Nereis tricirrata sp. nov.

Figs 2A-H, 3A-L, 4A-F

Material examined.

Holotype: TIO-BTS-Poly-137, complete, northern South China Sea, (17°33'N, 111°9'E), 1766 m depth, coll. Jun-Hui Lin, 16 June 2019 . Paratype: TIO-BTS-Poly-138, incomplete, northern South China Sea, (18°26'N, 112°26'E), 1157 m depth, coll. Jun-Hui Lin, 21 June 2019 .

Sequences.

OP292645, COI gene, 649 bp; OP292646, 16S gene, 437 bp; OP292647, 18S gene, 1330 bp; OP292648, histone H3, 308 bp; extracted from ethanol-preserved tissue of the holotype.

Diagnosis.

The new species is characterized by: (1) absence of eyes on the prostomium; (2) possession of three anal cirri instead of two on the pygidium; (3) few paragnaths on both rings of the pharynx; (4) notopodial and neuropodial ligules acutely conical; and (5) homogomph falcigers in posterior notopodia with several coarse teeth.

Description.

Holotype complete but broken into two fragments. Body tapering posteriorly. Anterior fragment 35.27 mm long for 44 chaetigers, remaining posterior fragment 7.28 mm long for 15 chaetigers (including regenerated segments), maximum width 2.1 mm (excluding parapodia) at chaetiger 7. Paratype incomplete, broken into three fragments with 45 chaetigers, 12 chaetigers and 7 chaetigers, respectively. Body in formalin light brown. Preserved specimens without pigmentation (Fig. 2A).

Prostomium pentagonal and slightly longer than wide, with one pair of digitiform frontal antennae (Fig. 2B). One pair of biarticulated palps arising antero-laterally, palpophores cylindrical, palpostyles globular. Eyes absent (Fig. 2B).

Peristomium apodous, 1.5 times as long as chaetiger 1. Four pairs of tentacular cirri slender, distally tapered (Fig. 2B); postero-dorsal pair the longest, extending to chaetiger 3.

Pharynx dissected, with dark brown jaws, distally curved, each with 15 blunt teeth on cutting edge. Small conical paragnaths sparse on both rings, arranged as follows: Area I = 0; II = 4 cones in a row; III = 0; IV = 2; V = 0; VI = 1; VII-VII = 2.

First two chaetigers uniramous, remaining ones biramous. Uniramous chaetigers with acutely conical dorsal ligules, subequal in length and of similar shape to ventral ligule (Fig. 2D). Dorsal cirri slightly longer than dorsal ligules.

Notopodia of biramous chaetigers with dorsal and ventral ligules, without notopodial prechaetal lobes. Notopodial dorsal ligules acutely conical (Fig. 2E-H), gradually becoming reduced towards posterior end (Fig. 2F-H). Dorsal cirri slender and attached to base of dorsal ligule throughout, subequal in length to notopodial dorsal ligules in anterior parapodia (Fig. 2D), and markedly longer than dorsal ligules in middle and posterior parapodia (Fig. 2G, H). Notopodial ventral ligules acutely conical, subequal in length to dorsal ligules in anterior parapodia (Fig. 2E), and 1.5-2 times length of dorsal ligules in posterior parapodia (Fig. 2G, H).

Neuropodia of biramous chaetigers with neuroacicular ligules subtriangular, postchaetal lobes rounded (Fig. 2E-H). Neuropodial ventral ligules acutely conical (Fig. 2E), longer than neuroacicular ones, decreasing in size to posterior end (Fig. 2E-H). Ventral cirri attached to ventral edge of parapodia, conical in anterior parapodia, becoming slender and cirriform from middle parapodia (Fig. 2F-H). Ventral cirri shorter than neuropodial ventral ligules in most chaetigers, but longer in chaetigers near pygidium (Fig. 2H).

In anterior parapodia, notochaetae with four homogomph spinigers (Fig. 3A, D); neurochaetae homogomph spinigers and heterogomph falcigers in dorsal fascicles (Fig. 3B, C, E, F), heterogomph spinigers and falcigers in ventral fascicles (Fig. 3B, E). In mid-body, notochaetae with two homogomph spinigers and one homogomph falciger (Fig. 3G); neurochaetae as in anterior parapodia (Fig. 3H, I). In posterior parapodia, notochaetae with two homogomph falcigers (Fig. 3J); neurochaetae as in anterior parapodia (Fig. 3K, L). Neurochaetae decreasing gradually in number towards posterior end.

