Capitella longqiensis, Sun & Bao & Chen & Wu & Ju & Liao & Zhou, 2025

Capitella longqiensis View in CoL sp. nov.

( Fig. 2)

ZooBank registration: urn:lsid:zoobank.org:act:.

Capitella sp. — Zhou et al. 2022: supplementary table S5.

Type material: Holotype ( RSIO POLY49001 ): Southwest Indian Ridge, Longqi vent field ( 49°38.88′E, 37°46.80′S), depth 2778 m, TV-guided grab, R / V Xiangyanghong 10, Cruise DY 49, DY49I-S011- TVG04 , 6 January 2018 GoogleMaps . Paratype 1 ( RSIO POLY49002 ) and paratype 2 ( RSIO POLY49003 ): collected from the same location as the holotype GoogleMaps .

Description: All specimens incomplete. Holotype 13 mm long, 1.3 mm wide, with 24 chaetigers. Paratypes ranging from 16 to 26.3 mm long, 0.9–1.4 mm wide, with 20–35 chaetigers. Colour in alcohol light brown to pale white ( Fig. 2A). Prostomium rounded, wider than long. Peristomium wider than prostomium, longer than chaetiger 1, forming a complete achaetous ring ( Fig. 2A–C). Eyespots absent. Nuchal organs not visible. Torax with nine segments, not distinctly biannulate. Chaetigers 1–5 slightly increasing in size; chaetiger 5 widest, with mid-ventral groove; chaetigers 6–7 gradually decreasing in size, narrower than chaetigers 1–5, with mid-ventral and lateral groove; chaetigers 8 and 9 similar in shape, narrower, with mid-ventral and lateral groove and running along abdomen ( Fig. 2D). All three specimens with unilimbate capillaries in notopodia and neuropodia of chaetigers 1–7 ( Fig. 2D) and with hooded hooks in notopodia and neuropodia of chaetigers 8 and 9. Notochaetae arranged in one or two irregular rows of 7–15 capillaries in chaetigers 1–7 and 20–22 hooded hooks in chaetigers 8 and 9 ( Fig. 2E, F); neurocheatae arranged in one or two irregular rows of 4–17 capillaries and 16–20 hooded hooks. Genital spines with tips sharply curved present on chaetigers 8 and 9: chaetiger 8 with two external genital spines, chaetiger 9 with three embedded genital spines ( Fig. 2G). Division between thorax and abdomen not prominent. Abdominal segments multi-annulated, as long as wide; chaetigers with 15–20 hooded hooks in a row. Hooks similar on thorax and abdomen, with long anterior shaf, a pointed and large main fang, and five or six teeth in two rows above main fang; hoods short, not extending beyond main fang ( Fig. 2H, I).

Methyl Green staining patern: Prostomium, peristomium, chaetigers 1–5 and 7 lightly stained with a dark ring, chaetigers 8 and 9 darkly stained, and abdominal segments staining darkly at notopodial and neuropodial tori, whereas the rest of the abdominal segmental region is largely unstained ( Fig. 2B).

Molecular identity: GenBank accessions PQ643863–PQ643865 for cox1, PQ651927 for 16S, PQ651921 for 18S, PQ651924 for 28S, and PQ651930 for H3 (also see Supporting Information, Table S2). No identical matches on GenBank for all five genes.

Etymology: Te specific epithet ‘ longqiensis ’ refers to the type locality, Longqi hydrothermal vent field, SWIR.

Type locality: Longqi hydrothemal vent field, SWIR, Indian Ocean ( 49°38.88′E, 37°46.80′S), depth 2778 m, in suspected inactive sulphide deposits peripheral to active areas at the Longqi hydrothermal vent field. Specimens were collected together with the neolepetopsid limpet Neolepetopsis ardua GoogleMaps .

Distribution: Known only from the type locality.

Remarks: Capitella longqiensis belongs to a group of species of Capitella with capillary chaetae on chaetigers 1–7 and hooded hooks on chaetigers 8 and 9. Capitella longqiensis shares features with Capitella aracaensis Silva & Amaral, 2017 , C. blakei , and Capitella singularis (Fauvel, 1932) by exclusive capillaries on both noto- and neuropodia of chaetiger 7, the peristomium that is clearly distinct from the prostomium and forming a complete ring, and the absence of eyespots. However, these species differ in the shape of the prostomium, which is conical and smooth in C. singularis and C. blakei , rounded and smooth in C. aracaensis , and rounded with a ventral groove in C. longqiensis . Differences are also seen in the number of hooded hooks and genital spines. Both C. longqiensis and C. blakei have more hooded hooks per fascicle than C. aracaensis and C. singularis : C. longqiensis has 16–22 hooded hooks per fascicle on chaetigers 8 and 9 and 15–20 on anterior abdominal chaetigers; C. blakei has 20–22 hooks on chaetigers 8 and 9 and 25–22 hooks on anterior abdominal chaetigers; C. singularis has 5–10 hooks per fascicle on chaetigers 8 and 9 and 12 on anterior abdominal chaetigers; and C. aracaensis has 4–6 hooks on chaetigers 8 and 9 and 8–11 on anterior abdominal chaetigers. Among the deep-sea species ( C. blakei , C. iatapiunai , C. longqiensis , and C. multibranchiata ), C. blakei and C. longqiensis have only capillaries on chaetigers 1–7, whereas C. iatapiunai has hooks on chaetiger 7, and C. multibranchiata has capillaries on chaetigers 1–8/9. Overall, C. longqiensis can be distinguished by the combined characteristics of having a rounded prostomium with ventral groove, a peristomium forming a complete ring, capillaries only on chaetigers 1–7, and 16–22 hooded hooks per fascicle on chaetigers 8 and 9.