5C, F), with 35–38 hooded hooks per fascicle. Hooded hooks with long anterior shaf, a pointed and large main fang, and multiple teeth above main fang; hoods moderate, not extending beyond main fang ( Fig. 5G). Te rest of the body unknown.

Methyl Green staining patern: More or less uniformly in chaetigers 1–9, stained light green, dark green in chaetigers 10 and 11; abdomen with dark stain rings around noto- and neuropodial lobes ( Fig. 5B).

Molecular identity: GenBank accessions PQ643869 for cox1, PQ651928 for 16S, PQ651922 for 18S, PQ651925 for 28S, and PQ651931 for H3 (also see Supporting Information, Table S2). No identical matches on GenBank for all five genes.

Distribution: Currently found only from sediments rich in organic mater on the SWIR. Te specimens were collected from sediments with organic mater accounting for 0.29% of the dry weight, two to three times higher than background sediments.

Remarks: Te specimen was assigned to Notomastus , the most species-rich genus, by having 11 thoracic chaetigers with capillaries only and followed by abdomen with hooded hooks only. Te phylogenetic analyses support the genus assignment. Notomastus sp. TVG12 falls into a well-supported clade including the type species N. latericeus . Given that the specimen is poorly preserved, with most of its chaetae lost, meaningful comparison with other species of Notomastus is not possible, hence the species is unidentifiable at the species level herein, and instead we use the collection number ‘TVG12’ as an informal name.

Phylogenetic and genetic distance

Alignment and concatenation of the five gene fragments of capitellids generated a dataset of 3062 bp for phylogenetic analyses. Te consensus tree from ML analyses recovered Capitellidae as monophyletic (BS = 95; Fig. 6). Within Capitellidae , two main clades (clades I and II; Fig. 6) were well supported, with BS values of 91 and 89, respectively. Among the genera included in the phylogenetic analyses, only Capitella was recovered as monophyletic (clade I-iii, BS = 93; Fig. 6). Te sequences from D. branchiatus (Edmond) and C. longqiensis . (Longqi) formed a well-supported clade, respectably, corresponding to the two new species. Decamastus branchiatus is sister to an undescribed ' Capitellidae sp.' from the interior of an inactive chimney at the rim of the Kebrit brine pool in the Red Sea ( Vestheim and Kaartvedt 2016) with full support (BS = 100). Capitella longqiensis grouped among species of Capitella , sister to Capitella sp. collected from the southern Chinese coast (BS = 100), and these two, in turn, grouped with C. aracaensis (BS = 99) from Brazil. Te single specimen collected from the organic mater-rich sediments on SWIR, tentatively named herein as ‘ Notomastus sp. TVG12’, nested in a clade of Notomastus (clade I-iv, BS = 81; Fig. 6), sister to an undescribed Notomastus sp. collected from the nodule province of the eastern Clarion-Clipperton Fracture Zone in the equatorial Pacific Ocean ( Bonifácio et al. 2020).

Te K2P genetic distances were estimated using 475 bp of cox1 and 259 bp of 16S (Supporting Information, Tables S3 and S4). Within the family Capitellidae , the average genetic distance between genera was 41.7% (range 20.8%–63.1%) for cox1 and 30.7% (range 26.8%–34.7%) for 16S, in comparison to the mean intra-genus genetic distance of 25.7% (range 19%–36.4%) for cox1 and 23.8% (range 22.5%–25.2%) for 16S. Without reliable sequences from the other species from the genus Decamastus , D. branchiatus showed the lowest genetic distance from C. aracaensis (26.4%) for cox1 and from Capitella teleta Blake, Grassle & Eckelbarger, 2009 (23.6%) for 16S.

Capitella View in CoL showed interspecific K2P genetic distances ranging from 17.9% ( C. aracaensis View in CoL vs. Capitella neoaciculata Silva & Seixas, 2017 View in CoL ) to 25.3% ( C. aracaensis View in CoL vs. C. longqiensis View in CoL ) for cox1, and from 9.4% ( Capitella biota Silva & Amaral, 2017 View in CoL vs. C. teleta View in CoL ) to 37.3% ( C. aracaensis View in CoL vs. C. neoaciculata View in CoL ) with the mean value of 22.5% for 16S. Te new species, C. longqiensis View in CoL , exhibited lowest genetic distance from C. teleta View in CoL (19.4%).

Species of Notomastus View in CoL showed a larger range of interspecific genetic distances than Capitella View in CoL in both genes, ranging from 23.4% ( Notomastus profondus Eisig, 1887 View in CoL vs. Notomastus sp. TVG12) to 53.2% ( Notomastus profondus View in CoL vs. Notomastus koreanus Jeong, Soh, Wi & Suh, 2018 View in CoL ) for cox1, and from 5.2% (between two unidentified species from different polymetallic nodule provinces of the CCZ to 43% ( Notomastus hemipodus Hartman, 1945 View in CoL vs. Notomastus cf. latericeus View in CoL from Emerald Basin) for 16S. Te genetic distances between Notomastus sp. TVG12 and its Notomastus View in CoL congeners ranged from 23.4% (vs. Notomastus profondus View in CoL ) to 50.1% (vs. Notomastus koreanus View in CoL ) for cox1, and from 9.6% (vs. Notomastus sp. collected from the nodule province of the eastern Clarion-Clipperton Fracture Zone in the abyssal equatorial Pacific Ocean) to 36.6% (vs. Notomastus hemipodus View in CoL ) for 16S.

Te intraspecific genetic distance of cox1 was 0.6% in C. longqiensis , whereas all three cox1 sequences of D. branchiatus were identical, much lower than the interspecific genetic distance in each of the two species.