All spinigers with long blades finely serrated (Fig. 4A-C); blade of notopodial spinigers shorter, but thicker than neuropodial ones. Notopodial falcigers commencing between chaetigers 20-30 (chaetiger 24 in paratype), with straight, finely serrated, blunt-tipped blade in mid-body (Fig. 4D), but with coarse teeth on relatively short blade in posterior parapodia (Fig. 4F). Neuropodial falcigers with relatively long, serrated, and blunt-tipped blade (Fig. 4E).

Posterior end with six or seven regenerated chaetigers (Fig. 2C), which are disproportionately smaller than normal chaetigers. Pygidium with three anal cirri, all filiform, one on mid-dorsal and one on each of the ventro-lateral sides (Fig. 2C).

Etymology.

The specific epithet tricirrata is composed by the Latin prefix tri -, meaning three, and the Latin noun cirrus, and refers to the three anal cirri present on the pygidium, one on the mid-dorsal and one on each of the ventro-lateral sides.

Distribution.

Currently only known from the deep-sea sedimentary habitat in the northern South China Sea.

Habitat.

Deep-sea soft sediments characterized by foraminiferal ooze at depths between 1100 m and 1800 m.

Phylogenetic analysis.

There are no identical sequence matches on GenBank for COI and 16S. The low 18S gene divergence (0-1.9% K2P) between Nereis tricirrata sp. nov. and other Nereis species revealed their close genetic relationship, including an eyeless species, Nereis sanderi Blake, 1985 (AM159579). The reconstructed phylogenetic tree (Fig. 5), using the maximum likelihood and Bayesian inference analyses, indicates that all Nereis species form a monophyletic clade with 100% nodal support and confirms the placement of Nereis tricirrata sp. nov. within the genus Nereis . Currently, limited sequences of eyeless Nereis species are available, which hinders a better understanding of the relationship among eyeless Nereis species. When comparing the new species to other described Nereis species with COI genes available in GenBank, the mtCOI-based genetic divergence (K2P) ranged from 23.1% to 37.3% (Table 2), which was comparable to that of previous studies on other nereidid genera, such as Alitta species (Villalobos-Guerrero and Carrera-Parra 2015), Neanthes species (Shimabukuro et al. 2017), and cryptic species of Nereis denhamensis (Glasby et al. 2013).

Remarks.

Nereis tricirrata sp. nov. is distinguished from most Nereis species around the world by the absence of eyes on the prostomium. With the new species in this study, seven other described Nereis species from the deep Pacific also lack prostomial eyes. Six of these species belong to a distinct group with greatly prolonged notopodia in the posterior parapodia, including N. profundi Kirkegaard, 1956, N. anoculis Hartman, 1960, N. anoculopsis Fauchald, 1972, N. sandersi Blake, 1985, N. piscesae Blake & Hilbig, 1990, and N. abyssa Imajima, 1990. Comparison of the two eyeless species bearing normal notopodia throughout the body showed that Nereis tricirrata sp. nov. differs from Nereis izukai Okuda, 1939 (Imajima 1996) from the Japanese Pacific in the arrangement of paragnaths on the pharynx and the nature of notopodial falcigers in the posterior parapodia. Nereis izukai possesses far denser paragnaths on the pharynx (Area I = 11; II = 52-56; III = ~ 70; IV = 50-60; V = 0; VI = 6-12; VII-VIII = ~ 62), and its notopodial falcigers lack coarse teeth on the cutting edge in the posterior parapodia. A not-formally-named Nereis species without prostomial eyes, labelled as Nereis sp. B, was recorded from off eastern Taiwan Island at depths of 2233-2551 m (Hsueh 2020). It was unclear whether Nereis sp. B possessed prolonged notopodia in the posterior parapodia as it was incomplete and lacked the posterior end. Despite this, Nereis sp. B is distinct from Nereis tricirrata sp. nov. in that the former possesses more paragnaths than the latter (Area I = 2; II = 21; III = 37; IV = 11-30; V = 0; VI = 5-7; VII-VIII = 74). Finally, it should be noted that the new species bears three slender anal cirri on the pygidium instead of two as usually occurs in nereidid species.